Tooth Movement - Critical Reviews in Oral Biology & Medicine

Critical Reviews in Oral Biology & Medicine 

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Tooth Movement 

Zeev Davidovitch 

CROBM 1991 2: 411 

DOI: 10.1177/10454411910020040101 

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Critical Reviews in Oral Biology and Medicine, 2(4):411-450 (1991) 

Zeev Davidovitch 

Department of Orthodontics, The Ohio State University College of Dentistry, Columbus, Ohio 

ABSTRACT: This article reviews the evolution of concepts regarding the biological foundation of force-induced 

tooth movement. Nineteenth century hypotheses proposed two mechanisms: application of pressure and tension 

to the periodontal ligament (PDL), and bending of the alveolar bone. Histologic investigations in the early and 

middle years of the 20th century revealed that both phenomena actually occur concomitantly, and that cells, as 

well as extracellular components of the PDL and alveolar bone, participate in the response to applied mechanical 

forces, which ultimately results in remodeling activities. 

Experiments with isolated cells in culture demonstrated that shape distortion might lead to cellular activation, 

either by opening plasma membrane ion channels, or by crystallizing cytoskeletal filaments. Mechanical distortion 

of collagenous matrices, mineralized or non-mineralized, may, on the other hand, evoke the development of 

bioelectric phenomena (stress-generated potentials and streaming potentials) that are capable of stimulating cells 

by altering the electric charge on their membrane or their fluid envelope. In intact animals, mechanical perturbations 

on the order of about 1 min/d are apparently sufficient to cause profound osteogenic responses, perhaps 

due to matrix proteoglycan-related "strain memory". 

Enzymatically isolated human PDL cells respond biochemically to mechanical and chemical signals. The 

latter include endocrines, autocrines, and paracrines. Histochemical and immunohistochemical studies showed 

that during the early places of tooth movement, PDL fluids are shifted, and cells and matrix are distorted. 

Vasoactive neurotransmitters are released from periodontal nerve terminals, causing leukocytes to migrate out 

of adjacent capillaries. Cytokines and growth factors are secreted by these cells, stimulating PDL cells and 

alveolar bone lining cells to remodel their related matrices. This remodeling activity facilitates movement of 

teeth into areas in which bone had been resorbed. 

This emerging information suggests that in the living mammal, many cell types are involved in the biological 

response to applied mechanical stress to teeth, and thereby to bone. Essentially, cells of the nervous, immune, 

and endocrine systems become involved in the activation and response of PDL and alveolar bone cells to applied 

stresses. This fact implies that research in the area of the biological response to force application to teeth should 

be sufficiently broad to include explorations of possible associations between physical, cellular, and molecular 

phenomena. The goals of this investigative field should continue to expound on fundamental principles, particularly 

on extrapolating new findings to the clinical environment, where millions of patients are subjected annually 

to applications of mechanical forces to their teeth for long periods of time in an effort to improve their position 

in the oral cavity. Recently developed research tools such as cell culture techniques and immunologic probes, 

are the best hope for enhancing this development. 

KEY WORDS: orthodontic forces, distortion of cells and matrix, neurotransmitters, cytokines, synergism. 

I. INTRODUCTORY REMARKS 

Throughout their natural history, teeth move 

and migrate. Prior to their eruption into the oral 

cavity, changes in the position of tooth buds occur 

primarily due to the growth of dental structures, 

and the concomitant remodeling of neighboring 

tissues, i.e., alveolar bone, gingiva, and 

periodontal ligament (PDL), including the dental 

follicle. Following their emergence into the oral 

cavity, teeth reach a position in the dental arch, 

dictated by the forces of the surrounding muscles 

of the tongue, cheeks, and lips, and by contact 

with teeth of the opposite jaw. During mastication, 

teeth can move slightly in the vertical and 

horizontal directions, within the constraints of the 

soft tissues of the PDL, and the bendability of 

the alveolar bone. Despite their large magnitude, 

masticatory forces do not alter the position of 

teeth, due to their short duration. However, in 

the presence of periodontal disease, when paradental 

tissues are gradually destroyed, teeth can 

1045-4411/91/$.50 

© 1991 by CRC Press, Inc. 411 



migrate into new positions, where the masticatory 

and parafunctional forces reach equilibrium. 

Often, these new positions are aesthetically 

undesirable. 

Tooth position may be deemed undesirable 

due to functional and aesthetic considerations, 

prompting patients to seek orthodontic care. In 

its most simplistic translation, "orthodontics" 

means straightening teeth. This "straightening", 

or movement of teeth into desirable positions, is 

accomplished by the application of forces to teeth, 

usually of small magnitude (on the order of a few 

grams per square centimeter of dental root surface) 

and long duration (usually about 2 years). 

Millions of people are subjected annually to orthodontic 

treatment worldwide, making this 

branch of dental care a widespread and lucrative 

specialty. The number of dentists in the U.S. who 

limit their practice to orthondontics is presently 

around 10,000. However, many general dentists 

worldwide provide orthodontic care to their patients, 

and all, specialists and generalists alike, 

base their therapeutic means upon the time-tested 

observation that teeth can be forced to move away 

from their position in the dental arch to new locations 

by means of applied mechanical forces. 

The following review focuses upon the phenomenon 

of tooth movement that can be brought 

about by the application of continuous mechanical 

forces to teeth. Excluded from this review 

are the phenomena of eruptive, pathological (periodontal 

disease related), and surgically induced 

tooth movements. Specifically, this review discusses 

biological aspects of force-induced tooth 

movement on the tissue, cellular, and molecular 

levels. 

II. HISTORICAL PATHFINDERS 

The first recorded recommendation to use 

force for orthodontic reasons was made around 

the year 1 A.D. by Celsus, who suggested the 

application of finger pressure to teeth for alignment 

purposes.1 Seventeen centuries later, Fauchard 

was the first to publish a description and 

an illustration of an orthodontic appliance, which 

generated forces by using ligatures to tie teeth to 

a rigid arch.2 In the 18th century, Hunter3 provided 

the first biological explanation for ortho- 

412 

dontic tooth movement: "To extract an irregular 

tooth would answer but little purpose, if no alterations 

could be made in the situation of the 

rest; but we find that the very principle upon 

which teeth are made to grow irregularly is capable, 

if properly directed, of bringing them even 

again. This principle is the power which many 

parts (especially bones) have of moving out of 

the way of mechanical pressure." 

Two significant observations were made during 

the 19th century concerning the biological 

nature of orthodontic tooth movement. In 1815, 

Delabbare4 remarked that pain and swelling of 

paradental tissues occur following the application 

of orthodontic forces to teeth. In contemporary 

terms, Delabbare introduced the notion that inflammation 

is an integral part of orthodontic tooth 

movement. In 1888, Farrar5 hypothesized that 

tooth movement is due, at least in part, to bending 

of alveolar bone by applied forces. This notion 

was supported by Wolffss6 proposition in 1892 

that the internal architecture of bone is dictated 

by the mechanical forces that act upon it. 

The first report on the histomorphology of 

tissues surrounding orthodontically treated teeth 

was published by Sandstedt in 1904 to 1905.7T8 

That landmark experiment, which was performed 

in one dog, concluded that force-induced tissue 

changes are limited to the PDL and its alveolar 

bone margin. At the end of 3 weeks of treatment, 

Sandstedt observed new bone growth in the 

stretched PDL, and bone resorption in the area 

of PDL compression. Cell death occurred in the 

compressed PDL when the applied force was excessive, 

and the alveolar bone resorbed as a result 

of osteoclastic activity in adjacent marrow spaces 

(undermining resorption). 

Six years later, Oppenheim9 reported on a 

histologic examination of the jaws of one juvenile 

baboon whose teeth had been treated by orthodontic 

forces for 40 d. In contrast to Sandstedt, 

Oppenheim saw no demarcation between the old 

and new alveolar bone near the moving teeth, 

but rather a trabecular structure that strongly suggested 

a complete transformation of the entire 

alveolar bone in that region. The bony trabeculae 

were all rearranged in the direction of the force. 

However, Oppenheim's conclusions that orthodontic 

forces were capable of transforming the 

entire alveolus were rejected by his contempor- 



aries as misinterpretations. He was also criticized 

for using an animal with deciduous teeth for his 

experiment, suggesting that the transformation he 

had seen was related to growth and development 

rather than being the outcome of applied mechanical 

forces. 

Oppenheim's "transformation" hypothesis 

might have supported Farrar's earlier contention 

that orthodontic forces bend the alveolar bone, 

and thus are able to stimulate all the cells in and 

around this bone. However, Farrar's clinical approach 

also was not popular, because he advocated 

the use of heavy forces that could indeed 

bend the bone. In fact, the pendulum swung furthest 

to the other side, when Schwarz10 recommended 

the use of light orthodontic forces. He 

defined those forces as being "not greater than 

the pressure in the blood capillaries" (15 to 20 

mmHg, or about 20 to 26 g/cm2 of root surface). 

A 1957 publication by Fukada and Yasuda"l 

attracted wide attention. They observed that 

bending of bone by mechanical means evokes the 

generation of measurable electric potential spikes 

in areas of compression and tension. While this 

observation caused the rebirth of the field of applied 

exogenous electricity to bone nonunion 

fractures, it also precipitated the reintroduction 

of the concept of alveolar bone bending by orthodontic 

forces!2,13 

III. HISTOMORPHOLOGY OF TOOTH 

MOVEMENT 

A. Observations by Light Microscopy 

The pioneering work of Sandstedt and Oppenheim 

opened the door for comprehensive efforts 

to explore in detail the morphological 

changes in the stressed PDL. For over 4 decades, 

Reitan'4-'9 spearheaded this thrust with authority 

and confidence. The strength of his work was 

derived primarily from the extensive use of human 

material, whereby teeth that were to be extracted 

for orthodontic reasons were subjected to 

a variety of orthodontic force systems, i.e., light, 

heavy, continuous, intermittent, tipping, and 

translatory. At the end of the experimental period, 

the teeth were removed together with their 

surrounding tissues, and processed for histologic 

evaluation. Moreover, Reitan studied paradental 

tissues of animals subjected to orthodontic forces, 

particularly dogs and monkeys, exploring the effects 

of age, function, type of bone, force magnitude, 

duration, and direction, on the morphological 

characteristics of the tissues. He concluded 

that PDL cells in sites of tension proliferate, and 

that newly formed osteoid in these areas resorb 

slowly when subjected to pressure. In examining 

tissues from different species,20 Reitan observed 

that their responses varied, and attributed this 

variability to the differences in their structural 

composition, i.e., alveolar bone density, frequency 

and distribution of marrow spaces, and 

the cell and matrix constitution of the PDL. 

The realization that the rate of orthodontic 

tooth movement in humans varies and is unpredictable 

prompted Storey to suggest that it depends 

upon the magnitude of the applied force,21'22 

or the presence of hormonal fluctuations, such 

as those associated with the menstrual cycle.23 

These clinical observations motivated Storey24-27 

to conduct a series of experiments in rodents in 

which he applied forces of different magnitudes 

to the maxillary incisors, causing lateral tooth 

movement and mid-premaxillary sutural widening. 

In the rabbit and rat the teeth moved faster, 

as the force was increased, but as in man, there 

seemed to be a range of force magnitudes that 

could be termed optimal.24 Near teeth treated with 

such a force, newly formed bone appeared more 

mature, while heavy forces were associated with 

the formation of a highly cellular, poorly calcified 

matrix. Heavy forces caused periodontal necrosis 

and other destructive changes in the PDL, 

while light forces appeared to be favorite when 

moving a tooth through a thin plate of bone, as 

apposition of bone on the labial surface seemed 

to lag behind the resorptive activity on the PDL 

side.25 In older animals, the rate of tooth movement 

decreased, perhaps due to reduced cellular 

activity.26 Based on these observations, Storey,27 

concluded that the process of tooth translation 

through bone consists of three different phenomena: 

bioelastic, bioplastic, and biodisruptive. The 

PDL and alveolar bone, due to their fluid-fiber 

composition, can be deformed elastically by external 

stresses, which also evoke cellular activities. 

When the tissue elastic limit is reached, it 

starts to deform plastically, with adaptive prolif- 


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413

erative and remodeling reactions. Prolonged 

forces that exceed the bioplastic limit result in 

biodisruptive deformation, with ischemia, cell 

death, inflammation, and repair. Clinically 

speaking, Storey asserted that light forces within 

the bioplastic range would cause slow tooth 

movement, while optimal forces that cause teeth 

to move faster are within the boundaries of the 

biodisruptive range.27 He concluded that within 

the optimum range, tooth movement is rapid, but 

the quality of remodeling bone is poor, increasing 

the potential for relapse, once external force application 

ceases. 

In contrast to Reitan, who was able to study 

human paradental tissues following the application 

of forces to teeth, Storey's histologic work 

was conducted solely on rodents. Storey observed 

that in humans teeth move at different 

rates when forces of various magnitudes are used, 

and he used rodent incisors to explore the reasons 

for this phenomenon. While generally Storey's 

observations correlated well with findings of other 

investigators, it is doubtful whether conclusions 

derived from experiments in rodents can be directly 

extrapolated to man, due to marked physiological 

differences, as pointed out by Reitan.20 

Nevertheless, Storey's reports are significant, as 

he emphasized the development of an inflammatory 

process in the stressed PDL, even when 

light forces are being used.27 

Reitan's and Storey's investigations demonstrated 

the complexity of the tissue reaction 

during tooth movement. It was no longer perceived 

as a simple phenomenon of applied force 

causing the tooth to move within the PDL, leading 

to tension and compression, and subsequent 

bone formation and resorption, but rather as a 

dynamic set of events that involved profound alterations 

in cellular functions and changes in matrix 

composition. Thus, histology facilitated the 

morphological description of changes in the dentoalveolar 

complex that followed the administration 

of orthodontic forces to teeth. While being 

unable to explain why alveolar bone and PDL 

cells are responsive to applied mechanical 

stresses, or how these physical entities evoke biochemical 

responses by the cells, the histological 

investigations unveiled the sites of cellular activity, 

and enabled other researchers to ask "why" 

and "how". These questions were explored by 

414 

the use of methods such as histochemistry, electron 

microscopy, autoradiography, and immunohistochemistry. 

In addition, PDL and alveolar 

bone, recognized as the prime targets for orthodontic 

forces, were obtained from animals and 

humans and were subjected to mechanical stresses 

in culture conditions, either in tissue form or as 

isolated cells. 

B. Histochemical Changes Associated 

with Force-Induced Paradental Tissue 

Remodeling 

Activities of oxidative enzymes and phosphatases 

were localized in the PDL of rats during 

force-induced tooth movement by Deguchi and 

Mori28 and Takimoto et al.20'30 They caused tooth 

movement by placing a piece of stretched rubber 

between the first and second molars, forcing the 

teeth to move in opposite directions. (This 

method, introduced by Waldo and Rothblatt,31 

was later adopted by a number of investigators 

who used rats as experimental animals in studying 

tooth movement. It does not allow measurements 

of force magnitude, and the rubber pieces 

can traumatize gingival and periodontal tissues.) 

They reported on an increase in the number of 

osteoclasts displaying high succinic dehydrogenase 

activity in PDL pressure zones after 24 h. 

In contrast, they observed no changes in the distribution 

of acid phosphatase and lactate dehydrogenase 

in the stressed PDL. 

Rats were also used by Lilja et al.32 to study 

the distribution and activity of a number of enzymes 

associated with alveolar bone resorption. 

One maxillary molar in each rat was moved bucally 

by light (5 g) or heavy (36 g) forces generated 

by a spring attached to the incisors for 

either 10 h or 1, 3, 4, or 6 d. The activities of 

acid phosphatase increased in PDL cells in 

compression zones, as well as in adjacent gingival 

cells and alveolar crest periosteum. Staining 

for acid phosphatase also increased in marrow 

cells and in osteocytes near the PDL pressure 

zone. Lactate dehydrogenase activity, a marker 

of vital cells, disappeared from areas of PDL 

pressure ("hyalinized zones"), where cells apparently 

died in both cases of light and heavy 

forces. Interestingly, prostaglandin synthetase 



activity was found in alveolar bone marrow cells 

and in gingival cells, but not in PDL cells of the 

rat. Unfortunately, quantitative analysis of the 

data in this study was impossible, because each 

group consisted of only one animal. 

In a related experiment, Lilja et al.33 examined 

the PDL pressure zone of human premolars 

following force application for 1 to 30 d. Dilated 

blood vessels were seen near the hyalinized zone 

throughout the experiment. Intense activities of 

arylsulfatase and prostaglandin synthetase and 

moderate activity of aminopeptidase M were 

found in macrophages degrading the hyalinized 

zone. Adjacent osteoclasts displayed intense activity 

for succinic dehydrogenase and acid phosphatase. 

Again, each treatment time group consisted 

of one to two subjects, precluding statistical 

analysis of the data. 

The generation of osteoclasts in the compressed 

PDL did not escape attention, leading 

investigators to examine this zone in an effort to 

elucidate specific histochemical aspects of bone 

remodeling. Kurihara and Enlow34 stained rat 

maxillary second molar sections with ruthenium 

red, which demonstrates the presence of glycosaminoglycans 

(GAG). In areas of alveolar bone 

resorption, reattachment of the PDL seemed to 

occur as a result of GAG secretion by fibroblasts 

and osteoblasts, serving to link new and old collagenous 

fibers. Here, too, quantitation was impossible 

due to inadequate sample size. In contrast, 

Martinez and Johnson35 were better able to 

assess the effect of orthodontic forces on alveolar 

bone GAG content in rats. They moved maxillary 

molars in groups of four rats, treated for 1, 3, or 

5 d with a spring (25 g). They found that GAG 

staining in alveolar bone at PDL tension areas 

(achieved with alcian blue) increased at day 3. 

However, in an external control group that received 

an inactive spring, the GAG staining was 

lighter than in the untreated side of the maxilla 

of the rats treated with an active spring. Thus, 

this well-planned experiment demonstrated the 

need for a control group in which the orthodontic 

appliance remains inactive. 

Compression zone osteoclasts in rats were 

also the targets of an investigation by Noda,36 

who sought to determine the effect of calcitonin 

on osteoclastic cytochrome c oxidase activity. 

First, maxillary molars were moved lingually with 

a spring for periods of time ranging from 15 min 

to 72 h. Enzymatic activity was localized in mitochondria 

of osteoclasts by electron microscopy. 

Calcitonin injections caused an early reduction 

in the number of mitochondria and enzymatic 

activity in detached osteoclasts, but this effect 

was abolished 72 h after calcitonin administration. 

These results demonstrate that locally induced 

bone resorption may be affected by a boneseeking 

hormone. 

The above-mentioned histochemical studies 

did not, for the most part, produce quantifiable 

data. Nonetheless, they painted a picture of enzymatically 

active cells, engaged in the remodeling 

of the stressed PDL and alveolar bone. In 

areas of PDL compression, oxidative enzymes 

and proteinases were localized in osteoclasts and 

in macrophages removing necrotic tissue from 

hyalinized zones, demonstrating heightened metabolic 

rates in cells involved in alveolar bone 

resorption and degradation of PDL matrix and 

dead cells. Further details on the activities of 

these cells, as well as those located in PDL tension 

sites, are derived from experiments in which 

electron microscopy was utilized as the investigative 

tool, as discussed in the following section. 

C. Ultrastructural Changes in Paradental 

Tissues During Tooth Movement 

Despite Reitan's reported observation'9 that 

paradental tissues of the rat are different than 

those of man in many respects, as, for example, 

morphologically and physiologically, rats remained 

the experimental animal of choice in 

transmission electron microscopic (TEM) studies 

of paradental tissues during tooth movement. 

Rygh and Selvig37 described finding degradation 

products of erythrocytes in enlarged blood vessels 

and in the extravascular space of the compressed 

PDL. The tension site in the PDL was 

studied by Ten Cate et al.,38 who later also examined 

stretched cranial sutures in rats.32 In both 

PDL and suture they observed that fibroblasts 

were apparently engaged in synthesizing as well 

as degrading collagen. Fibroblasts entering areas 

of matrix disruption were termed "pioneers", a 

term introduced earlier by Rygh40 for identifying 

cells entering hyalinized zones in the compressed 



415

PDL. In the latter site, Rygh was undecided on 

whether these phagocytic cells were primarily 

macrophages or fibroblasts. In examining PDL 

tension zones in rats that had been subjected to 

force application (5 to 10 g) to a maxillary molar, 

Rygh41 observed distended blood vessels and mitotic 

PDL cells. Spaces appeared in the stretched 

PDL, which were filled with flocculent material. 

Collagen fibers were primarily oriented in the 

direction of tension, but many nonoriented fibrils, 

as well as elastic-like (oxytalan) fibers, were 

visible. The latter were detected earlier by 

Edwards42 in dogs. 

Teeth are attached to the alveolar bone with 

embedded PDL fibers (Sharpey's). Martinez and 

Johnson43 examined this attachment in rats using 

scanning electron microscopy (SEM). They reported 

that 5 d of tooth movement significantly 

reduced the diameter of the attached fibers, suggesting 

a reduction in the mechanical strength of 

the PDL. Kurihara and Enlow44 used TEM in an 

attempt to elucidate the nature of the attachment 

of PDL fibers to bone during resorptive activities. 

They concluded that the most prevalent type of 

attachment in resorptive sites is adhesive in nature. 

It consists of a layer of ground substance, 

deposited by fibroblasts on the naked surface of 

recently resorbed bone. Later, collagen fibrils are 

secreted into this layer, coalescing into fibers. In 

this fashion, partially released old bone matrix 

fibers intermingle with the newly formed PDL 

fibers. 

Since Kethcham's report in 1927 that orthodontic 

treatment is associated with radiographic 

evidence of significant dental root resorption in 

many moving teeth, this phenomenon was explored 

by numerous investigators in man and experimental 

animals using light microscopy, TEM, 

and SEM. Via SEM, Kvam45 examined human 

premolars that had been exposed to orthodontic 

forces. After 5 d of treatment, small areas of root 

resorption were found on the margins of the compressed 

PDL of all teeth, and after 25 d all treated 

teeth displayed resorption lacunae penetrating 

through the cementum into the dentin. Extensive 

external root resorption was observed in the teeth 

of patients whose palates had been expanded rapidly 

by Barber and Sims46 and by Langford and 

Sims.47 In this procedure, heavy forces were applied 

for about 14 d to teeth anchoring the ex- 

416 

pansion device, and the teeth were then retained 

in their new position for a few months to allow 

bone to fill the expanded mid-palatal suture. All 

anchor teeth exhibited root resorption lacunae, 

and the degree of resorption was directly related 

to the length of the retention period. Repair of 

root defects by cellular cementum was observed, 

but with little evidence of PDL fiber reattachment. 

Using light microscopy and TEM, Rygh48 

investigated the PDL compression zones in rats 

whose molars were subjected to orthodontic forces 

(5, 10, or 25 g) for 2 h to 28 d. Root resorption 

lacunae were seen near the hyalinized zone, in 

close proximity to a rich PDL vascular network. 

Rygh suggested that root resorption might be a 

side effect of the cellular activity associated with 

the removal of the necrotic tissue of the hyalinized 

zone, and described the removal of the cementoid 

layer as the elimination of the defense 

against resorption in an area that is strongly proresorptive. 

This proposed association between 

root resorption and PDL injury was supported 

recently by the results of an experiment in rats 

by Nakane and Kameyama.49 In that study the 

gingiva and PDL of a maxillary molar were injured 

repeatedly three times at 3-h intervals, by 

insertion of a 2-mm-long needle. Root resorption 

developed within 1 d near the traumatized, inflammatory 

PDL and continued through the 21 

d experimental period, with concomitant repair 

by cementoblasts. 

Since mineralized tissues remodel under the 

influence of systemic and local factors, it was 

suggested that factors associated with maintenance 

of calcium homeostasis might regulate the 

activity of root resorbing cells. To test this hypothesis, 

tooth movement was performed by researchers 

in hypocalcemic rats. Goldie and King50 

created calcium deficiency in adult lactating rats 

and applied 60 g force to a maxillary molar for 

1 to 14 d. The teeth in these rats moved significantly 

faster than in the control animals, as their 

bones underwent extensive resorption. However, 

SEM measurements determined that the extent 

of root resorption was decreased in the calciumdeficient 

rats, suggesting that bone resorting cells 

are more responsive to bone seeking hormones 

than cells that resorb roots of teeth. In a more 

recent experiment, Engstrom et al.51 moved apart 

maxillary incisors in growing rats (30 d old) who 



were being fed a calcium- and vitamin-D-deficient 

diet. Using light microscopy, they determined 

that root resorption in the moving teeth 

was enhanced in the hypocalcemic rats, in contrast 

to the finding of Goldie and King.50 This 

discrepancy may be a result of the age difference 

between the two groups of hypocalcemic rats, as 

well as because molars were tested in one study, 

while incisors were examined in the other. 

D. Autoradiographic Examination of the 

PDL During Tooth Movement 

The PDL contains a mixed population of cells 

that can synthesize or degrade bone, cementum, 

and the nonmineralized PDL itself. Some of the 

metabolic processes associated with these activities 

have been investigated by the use of autoradiography. 

In this method, radiolabeled amino 

acids are injected into animals, and their timerelated 

location is determined by exposing tissue 

sections to radiation-sensitive photographic 

emulsions. The resulting silver-bromide salt 

crystals that form over radioactive sites can be 

localized microscopically. In tooth movementrelated 

studies, investigators utilized tritiated 

proline (3H-proline) to study the kinetics of collagen 

synthesis and tritiated thymidine (3H-Tdr) 

to study cell proliferation. 

Garant and his collaborators52-55 have studied 

extensively the process of collagen remodeling 

by PDL fibroblasts. They administered 3H-proline 

into mice and rats to determine the pattern 

of collagen synthesis by PDL fibroblasts by using 

both light microscopy and TEM. They described 

PDL fibroblasts as being elongated, polarized 

cells, with the nucleus positioned at one end and 

the cytoplasmic and secretory part at the other 

pole. Fibroblasts were found to be distributed 

evenly throughout the rodent PDL, and to migrate 

between the fibers, interacting during motion with 

adjacent matrix and cells. This motion appears 

to be facilitated by cellular microfilaments (actin 

and myosin), and by attachment to the matrix 

with glycoproteins (mostly fibronectin).56 Garant 

and Cho concluded that in tooth movement, PDL 

fibroblasts in tension sites express the phenotype 

of actively migrating and matrix-secreting cells. 

Cells in the normal PDL proliferate and die 

regularly,57 but these events are markedly increased 

during tooth movement.58 Proliferative 

activities in the mechanically stressed rat PDL 

were studied extensively by Roberts et al.59-66 and 

by Yee et al.67'68 Uptake of 3H-Tdr by PDL cells 

in tension sites was increased significantly within 

2 h of the insertion of an elastic material between 

the first and second maxillary molars. Most of 

the mitotic activity occurred near the bone and 

the middle of PDL, but not near the dental root. 

Some of the newly divided cells appeared to migrate 

in the direction of the alveolar bone, perhaps 

because they were preosteoblasts.61 Based 

on these observations, Roberts and his associates 

concluded that stretching the PDL causes G,arrested 

cells to enter mitosis, while G1 cells are 

stimulated to start synthesizing DNA. The latter 

cells are readily labeled by 3H-Tdr. 

In an effort to identify and classify PDL cells 

at different stages of differentiation, Roberts and 

Cox69 resorted to measurements of nuclear volume 

in these cells. This tedious method revealed 

that the nuclei of osteoblasts are larger than those 

of fibroblasts, a fact that can be used to identify 

committed osteoprogenitor cells. While fibroblasts 

were found predominantly near PDL blood 

vessels, osteoblastic progenitors were located 

further away from the vessels and closer to the 

bone and cementum surfaces. These reports create 

the impression that the PDL preosteoblastic 

population resides solely within the boundaries 

of the PDL. However, McCulloch et al.70 and 

McCulloch and Heersche71 have attracted attention 

to the finding that in mice many alveolar 

bone marrow spaces are directly connected 

through vascular channels with the PDL. Moreover, 

frequent injections of 3H-Tdr into large 

groups of mice, and subsequent autoradiographic 

examination of their mandibles revealed that the 

endosteal spaces contain many labeled progenitor 

cells. Thickened areas of cementum were found 

opposite openings of these channels in 64% of 

the examined specimens. Although it is presently 

unknown whether progenitor cells that originate 

in alveolar bone marrow spaces participate in the 

PDL response to mechanical stress, it is tempting 

to speculate that such an association indeed exists. 



417

E. Tooth Movement: Visible Tissue 

Changes 

Clinicians realized 2000 years ago (perhaps 

earlier) that teeth can be moved from one position 

in the mouth to another by being subjected to 

persistent mechanical forces. Merely 250 years 

ago, Hunter made the first attempt to explain the 

biological basis for this dental movement. Without 

having seen the involved tissues magnified 

by a microscope, he postulated that force-induced 

tooth movement was facilitated by what we may 

term today as bone remodeling. This notion was 

verified in the early years of the 20th century by 

Sandstedt, who described in detail the effects of 

mechanical force on the PDL-alveolar bone interface. 

His work illustrated clearly that orthodontic 

tooth movement is made possible by the 

resorption of alveolar bone near sites of compression 

in the PDL, while bone apposition occurs 

where the PDL is stretched. A controversy erupted 

a few years later when Oppenheim suggested that 

the entire alveolar bone near a moving tooth remodels, 

including endosteal surfaces. Most, if 

not all, of the investigators who were engaged in 

exploring this issue during the first half of the 

20th century overwhelmingly supported Sandstedt's 

hypothesis because, clearly, the most dramatic 

initial histologic changes could be seen in 

the stressed PDL and its immediate bordering 

mineralized tissue surfaces, bone and dental root. 

Reitan, whose comprehensive histologic explorations 

spanned over 4 decades into the second 

half of this century, reported on bone remodeling 

in alveolar bone marrow spaces and gingival periosteum, 

both at a distance from the stressed PDL. 

These observations appeared to support Oppenheim's 

transformation hypothesis, and compelled 

Reitan to accept the century-old proposition of 

Farrar that orthodontic forces bend the alveolar 

bone. In the late 1960s Baumrind succeeded in 

demonstrating that such a bending effect indeed 

takes place, while others have measured spikes 

of altered electric potentials in teeth, PDL, and 

alveolar bone that had been subjected to orthodontic 

forces. 

The search for an optimal orthodontic force, 

a force that would be most efficient in moving 

teeth, led two investigators who examined paradental 

tissues microscopically to make contrast- 

418 

ing recommendations. Schwarz warned against 

using "heavy" forces, forces that occlude PDL 

capillaries and thus can damage the tissue. However, 

Storey recommended utilizing forces that 

do cause damage to the PDL, biodisruptive forces 

that introduce inflammation into this tissue. He 

also showed that within a certain range, tooth 

movement could be accelerated concomitantly 

with an elevation in force magnitude. Like Reitan, 

Storey associated slow tooth movement in 

adults with a slow rate of cellular activity. 

With the advent of electron microscopy, detailed 

information emerged in the last 2 decades 

on the ultrastructure of dento-alveolar tissues. 

Cells and matrices were studied in great detail 

by the use of TEM and SEM. Moreover, histochemical 

and autoradiographic investigations shed 

new light on biochemical events that occurred in 

the dento-alveolar tissue complex during normal 

existence and while under altered states of mechanical 

stress. It became evident that cells that 

remodel the dento-alveolar complex are equipped 

with an elaborate system of cytoplasmic organelles 

that enable them to synthesize and secrete 

matrix components and the enzymes that participate 

in the degradation of this matrix. Garant 

and Ten Cate and their associates demonstrated 

that PDL fibroblasts can readily remodel the matrix, 

as well as migrate through it, while Jee and 

Roberts and their collaborators identified preosteoblasts 

both in the PDL, following their cell 

cycle kinetics, and through the stretched PDL. 

In the compressed PDL, Rygh, Kvam, and others 

have identified macrophages that seemed to remove 

necrotic tissue. 

Taken together, the above microscopic studies 

on both light and electronic levels have described 

in great detail morphological changes, 

and some fundamental physiological alterations 

that seem to occur in dento-alveolar tissues during 

tooth movement. However, with the exception 

of the proponents of the bone bending hypothesis, 

none of the above investigations have 

addressed the question of the mechanism of transduction 

of physical stimuli to biological reactions. 

Those who advocated the idea that bent 

bone generates electric potentials proposed that 

these potentials somehow stimulate cells in a mechanically 

stressed area by causing structural and 

enzymatic changes in the cellular plasma mem- 



ane. However, interest in this question has increased 

rapidly in recent years, far beyond the 

boundaries of orthodontic tooth movement. 

Moreover, it appears increasingly evident that the 

regulation of cellular functions in most, if not 

all, tissues is under the influence of local and 

systemic factors that are derived from the endocrine, 

nervous, and immune systems. Consequently, 

Section IV reviews evidence in support 

of the hypothesis that dento-alveolar tissue remodeling 

during tooth movement is an outcome 

of cellular activities that are regulated by interactions 

between physical distortions and locally 

distributed humoral factors that act as endocrines, 

paracrines, or autocrines. 

IV. MECHANISMS OF CELLULAR 

STIMULATION IN TOOTH MOVEMENT 

A. The Effect of Mechanical Stress on 

Cells 

Cells of all kinds are subjected at one time 

or another to compression, stretch, and shearing. 

There is growing evidence that most cells have 

ion channels that are potentially capable of regulating 

active and passive variations in cellular 

mechanics. According to Morris,72 these ion 

channels are mechanosensitive, i.e., their openstate 

probability depends on stress at the membrane. 

Such channels were postulated decades 

ago as a means of mechanoelectrical transduction 

in muscle and nerve cells. However, it now seems 

that most other cell types have such channel components 

in their membranes. These channels are 

ubiquitous, occurring at uniform density, on the 

order of 1/jim2 and in every cell.73-75 Calcium 

ions may enter cells in significant amounts through 

these channels.76-78 According to Morris and Sigurdson,79 

tensions generated in patch electrodes 

to activate stretch sensitive channels are of the 

same order of magnitude as those measured in 

migrating fibroblasts.80 

Cell membrane tension may result from intracellular 

osmotic changes, contraction of cytoskeletal 

elements, or physical changes in the 

extracellular matrix. In 1985, Ingber and 

Folkman81 constructed three-dimensional cell 

models comprised of a discontinuous array of 

compression-resistant struts, pulled open by connections 

with tension elements. The stability of 

such a structure depends on maintenance of tensional 

integrity. Based on these models, and observations 

of cellular behavior in vitro, these authors 

concluded that important functions are 

regulated by alterations in the integrity and composition 

of the extracellular matrix. Interconnections 

between mammalian cellular nuclei and the 

plasma membrane, as well as with the extracellular 

matrix, are through the continuous system 

of cytoskeletal filaments and cell surface transmembrane 

receptors. Physical forces, either those 

generated by the cytoskeleton or in the matrix, 

appear to be important regulators of cell and tissue 

growth. This interaction between force and 

cell function was observed to exist in skeletal 

myotubes,82 lymphocytes,83 arterial smooth muscle 

cells,84 osteosarcoma cells,85 and endothelial 

cells.78 

Ingbar and Folkman81 hypothesized that if 

physical stimuli can be translated into metabolic 

alterations through changes of intracellular structure, 

then mechanochemical transduction of these 

signals is most likely mediated by the structural 

linkages that join the cytoskeleton with the external 

milieu. In 1985, Ingber and Jamieson86 

proposed that the cellular mechanism of mechanochemical 

stimuli is transduced into chemical 

information through local changes in thermodynamic 

parameters. In this fashion, activation 

energy of a reaction is produced by pressure and 

volume alterations, and various chemical reactions 

and macromolecular polymerization processes 

can be selectively promoted or inhibited 

as a result of mechanical perturbation of the cell 

surface. Indeed, Joshi et al.87 have been able to 

demonstrate that intracellular cytoskeletal polymerization 

can be modulated by mechanical forces 

applied to the cell surface in neurites. Similar 

changes may be caused by cell growth factor 

interactions. For instance, Herman and Pledger88 

reported on alterations in the distribution of actin 

and vinculin in fibroblasts as a result of exposure 

to platelet-derived growth factor. Likewise, the 

arrangement and function of steroid hormone receptors 

may be very sensitive to mechanical perturbation 

because they appear to be associated 

physically with the nuclear protein matrix.89-91 

Nicolini et al.92 reported that a specific in- 



419

crease in nuclear size is necessary for S phase 

initiation, an observation that was reported earlier 

by Roberts and Cox69 to occur in PDL cells. 

Nuclear shape seems also to play an important 

role in regulating nuclear transport. For instance, 

Jiang and Schindler93 studied the effect of various 

growth factors on nuclear transport, and suggested 

that changes in nuclear shape may be permissive 

for delivery of growth factor-receptor 

complexes to their site of action in the nucleus. 

Experimenting with mammary epithelial 

cells, Emerman and Pitelka94 observed that cell 

rounding is usually associated with inhibition of 

cell growth and with promotion of cytodifferentiation. 

Thus, cells that produce specialized 

products usually appear round, a shape that might 

facilitate exposure of specific parts of their genome. 

In orthodontic tooth movement, such transformations 

in cellular shape are readily visible in 

mechanically stressed paradental cells (Figures 1 

to 3). In unstressed PDL sites (Figure 1), alveolar 

bone osteoblasts appear flat, while those in areas 

of PDL tension (Figure 2) seem large and round. 

In areas of PDL compression (Figure 3), PDL 

fibroblasts assume a round shape. Histologic 

studies by Reitan'4-16 and Rygh41 have demonstrated 

that activated osteoblasts in PDL tension 

sites are engaged in producing a new bone matrix, 

while PDL cells in compression sites are 

primarily involved in enzymatic degradation of 

the compressed extracellular matrix. 

B. The Effect of Mechanical Stress on 

Mineralized and Nonmineralized 

Connective Tissues 

1. Regulation of Bone Remodeling In 

Vivo by Applied Stresses 

Typically, orthodontic forces are applied 

continuously to teeth and their surrounding tissues. 

These forces evoke cellular activity, as has 

been demonstrated by investigators who examined 

affected tissues mircoscopically. However, 

it is unclear how long a force should be applied 

to stimulate target cells in particular areas of PDL 

and alveolar bone. Lanyon and his associates have 

addressed this issue in a series of experiments 

whereby controlled strains were applied to avian 

420 

FIGURE 1. "Flat" alveolar bone osteoblasts (arrows) 

in a 5-[pm horizontal section of cat maxilla, stained 

immunohistochemically for cGMP. Tissue near control, 

nonorthodontically treated canine. B, alveolar bone; P, 

periodontal ligament. (Magnification x 1400.) 

bone in vivo, allowing the investigators to examine, 

radiographically, histologically, and histochemically, 

the effects of various strains on 

bone remodeling95-04 Their experimental model 

consisted of surgical removal of bone from both 

proximal and distal epiphyses of the ulna in turkeys 

and roosters, freeing the entire diaphysis 

from regular functional strains, leaving its nervous 

and vascular supplies intact.95 Mechanical 

loads are then introduced to this bone through 

stainless steel pins attached to an external loading 

apparatus. The operation caused removal of loadbearing 

by the ulna, followed by a loss of bone 

mass.96 This loss was prevented by 4 cycles per 

day of an externally applied loading regimen 

(10,000 to 12,000 microstrain, 0.5 Hz), for 42 

d. When the number of cycles was increased to 

36 per day, bone formation increased significantly. 

Static (continuous) loads had no effect in 



FIGURE 2. Alveolar bone osteoblasts (arrows) at PDL 

tension site after 14 d tipping force application to cat 

maxillary canine. Horizontal section, 5 ixm thick, stained 

immunohistochemically for cGMP. Notice the round appearance 

of the cells. B, alveolar bone; P, periodontal 

ligament. (Magnification x 1400.) 

this preparation on bone remodeling, while similar 

loads applied intermittently for a total of only 

a few minutes daily increased bone mass substantially.97 

The magnitude of the strain in the 

loaded bone appeared to be directly associated 

with the nature and degree of the remodeling 

response.4 Peak longitudinal strains below 0.001 

were associated with bone loss, while peak strains 

above this level were associated with substantial 

enhancement of periosteal and endosteal bone 

formation.98 These effects were found also in 

birds suffering from calcium deficiency." 

These experiments demonstrated that bone 

cells in vivo are very sensitive to a small number 

of strain cycles daily. A maximal osteogenic response 

was obtained by only 72 s of load bearing. 

Moreover, it seemed like the creation of a static 

load environment is essentially ignored as an os- 

FIGURE 3. Periodontal cells in PDL compression zone 

at the border of the necrotic "hyalinized zone" after 14 

d of tipping force application to cat maxillary canine. 

Horizontal section, 5 ipm thick, stained immunohistochemically 

for cGMP. Notice the enlarged size of the 

cells in comparison to the thinner PDL cells seen in 

Figure 1. (Magnification x 1400.) 

teoregulatory stimulus, suggesting that functional 

influence on bone architecture is derived solely 

from intermittent loading. 00 Lanyon101 then proposed 

a hypothesis to explain the mechanism by 

which bone adapts to functional load bearing. In 

his opinion, the osteocytes are the most likely 

candidates to sense the distribution, rate of 

change, and magnitude of strain in the bone matrix. 

Following their recognition of a change in 

the strain situation, osteocytes communicate with 

the bone surface cells that remodel the bone. It 

seems like the important feature of strain in this 

respect is in the occurrence of an abnormally 

distributed strain rather than an unusually large 

strain. Evidence for osteocytic response to applied 

stress was found in the higher level of glucose-6-phosphate 

dehydrogenase in these cells, 

and a sixfold increase in the number of osteocytes 



421

that have incorporated 3H-uridine into their 

RNA.102 Furthermore, Lanyon hypothesized that 

osteocytes respond not only to the transient effects 

of mechanical strain, but also to the persistent 

effect of mechanical strain on the matrix. 

As a candidate for such strain-sensitive matrix 

components, he chose proteoglycans.103 Examining 

bone sections in a polarizing microscope, 

Lanyon quantified their birefringence and observed 

matrix proteoglycan reorientation following 

short mechanical loading. These large, highly 

charged matrix molecules could be forced by 

strain to attach to cell surface receptors, or pass 

into the cell and attach to its cytoskeleton. Since 

the proteoglycan reorientation persists for 1 to 2 

d, it could provide a physical basis for a "strain 

memory" in bone.'03 In a more recent study, 

Skerry et al.104 found that reversal reorientation 

of bone matrix proteoglycans also occurs in vitro, 

not only in avian bone, but in bones from rats 

and dogs as well. They attributed this molecular 

distortion to strain-generated fluid flow, usually 

preferentially oriented with the direction of collagen 

fibers. 

The experiments of Lanyon and his associates 

provide an attractive explanation of the longlasting 

effects of short-lived strains on bone cells. 

However, the exact mechanism by which proteoglycan 

reorientation could influence the activity 

of target cells in bone remains unknown. In 

orthodontics, functional appliances subject teeth 

to intermittent forces, leading to their gradual 

movement. In this situation, bone matrix distortion 

could perhaps be evoked and act in the fashion 

proposed by Lanyon and co-workers. In contrast, 

fixed-appliance orthodontics resorts to 

continuous force applications. In this mode of 

treatment, tissue effects often contain widespread 

damage, intimately associated with inflammatory 

and reparative responses. Moreover, orthodontic 

tooth movement is materialized by intense resorptive 

activity of alveolar bone, while Lanyon 

and associates' short-term strain application did 

not evoke any resorptive activity, but rather extensive 

formative function by bone cells. Nonetheless, 

the implication of bone matrix in the 

transduction of mechanical stimuli to physiological 

responses by bone cells serves to broaden 

our understanding of the mechanism of cell stimulation 

by externally applied forces. This seem- 

422 

ingly critical involvement of bone matrix in the 

response of bone cells to mechanical stress can 

explain, at least in part, Oppenheim's9 earlier 

observation of a transformation of the entire alveolar 

process during tooth movement. 

2. Bioelectric Phenomena in Bone 

The dependency of bone tissue integrity and 

metabolism on mechanical stress has long been 

recognized. At the present time, growing evidence 

strongly suggests that osteoporosis can be 

curtailed or reversed by regular physical exercises, 

which subject skeletal elements to musclederived 

forces. 05106 Astronauts and animals that 

have participated in space flights or in experiments 

on simulated weightlessness demonstrated 

continuous loss of skeletal tissue due to the lack 

of gravitational forces.107"' These observations 

imply that mechanical stresses regulate the activity 

of skeletal cells, confirming Wolff s6 proposition 

that the structural architecture of bone 

depends on the nature of the mechanical stresses 

applied to it. Applying pressure and tension to 

chick embryo long bone rudiments in vitro, 

Glucksmannll2 observed in 1942 that optimal cartilaginous 

tissue structure was obtained only in 

the presence of mechanical stress. Experiments 

of this sort, which have persisted, demonstrated 

that skeletal tissues and cells can respond to applied 

mechanical stresses in vitro, even in the 

absence of other seemingly important systems, 

such as the nervous and vascular systems. Thus, 

bone loomed brightly as a self-contained tissue, 

whose response to mechanical stress is independent 

of any other tissue system. An example 

for this rather narrow approach, which attributed 

most of the control of the response of bone to 

mechanical stress on the bone cells themselves, 

can be found in the proceedings of a recent conference 

on functional adaptation in bone tissue. 13 

Isolated bones or bone cells were presented at 

that conference as being fully capable of responding 

biochemically to applied mechanical stresses. 

A proposed major link in the cascade between 

the applied force and the biological response was 

stress-generated electric potentials.114,115 

The concept of the inherent ability of bone 

to respond to applied mechanical stress was 



oosted by Fukada and Yasuda's" report in 1957 

that measurable electric potentials are evoked 

temporally in bent bone. They investigated dry 

specimens cut from femora of man and ox, and 

were able to measure direct and converse piezoelectric 

effects in those bones. They concluded 

that the piezoelectric effect appears only when a 

shearing force is applied to the collagen fibers of 

the bone lattice, causing them to slip past each 

other. More recently, Marino et al."6 investigated 

the piezoelectric characteristics of collagen 

films and concluded that these effects originate 

at the level of the tropocollagen molecule, or in 

molecules no larger than 50 A in diameter. Further 

support for the concept that collagen is the 

source of the piezoelectric effect came from studies 

on tendons17'118 and on decalcified bone. 19 

Bassett and Becker120 reported that a net negative 

potential and a net positive potential appear, respectively, 

on the compression and tension sides 

of bone. These phenomena were recorded later 

by Cochran et al.'21 in a segment of a bovine 

mandible, and by Gillooly et al.12 and by Zengo 

et al. 122,123 in dog mandibles. Marino and Gross'24 

recently compared piezoelectric surface charges 

of human bone with those of the cementum and 

dentin of whale teeth. They found that cementum 

and dentin were capable of producing only about 

12% of the surface charge produced by cortical 

bone, and concluded that "piezoelectricity mediates 

alveolar (bone) remodeling". This is a typical 

statement, which narrows the effect of mechanical 

stress on bone to the generation of electric 

potentials by the stressed collagen. 

The possibility that alveolar bone is indeed 

bent by the application of tipping or translatory 

forces to teeth was first suggested by Farrar.5 

This event was later confirmed by Baumrind13 in 

rats and by Grimm125 in humans. It led De- 

Angelis'26 to propose that the alteration of the 

electric environment within the stressed alveolar 

bone may regulate differentiation of bone progenitor 

cells. A mechanism by which these potentials 

may reach the surface of bone cells was 

suggested by Pollack et al. 15 According to this 

concept, bone is surrounded by an electric double 

layer in which electric charges flow in accordance 

with a stress-related fluid flow. This stress-generated 

potential may affect the charge of cell 

membranes, as well as that of macromolecules 

in their vicinity. Borgens,127 examining intact and 

damaged mouse bones, detected endogenous ionic 

currents that he attributed to streaming potentials, 

rather than to piezoelectricity, due to the long 

(up to 30 min) current decay period. He suggested 

that the source of current in mechanically 

stressed bone is cells rather than matrix. Otter et 

al.,128 studying dry and wet specimens of bovine 

tibia, concluded that while in the dry state the 

current is primarily piezoelectric, in wet bone the 

dominant mechanism is streaming potentials. 

Bioelectric measurements in alveolar 

bone'22,'23 have demonstrated that the compressed 

(concave) side of the orthodontically 

treated bone is electronegative with respect to the 

tension (convex) side, suggesting that negative 

potentials during bone bending can generate bone 

deposition, while positive potentials are responsible 

for bone resorption. However, Borgens' experiments 

in fracture sites'27 failed to find such 

a correlation. Rather, his measurements showed 

that current enters the lesion, where its dispersion 

(i.e., its pathway and density) remains unknown 

due to the complexity of the distribution of mineralized 

and nonmineralized matrices. 

Whichever is the source of electric potentials 

in bone, it seems that these endogenous currents 

are involved in bone repair, remodeling, and perhaps 

growth. This conclusion led numerous 

investigators129-"33 to apply weak currents to nonunion 

bone fractures in an effort to facilitate healing. 

Clinical successes in orthopedics prompted 

orthodontists to combine orthodontic force application 

with administration of weak electric 

currents to jaw tissues in an effort to determine 

whether a synergistic effect on the rate of tooth 

movement would be achieved. In the first experiment 

of this kind, Beeson et al. 134 implanted 

electrodes in cat mandibles and applied a 10-1pA 

direct current constantly for 5 weeks. No significant 

differences between electrically treated and 

control animals were found, regardless of whether 

the cathode or the anode were placed in the vicinity 

of the moving tooth. This absence of an 

effect seems to have stemmed from the placement 

of the electrodes near the apex of the moving 

teeth, rather than near the alveolar crest where 

most of the force-induced bone remodeling occurs 

when teeth are tipped. Different results were 

obtained by Davidovitch et al. ,135 136 who applied 



423

a current of 20 ILA to gingiva near orthodontically 

tipped maxillary canines in young adult cats. Significant 

acceleration of the rate of tooth movement 

was observed after 7 and 14 d of combined 

application of force and electricity. This potentiating 

effect was explained by the placement of 

the anode very close to the site of PDL compression, 

where alveolar bone resorption occurs, while 

the cathode was placed in close proximity to the 

site of PDL tension, where new bone is deposited. 

3. Biochemical Events in Cells Affected 

by Mechanical Forces In Vitro 

The ability to maintain cells and organs in 

culture permitted investigators to subject them to 

mechanical stress and to monitor their response 

to either tensile or compressive forces. These 

experiments targeted specific cellular functions, 

such as proliferation, synthesis, and secretion of 

extracellular matrix components, or the enzymes 

that degrade them. The rationale for these experiments 

is derived from the assumption that in 

vitro conditions facilitate the exposure of sensitive 

target cells to a specific stimulus, in the 

absence of any other factors that might exist in 

vivo, which would mask or "confound" the response 

that is observed in vitro. As discussed 

below, the situation in the intact mammalian organism 

differs from that of the culture system in 

that bone cells in the living animal may become 

subjected to mechanical stresses simultaneously 

with signal molecules derived from neighboring 

endothelial cells, fibroblasts, or migratory leukocytes.These 

molecules may amplify or diminish 

the effect of mechanical stress on the shape 

and cytoskeletal structure of the bone cells. Thus, 

for an in-depth understanding of the nature of the 

response of cells to mechanical stress it is essential 

to study combinations of these interactions, 

a design that is rarely performed for in vitro 

investigations. 

Among the main investigative yardsticks that 

were utilized to explore the mechanism of activation 

of cells by mechanical forces were cyclic 

nucleotides, prostaglandins, DNA, phosphatases, 

and metalloproteinases. Adenosine 3',5'monophosphate 

(cyclic AMP, or cAMP) and 

guanosine 3',5'-monophosphate (cyclic GMP, 

424 

or cGMP) have been identified as mediators of 

the effects of external stimuli on bone cells in 

vivo'37-'39 and in vitro.'40'141 Fluctuations in the 

levels of these substances have been found to 

occur in bone treated by parathyroid hormone 

(PTH),120 calcitonin,138 and vitamin D3,139 as well 

as electric currents'40 and mechanical force.'41 

Prostaglandins, particularly those of the E series, 

have been associated with bone remodeling 

activities resulting from malignancies,142 gingival 

inflammation,143 rheumatic joint disease,'44 

and fracture.'45 Thus, fluctuations in the levels 

of cAMP and cGMP in mechanically stressed 

cells and PGE2 in their culture media were used 

frequently as indicators of cellular responsiveness. 

In 1975, Rodan et al.141 applied compressive 

forces to chick cartilaginous bone rudiments and 

observed a reduction in the levels of cAMP and 

cGMP in these cells within 15 min. Uchida et 

al.146 stretched rat costochondral chondrocytes 

from 1 min to 24 h, and reported an initial increase 

in cAMP content at 3 to 5 min, with a 

decline to control levels shortly thereafter. The 

incorporation of 35S into GAG by the stretched 

cells increased significantly, but their rate of DNA 

synthesis was not altered. Interestingly, when 

stretched chondrocytes were incubated in the 

presence of calcitonin, their cAMP levels at 24 

h were much higher than those of control cells 

or cells treated with PTH. 

In 1980, Somjen et al.'47 stretched rat embryonic 

calvarial cells for 1 to 60 min. They 

observed sharp increases in cAMP and PGE2 levels 

in cells and media, respectively, with peaks 

at 15 to 20 min and subsequent declines. Both 

cAMP and PGE2 levels failed to increase when 

bone cells were stressed in the presence of indomethacin, 

a potent inhibitor of prostaglandin 

synthesis. More recently, Shen et al.'48 exposed 

osteoblasts cultured from rat fetal calvaria to different 

concentrations of PGE2, and within 10 min 

observed a transient change in shape from an 

epithelioid to stellate, with markedly increased 

numbers of gap junctions. 

Rat calvarial cells were also used by Hasegawa 

et al.149 in an effort to determine the effect 

of stretching on DNA and matrix component synthesis. 

Continuous or intermittent stretching for 

2 h increased both the number of DNA synthes- 



izing cells (by 64%) and the production of osteonectin-like 

molecules. These results suggest 

that bone cells respond to mechanical stress by 

increasing their numbers and by rearranging their 

contacts to neighboring structures. Similar effects 

on DNA synthesis were obtained with avian 

calvarial osteoblasts that were stretched for up to 

5 d by Buckley et al. 50 In addition, the stretched 

cells were uniformly aligned perpendicular to the 

direction of the strain field. 

Compressive forces were applied to bone cells 

in a number of studies, usually by compressing 

the gas phase above the culture medium. In this 

fashion, Klein-Nulend et al.'51 applied intermittent 

pressure to mouse calvaria for 5 d. This 

treatment increased alkaline phosphatase activity 

and 45Ca uptake by the calvaria, while resorptive 

activities decreased. The net result was a 16% 

increase in the calvarial mineral content. Similar 

effects were observed by these investigators following 

the application of intermittent compressive 

forces (132 g/cm2, 0.3 Hz) to fetal mouse 

metatarsal bone rudiments.'52 They concluded that 

compression inhibits the migration and activity 

of osteoclasts and their precursors. Heavier, continuous 

compressive forces (3 atm) were applied 

by Ozawa et al.'53 to osteoblast-like cells, resulting 

in suppression of osteoblastic activities 

and marked enhancement of PGE2 production. 

Unquestionably, the main arena of tissue remodeling 

during tooth movement is the PDL. It 

is the prime target of tooth-moving mechanical 

forces, and has been the subject of numerous 

investigations aimed at elucidating details of the 

biological response of its cells to applied mechanical 

stress. Duncan et al.154 applied mechanical 

forces to mouse molars in vivo as well as in 

vitro. After 3 to 5 d in culture, large amounts of 

PGE2 and type II collagen were synthesized by 

the ligaments. A substitute model for the PDL 

was introduced by Meikle et al.,155 who used a 

spring to apply tensile stress to rabbit calvarial 

sutures in vitro. They reported an increase in the 

tissue levels of collagenase and a reduction in 

the level of tissue inhibitors of metalloproteinases 

(TIMP) in these sutures. 

Fibroblasts isolated from chick embryos were 

grown and stretched on nylon meshes by Curtis 

and Seehar.'56 Intermittent stretching at 0.25 to 

1.0 Hz caused significant increases in mitotic 

frequency and in the proportion of cells in S 

phase. It was concluded that stress speeds up the 

mitotic cycle of fibroblasts, rather than switching 

cells from G, to S. However, Norton et al.157 

reported recently that tensile forces failed to 

change the cytoskeletal configuration in PDL fibroblasts 

(as determined by immunofluorescence 

of tubulin, vimentin, and actin), suggesting that 

these cells are not responsive to tensile forces per 

se, but rather to injurious effects resulting from 

these forces. Human gingival fibroblasts were 

stretched by Ngan et al.,158 who reported that a 

5% increase in cellular surface area for 5 min to 

2 h caused significant elevations in the levels of 

cAMP and PGE2 in the cells and their media, 

respectively. In a related study, Ngan et al.'59 

reported recently on similar effects in stretched 

human PDL fibroblasts. 

Despite the major role played by PDL fibroblasts 

in tooth movement, a surprisingly small 

number of investigations on the response of these 

cells to mechanical forces in vitro have been performed 

to date. However, the relative ease of 

obtaining these cells from extracted healthy human 

teeth,l60 and the availability of a number of 

mechanical systems that can apply various modes 

of compressive and tensile stresses to cells in 

culture promise to facilitate new experiments in 

the near future. However, since the PDL fibroblast-like 

cell population is heterogeneous, consisting 

of cells with differing phenotypes, and, 

as the PDL also includes numerous precursors of 

osteoblasts as well as epithelial and endothelial 

cells, strict identification and phenotyping of these 

cells will be required so that the data may be 

interpreted more meaningfully. 

Of great curiosity and perhaps importance is 

the possible role of the epithelial rests of Malassez 

in tooth movement. These clusters of epithelial 

cells, left behind during the growth of the 

dental root, are distributed throughout the PDL 

in close proximity to the root surface cementum 

layer. Their functional role continues to be an 

enigma. Reitan'7 noted that these epithelial clusters 

are eliminated from necrotic areas of compressed 

PDL and do not regenerate. He speculated 

that these cells may play a protective role 

in preventing force-induced root resorption. Brunette 

et al.'61 cultured monkey epithelial cells 

derived from rests of Malassez, together with 



425

PDL fibroblasts. They observed no junctional 

structures between epithelial cells and fibroblasts, 

but in some cases islands of epithelial cells 

were sandwiched between two layers of fibroblasts. 

Later, Brunette et al.'62 detected large 

quantities of PGE and PGF in media in which 

epithelial cells derived from porcine rests of Malassez 

had been incubated. They proposed that 

these prostaglandins might participate in the regulation 

of alveolar bone remodeling, either by 

affecting bone cells directly or indirectly by interacting 

with PDL fibroblasts and endothelial 

cells. Support for the concept of interaction between 

PDL fibroblasts, epithelial, and endothelial 

cells was provided by Merrilees et al. 163 They 

found that the GAG synthesized in vitro by PDL 

fibroblasts is predominantly chondroitin sulfate, 

whereas epithelial cells produced primarily hyaluronic 

acid. However, endothelial cells or their 

conditioned media, when cocultured with fibroblasts, 

stimulated increased GAG synthesis, particularly 

hyaluronic acid. 

The question of whether epithelial cells from 

the rests of Malassez can produce factors that 

enhance bone resorption was addressed by Birek 

et al.'64 They cultured epithelial cells or their 

conditioned media with mouse calvaria for 4 d, 

causing significant increases in calcium release 

from the bones. Indomethacin inhibited this resorptive 

effect only partially, suggesting that factors 

other than prostaglandins are synthesized by 

the epithelial cells, which may account for the 

osteolytic effects of the epithelial cells. The fact 

that epithelial cells from the rests of Malassez 

respond in vitro to tensile forces was demonstrated 

by Brunette. 65 In his experiment the number 

of 3H-Trd labeled cells doubled after 2 h of 

stretching. Moreover, stretched cells had a higher 

volume of filamentous structures and more desmosomes 

per unit length of cell membrane than 

unstretched cells. 

The above review indicates that bone cells, 

PDL fibroblasts, and epithelial cells from the rests 

of Malassez respond readily to applied mechanical 

stresses. These cellular responses include 

identifiable biochemical events that span the entire 

cellular domain. The plasma membrane, the 

cytoplasmic organelles, the filamentous skeleton, 

and the nucleus all seem to participate in the 

physical-to-chemical transduction process. How- 

426 

ever, one of the main foci of attention in this 

investigative field has been the role of prostaglandins 

in this process. The following section 

briefly reviews this issue. 

4. Prostaglandins and Force-Induced 

Bone Remodeling 

Since the report by Klein and Raisz'66 in 1970 

that prostaglandins stimulate bone resorption in 

tissue culture, numerous publications have implicated 

prostaglandins, particularly of the E series, 

in the response of bone cells to chemical 

and mechanical stimuli. Out of the plethora of 

investigations emerged a hypothesis, introduced 

by Rodan and Martin,'67 which proposed that 

osteoblasts regulate the resorptive activities of 

osteoclasts. This hypothesis was based upon the 

findings that osteoblasts carry receptors to all the 

hormones involved in the maintenance of calcium 

homeostasis, such as PTH, calcitonin, and vitamin 

D3. Osteoblasts respond to these endocrine 

molecules, as well as to locally produced agents 

such as growth factors, with elevations in cAMP 

contents and prostaglandin synthesis. Thus, prostaglandins 

could serve as a stimulatory link or a 

coupling factor between osteoblasts and osteoclasts. 

Applying tensile forces to cells derived 

from mouse embryo calvaria, Binderman et al.168 

stimulated the production of PGE2 and cAMP by 

these cells. This effect was abolished by agents 

that bind to membrane phospholipids (gentamicin 

and antiphospholipid antibodies), and thus reduce 

their availability for enzymatic changes. 

They concluded that mechanical forces exert their 

effect on bone cells by the following chain of 

events: activation of phospholipase A2, release 

of arachidonic acid, increased PGE synthesis, 

and elevated cAMP production. Taken together, 

these observations assign to PGE2 a central role 

in the regulation of force-induced bone cell activation. 

This is clearly an oversimplified concept 

that may apply to cell culture conditions, in which 

many factors found in the intact, living mammalian 

organism are absent. PGE was localized 

immunohistochemically in the cat PDL by Davidovitch 

et al.169'170 The application of tipping 

forces to cat canines for periods of time ranging 

from 1 h to 14 d caused a significant increase in 



the staining for PGE in PDL and alveolar bone 

cells at sites of both tension and compression, 

suggesting that PGE may indeed be involved in 

the response of PDL and bone cells to mechanical 

stress. However, as discussed below, other local 

factors with bone cell stimulation capabilities have 

been localized recently in the mechanically 

stressed PDL, suggesting that prostaglandins may 

be only one agent in a battery of factors that 

regulate the response of cells to force. 

The great emphasis in the literature on the 

role of prostaglandins in the response of bone 

cells to applied force prompted Yamasaki and his 

associates to perform a series of experiments on 

rats, monkeys, and humans, where PGE was injected 

into the gingiva near orthodontically treated 

teeth.171-174 In rats,171 PGE, or PGE2 administration 

increased the number of osteoclasts in mechanically 

stressed alveolar bone within 12 h. In 

monkeys,'72 the rate of tooth movement doubled 

during 18 d of treatment. The drawback of this 

experiment was the sample size, which consisted 

of two subjects. Similar results were obtained in 

a study on human patients. 173174 Here, PGE, was 

injected monthly for 5 months into the gingiva 

near orthodontically treated canines, resulting in 

doubling of the rate of tooth movement. In explaining 

why they chose PGE administration as 

an adjunct to orthodontic tooth movement, Yamasaki 

et al. stated that the rationale for the decision 

had been the reported evidence, primarily 

from tissue-cell culture experiments, that implicated 

PGE2 in bone resorption. However, PGE2 

has been shown to enhance metaphyseal bone 

growth in young rats,175 and PGE1 infusion for 

3 weeks enhanced alveolar bone growth in beagle 

dogs.'76 Thus, in vivo, PGE may regulate bone 

resorption and formation. To investigate this dual 

role of PGE in bone as a stimulator of both resorption 

and formation, Nefussi and Baron'77 cultured 

45Ca-labeled rat fetal long bones with PGE2 

for 4 d. This treatment caused enhanced periosteal 

osteoblastic activity (in terms of percentage 

of osteoblastic surfaces), but increased osteoclastic 

resorption in medullary cavities. Thus, 

the effect of PGE on bone cells may differ, depending 

on their location. 

Pursuing this avenue further, Lee178 moved 

teeth in rats by inserting an elastic band between 

the first and second maxillary molars, according 

to the method of Waldo and Rothblatt,31 for periods 

of 6 h to 5 d. In addition, the animals 

received either local gingival injections of PGE1 

twice daily, or a constant systemic administration 

by a mini-osmotic pump. In both cases the number 

of alveolar bone osteoclastic lacunae in PDL 

pressure sites was increased significantly, compared 

to non-PGEl-treated animals, but the effect 

was more pronounced in the animals receiving 

PGE, systemically. This finding raises the possibility 

of administering PGE, or PGE2 systemically 

during orthodontic treatment in an effort 

to enhance the rate of tooth movement. However, 

side effects of such a treatment must not be ignored. 

These effects may include diarrhea, vomiting, 

corneal congestion, and phlebitis. 

Another feature of prostaglandins related to 

bone remodeling has come to the foreground in 

recent years. Prostaglandins have long been 

known as being potent mediators of the inflammatory 

process in many tissues, including cartilage 

and bone. This fact led to the widespread 

use of nonsteroidal anti-inflammatory drugs in 

combatting rheumatoid arthritis'79'180 and jawbone 

destruction due to endodontic lesions'8' and 

periodontal disease.182-'84 If tooth-moving forces 

indeed cause an inflammatory reaction in the PDL, 

then it should be expected that not only would 

prostaglandins be found there, but other inflammatory 

mediators as well. The following sections 

examine this issue. 

V. REGULATION OF TOOTH MOVEMENT 

BY INFLAMMATORY MEDIATORS 

A. The Cells and Fluids of the 

Periodontal Ligament 

The PDL is a soft tissue envelope separating 

the tooth from the alveolar bone. Like all other 

connective tissues, it is comprised of cells and 

extracellular matrix, which consists of collagen 

and ground substance.'85 It contains an intricate 

network of blood vessels and nerve endings, and 

is very cellular. The majority of the PDL cells 

are fibroblasts, but some of these fibroblast-like 

cells are actually osteoprogenitor cells.65 Osteoblasts, 

either active or in the form of lining cells, 

occupy the alveolar bone surface bordering the 



427

PDL, while cementoblasts cover the dental root 

surface that interfaces with the PDL. Clusters of 

epithelial cells, the rests of Malassez, are spread 

in the PDL in the vicinity of the root surface, 

while capillaries are usually more numerous in 

the center of the ligament and in the zone closer 

to the alveolar bone. Cells migrating out of these 

capillaries, such as lymphocytes, macrophages, 

and mast cells, can be observed throughout the 

PDL. Cells may also migrate into the PDL from 

neighboring marrow spaces in the alveolar 

bone.70,71 

The PDL contains an elaborate network of 

neural filaments'86 that arise from the trigeminal 

nerve and send neural bundles through the apical 

alveolar bone in a coronal direction to the gingiva 

and PDL. Myelinated and unmyelinated fibers 

are found in the PDL, some terminating as "free" 

nerve endings, mostly in the inner part of the 

PDL, while others terminate as knob-like enlargements 

or as coiled nerve endings. Unmyelinated 

fibers usually follow PDL blood vessels 

and may have a vasomotor function. In this capacity, 

PDL nerve fibers may release, when 

stressed mechanically, vasoactive peptides that 

regulate movement of leukocytes out of 

capillaries. 

Nerve impulses resulting from tooth movement 

can be detected in afferent fibers. These 

impulses originate primarily in the PDL and not 

in the dental pulp, as shown in experiments where 

the pulp had been removed.'87 Mechanical stimulation 

activates PDL fibers that are associated 

with large-sized myelinated fibers. 188 The smaller 

C fibers also react to mechanical forces, but with 

a larger magnitude or longer duration.189 In forceinduced 

tooth movement, the PDL nerve fibers 

perform two main functions: transmission of nociceptive 

impulses centrally and release of neuropeptides 

peripherally. The latter (discussed below) 

may have an important role in regulating 

the local inflammatory response, primarily by 

interacting with cells of the vascular system. 

Examining the PDL in mouse molars, Freezer 

and Sims'90 observed that 88% of the blood vessels 

consisted of venules and 12% were capillaries. 

Gould et al. 91 and McCulloch and 

Melcher'92 found that 75 to 80% of the blood 

vessels were positioned in the bony portion of 

the PDL. To test the effect of orthodontic forces 

428 

(1 to 7 d) on the PDL vascular bed, Khouw and 

Goldhaber'93 perfused dogs and monkeys with a 

colloidal suspension of carbon particles. They 

reported on dilation of vessels in both areas of 

PDL tension and compression, in close proximity 

to sites of alveolar bone apposition and resorption, 

respectively. Examination by TEM of severe 

PDL compression sites in rats94 revealed 

stasis and erythrocytic breakdown, with disintegration 

of vessel walls. However, this initial 

response was followed by a repair process, typified 

by an invasion of the hyalinized zone by a 

front of cellular and vascular elements. A markedly 

increased PDL vascularity was also detected 

in the inflamed PDL of beagles by Jeffcoat et 

al.195 using an angiographic method. 

In addition to providing the PDL with a variety 

of leukocytes, the vascular system also contributes 

to its fluid composition. Bien'96 thoroughly 

analyzed the dynamics of PDL fluid in 

relation to tooth movement, and identified three 

sources of fluid in the PDL: cellular, vascular, 

and interstitial. The latter is localized in the ground 

substance and acts as a thixotropic gel, which is 

jelly-like when not in motion, and flows quite 

easily under pressure. When subjected to a steady 

force, this fluid flows within the PDL out of areas 

of compression and into areas of tension. This 

fluid flow, which starts as soon as the force is 

applied to a tooth and is maintained over extended 

periods of time, is apparently a crucial 

step in the physicochemical behavior of the PDL. 

This fluid motion and rearrangement signifies the 

onset and progress of distortion of PDL cells and 

fibers. This distortion of PDL, which is seen 

microscopically as widening in areas of tension 

and narrowing in sites of compression, may result 

in the release of vasoactive neuropeptides, appearance 

of stress-generated potentials, and alterations 

of cellular shape. Storey27 observed vasodilation 

and migration of carbon particles out 

of capillaries in stressed PDL within 20 min of 

the application of an orthodontic force to guinea 

pig incisors. Thus, orthodontic forces seem to 

evoke an early response in the stressed PDL, 

which encompasses fluids, matrix fibers and 

ground substance, and cells. 

The chain of events above-reported led us to 

propose the following scheme to describe the initial 

effects of force on paradental tissues (Figure 



4): (1) movement of fluids within the PDL; (2) 

gradual distortion of the PDL; (3) generation of 

streaming potentials; (4) alteration of cellular 

shape and ion channel permeability; (5) neuropeptide 

release from nerve endings; (6) capillary 

vasodilation and migration of leukocytes into extravascular 

areas; and (7) bending of the alveolar 

bone and generation of piezoelectric spikes. 

The outcome of such events is the introduction 

into the stressed PDL of agents derived from 

cells of the nervous and immune systems. In addition, 

products of the endocrine system are routinely 

delivered to the PDL through the circulation. 

Thus, on the biochemical level, mechanical 

forces can result in the simultaneous exposure of 

PDL cells to signals from the nervous, immune, 

and endocrine systems, leading to intricate and 

fascinating interactions and cellular responses. 

ORTHODONTIC FORCE 

Movement of PDL fluids 

B. Interactions between Cells and 

Products of the Nervous, Immune, and 

Endocrine Systems 

Recent attempts to understand the mode of 

regulation of cellular activities in various tissues 

have resulted in a rapidly growing volume of 

evidence in support of the contention that interactions 

between cells of the nervous, immune, 

and endocrine systems are pivotal parts of this 

mechanism. In our own research we have focused 

on the possibility of such interactions in the 

stressed PDL between neurotransmitters, cytokines, 

and hormones. The neurotransmitters we 

targeted are substance P (SP), vasoactive intestinal 

peptide (VIP), calcitonin gene-related peptide 

(CGRP), and methionine enkephalin (ME). 

The cytokines we chose are interleukin-lao and - 

Gradual Distortion of Generation of streaming 

PDL matrix and cells potentials that affect PDL 

~and alveolar bone cells 

|1~~~~~~~\^~ 

Alteration of cellular 

shape, cytoskeletal Neuropeptide release from 

configuration, and ion PDL afferent nerve endings 

channel permeability 

Capillary vasodilation, 

Bending of the alveolar migration of leukocytes into 

bone extravascular areas 

Piezoelectric effects Synthesis and release of 

cytokines, growth factors, PG's 

FIGURE 4. Initial effects of orthodontic forces on paradental tissues. 



429

113 (IL-la and IL- 13), interleukin-2 (IL-2), tumor 

necrosis factor-a (TNF-a), and gamma interferon 

(IFN--y). The reason for choosing these 

particular molecules was the existing evidence 

implicating them in bone remodeling. 

Incubation of human blood monocytes with 

SP by Lotz et al.197 induced the release of IL-1, 

TNF-a, and IL-6 by these cells, demonstrating 

recognition of neurotransmitters by immune cells. 

Similar effects were seen in B lymphocytes that 

were stimulated to differentiate by SP,198 and in 

neutrophils that were stimulated by SP to enhance 

their oxidative metabolism. 199 ME had a biphasic 

effect on the proliferation of peripheral blood 

monocytes200 and stimulated 02 release by polymorphonuclear 

cells.201 

Cells of the immune system were found to 

synthesize neurotransmitter-like molecules. Substance 

P was extracted from mouse liver granulomas 

by Weinstock et al.,202 203 and its messenger 

RNA (mRNA) was localized to the granuloma 

eosinophils by in situ hybridization. Roth et al.24 

reported that the opioid precursor proenkephalin 

was secreted by activated T helper cells. Cells 

of the nervous system were reported to be reactive 

to products of immune cells.205 For instance, 

incubation of mouse anterior pituitary cells 

with IL-1 induced protein phosphorylation, but 

without cAMP elevations, which appears to be 

an early signal for the secretion of 13-endorphin. 

In another experiment, Fagarasan and Axelrod206 

treated pituitary cells with IL-I in the presence 

of norepinephrine or isoproterenol, causing an 

additive effect on 3-endorphin secretion. Su et 

al.207 found identical steroid receptors in guinea 

pig brain and spleen, postulating that steroids 

can, in this fashion, alter the immune function 

and cause psychological changes. 

C. Neuropeptides and Mineralized 

Tissues 

SP, a neuropeptide present in parts of the 

CNS and PNS, such as C- type sensory fibers 

and autonomic afferents and efferents, is released 

from nerve endings in response to various stimuli. 

Its effects in peripheral tissues include vasodilation 

and an increase in capillary permeability. 

These effects may contribute to the plasma 

430 

extravasation and increased local blood flow that 

accompany inflammation. Moreover, SP can 

stimulate histamine release from mast cells at 

sites of injury and inflammation. These biological 

capabilities turned SP into a suspected prime 

contributor to the pathogenesis of rheumatoid arthritis. 

In 1984, Levine et al.208 caused adjuvant 

arthritis in rats, and observed that the severity of 

the disease was pronounced in joints that were 

densely innervated by SP-containing afferent 

neurons. Injection of SP into joints increased the 

severity of arthritis. Lotz et al.209 incubated synoviocytes 

from human arthritic joints with SP, 

causing increases in PGE2 and collagenase release. 

Yokoyama and Fujimoto210 demonstrated 

that lymphocytes from rheumatoid arthritis patients 

could be activated by SP to present high 

levels of HLA markers. Monocytes of these patients, 

when treated with SP, released high 

amounts of oxygen intermediates. 

A significant observation was made by 

O'Bryne et al.,211 who injected IL-la into rabbit 

knees and measured SP and PGE2 in the joint 

fluid at 4, 24, and 48 h. By 4 h, SP was increased; 

it further increased by 24 h and remained elevated 

at 48 h. PGE2 levels were highest at 4 h and 

remained elevated at 48 h. These results highlight 

the intimate association, at sites of inflammation, 

between cytokines, neurotransmitters, and prostaglandins. 

The neurogenic component of joint 

inflammation was demonstrated by Lam and Ferrell, 

who in one experiment212 have inhibited carrageenan-induced 

knee joint inflammation in rats 

by denervation, while in the other,213 abolished 

it by an intraarticular injection of capsaicin, a SP 

releaser and inhibitor. 

Immunolocalization of SP in dental tissues 

was first reported by Olgart et al.,214'215 who 

observed its presence in the feline dental pulp. 

In a more recent series of articles, Wakisaka et 

al.216-2'8 reported on the distribution and origin 

of SP in the rat molar pulp and PDL. They 

observed SP-containing nerves along blood vessels, 

primarily in the middle and apical regions 

of the PDL. During orthodontic tooth movement 

in cats, intense immunohistochemical staining 

for SP in PDL tension sites was seen by Davidovitch 

et al.219 within 1 h of treatment. Furthermore, 

administration of SP to human PDL 

fibroblasts in vitro significantly increased the 



concentration of cAMP in the cells and PGE2 

in the medium within 1 min.219 

Another neurotransmitter that has been implicated 

recently as a stimulator of bone resorption 

is VIP, a 28-amino acid residual peptide, 

which was originally extracted from porcine duodenum.220 

In vitro studies have demonstrated that 

VIP stimulates bone resorption dramatically, and 

that this activity is not mediated by PGE2.221 

Moreover, this effect of VIP involves an 8- to 

13-fold increase in bone cAMP levels.221 Binding 

studies222 revealed high-affinity receptors for VIP 

on human osteosarcoma cells. Hohmann et al.223 

localized VIP in nerve fibers in the periosteum 

of porcine rib, tibia, and vertebra. Here, VIP was 

traced to sympathetic postganglionic neurons. 

Herness224 localized it in mouse PDL, mainly in 

the apical part around the blood vessels. During 

orthodontic tooth movement in cats,22 intense 

staining for VIP was localized in the compressed 

PDL near sites of bone resorption and in the pulp 

of moving teeth. 

The recent discovery of CGRP by Rosenfeld 

et al.226 raised the interest of investigators of neural 

control of bone remodeling. This neurotransmitter 

was localized in small to medium diameter 

sensory ganglion neurons and appeared to coexist 

with SP.226 In the cat, local intraarterial infusion 

of SP or CGRP caused a concentration-dependent 

increase in nasal blood flow.227 The widespread 

distribution of this 37-amino acid peptide has 

been demonstrated in cardiovascular tissues of 

several species, as well as in perivascular nerves 

in the rat mesenteric artery.22 In arthritic patients, 

Larsson et al.229 detected CGRP-like immunoreactivity 

in the knee synovial fluid; however, 

similar amounts of CGRP were found in 

synovial fluids of control patients. Recently, Silverman 

and Kruger230 localized CGRP in many 

rat orofacial sensory structures, while Wakisaka 

et al.231 studied its distribution in the feline dental 

pulp. In the latter tissue CGRP-like immunoreactivity 

was observed in nerves along blood 

vessels as well as in the subodontoblastic zone, 

while dentinal injury in rat molars evoked sprouting 

of CGRP-containing nerve fibers.232 CGRP 

was also localized in the rat PDL233 and mandibular 

periosteum.234 In the cat dental pulp, intraarterial 

infusion of SP and CGRP produced vasodilation, 

as measured by both laser Doppler 

flowmetry and 125I clearance.235 The effect of 

CGRP was ten times larger when given after SP 

than before it. Regarding an association between 

CGRP and acute inflammation, Takahashi et al.236 

found CGRP-containing nerves in contact with 

dynorphin-containing dendrites in acutely inflamed 

rat hindpaw. 

In relation to tooth movement, Kvinnsland 

and Kvinnsland237 localized CGRP in the pulp 

and PDL of rats receiving orthodontic forces to 

maxillary molars for 5 d. In unstressed teeth, 

CGRP immunoreactivity was localized primarily 

in pulp and PDL nerves surrounding blood vessels. 

In moving teeth, the number of CGRP-containing 

nerves in both pulp and PDL increased, 

and their staining intensified, particularly in PDL 

tension sites. In these areas, dark "spots" were 

seen, which were probably fibroblasts that have 

bound CGRP released from stressed sensory nerve 

endings. Such a pattern of cellular staining for 

CGRP was observed in the pulp and PDL of 

moving teeth in cats by Okamoto et al.238 in the 

author's laboratory. In this group of 28 animals 

that had been treated by a translatory force application 

to one maxillary canine for 1 h and/or 

2, 7, or 28 d, CGRP immunoreactivity in the 

PDL intensified within 1 h in tension sites, but 

was particularly intense in compression areas at 

day 28, in PDL cells located at the edge of the 

hyalinized zone and near osteoclasts (Figures 5 

to 8). In the pulp of nonmechanically stressed 

teeth, staining for CGRP was widespread and 

distinct, localized in perivascular fibers and in 

numerous fibroblasts (Figure 9). Force application 

to a tooth did not seem to alter the CGRP 

immunoreactivity in dental pulp cells (Figure 10). 

While SP appears to be intimately involved 

in the transmission of painful sensations in the 

primary afferent system, enkephalins are related 

to morphine- and stimulation-produced analgesia. 

Both of these neuropeptides have been shown 

immunohistochemically to have a similar distribution 

in many regions of the rat CNS.239 This 

relationship between SP and ME could be of interest 

in investigating the regulation mechanism 

of bone remodeling, since Jessell and Iversen240 

have reported that opiate analgesics inhibit SP 

release in the rat trigeminal nucleus. One group 

recently focused on this issue.241-243 In one 

experiment241 they measured ME levels in dental 



431

432 

FIGURE 5. Immunohistochemical localization of CGRP in unstressed cat PDL. Notice 

dark staining of nerve fiber, but light staining of PDL cells (arrows). (Magnification x 1400.) 

FIGURE 6. Localization of CGRP in PDL tension site after 1 h of application of a translatory 

force to a cat maxillary canine. Notice dark staining over cells' periphery and cytoplasm 

(arrows). (Magnification x 1400.) 



FIGURE 7. Dark staining for CGRP of cells in PDL tension zone after 28 d of translatory 

force application to cat maxillary canine (arrows). (Magnification x 1400.) 

FIGURE 8. Cells in compressed PDL at edge of hyalinized zone, stained very darkly for 

CGRP, after 28 d of translatory force application to cat maxillary canine. (Magnification x 

1400.) 



433

434 

FIGURE 9. Nerve fiber in unstressed cat canine pulp, intensely stained for CGRP, approaching 

blood vessel wall (B). (Magnification x 1400.) 

FIGURE 10. Immunohistochemical localization of CGRP in pulp of cat canine subjected 

to translator force for 28 d. Staining deposits are seen in nerve fiber approaching blood 

vessel (B), as well as in adjacent cells. (Magnification x 1400.) 



pulps of human teeth that had been moved by an 

orthodontic spring for a few hours prior to extraction. 

They found drastic declines in the concentrations 

of ME in the moved teeth, perhaps 

correlated with the early development of painful 

sensations in orthodontic treatment. In another 

study242 ME was extracted from pulpless, decalcified 

human third molars, suggesting that ME 

is present in dentinal fibers that connect with the 

dental pulp. In a more recent investigation Robinson 

et al.243 measured the concentrations of 

P-endorphin, another opioid-like neuropeptide, 

in premolars of 30 young patients whose premolars 

were to be extracted for orthodontic reasons. 

As in the case of ME, an acute mechanical 

stress caused a gradual decrease in the level of 

3-endorphin in the pulp. In an effort to determine 

whether ME fluctuates in the PDL of moving 

teeth, we attempted to localize it immunohistochemically 

in orthodontically treated teeth; however, 

only scarce and faint immunoreactivity for 

ME could be detected in the unstressed PDL, 

with a similar appearance in the PDL of the moving 

tooth (unpublished results). 

D. Cytokines and Mineralized Tissues 

The evidence implicating cytokines in the 

regulation of the activities of mineralized and 

nonmineralized connective tissue cells is overwhelming. 

Following their initial discovery, cytokines 

were believed to be signal molecules produced 

by leukocytes, serving primarily as 

communication links between cells of the immune 

system. However, many other cell types 

were later found to synthesize cytokine-like molecules 

that could function in an autocrine or paracrine 

capacity. With respect to bone metabolism, 

cytokines with demonstrated or suspected 

effects are IL-1, IL-2, IL-3, IL-6, TNF-a, and 

IFN--y. Of these cytokines, the most potent stimulator 

of bone resorption in vitro is IL-1. It is 

produced by many cell types, including 

osteoblasts244.245 and chondrocytes.246 Secretion 

of IL-1 is triggered by a variety of stimuli, including 

other cytokines and whole microorganisms. 

It has two distinct forms, IL-la and IL- 

1p, which are coded by separate genes, but both 

forms have similar biologic actions. In a recent 

report Kaplan et al.247 listed the systemic effects 

of IL-1. This long list encompasses the CNS, as 

well as the vascular and immunologic constellations. 

On the local level, IL-1 attracts leukocytes, 

stimulates fibroblast proliferation, and enhances 

bone resorption. It thus seems to be one 

of the major components of the inflammatory 

response. 

Osteoblast-like cells, derived from human 

trabecular bone, were incubated for 1 to 3 d with 

various doses of IL-1 by Gowen et al.48 They 

reported a significant increase in the uptake of 

[3H] Tdr by these cells in comparison to controls, 

and a marked increase in cell counts at day 3. 

Infusion of IL-1 into normal mice by Boyce et 

al.249 first resulted in a PGE2-related hypocalcemia 

after 3 h, followed by a hypercalcemia at 

24 h. In another related experiment Sabatini et 

al.250 infused IL-1 into mice, causing hypercalcemia 

at 72 h, with increased numbers of osteoclasts 

and bone resorption surfaces. Continuous 

infusion of IL-1 for 14 d into rabbit knee joints 

by Feige et al.251 induced severe arthritic changes. 

In vitro, synovial fibroblasts were stimulated by 

IL-1 to produce PGE2 and collagenase,252 and 

Rafter253 proposed that polymorphonuclear leukocytes 

are the main source for IL-1 in arthritic 

joints. However, in a recent interesting experiment 

Johnson et al.254 discovered that rat synovial 

fibroblasts could be stimulated by lipopolysaccharides 

to produce and secrete IL-1, only following 

an initial exposure of the cells to IFN-y. 

In samples of gingival cervicular fluid and in 

gingival tissue, IL-la and IL-113 were detected 

in patients with periodontal disease.255 Marked 

reductions in IL-1 levels followed effective periodontal 

treatment. 

In bone, target cells for IL-1 appear to be 

osteoblasts, according to Thomson et al.,256 who 

incubated neonatal mouse tibial osteoclasts with 

human cortical bone and with IL-1 in the presence 

or absence of calvarial osteoblasts. Resorption 

of bone occurred only when osteoclasts and 

osteoblasts were present. When fetal rat long 

bones were incubated with IL-l a or IL-13p and 

with PTH by Dewhirst et al.,257 a synergistic 

effect on 45Ca release was recorded when both 

cytokine and hormone were introduced simultaneously. 

Moreover, the presence of small concentrations 

of IL-1 necessitated only a very small 



435

amount of PTH to cause a marked resorptive 

effect. This finding is potentially very significant, 

because it means that in cases of induced 

bone resorption, such as that occurring during 

tooth movement, suboptimal amounts of key local 

mediators in the PDL may be sufficient to 

evoke alveolar bone resorption, provided that 

minute amounts of systemic bone-seeking hormones 

also happen to be present in the same site 

at that time. 

The effects of interleukins on bone resorption 

were investigated by Gowen and Mundy258 using 

the mouse calvarial assay system. They observed 

marked stimulation of resorption by IL-1, but not 

by IL-2. Tatakis et al.259 administered IL-1, IL- 

2, or IL-3 to osteoblasts isolated from rat fetal 

calvaria. Only IL-1 stimulated the release of large 

amounts of PGE2 by the cells, and this effect was 

augmented by PTH. However, when osteoclasts 

were incubated with mouse calvaria in the presence 

of IL-2, their acid production increased.260 

During tooth movement in cats261 we did not observe 

immunohistochemical staining for IL-2 in 

the unstressed PDL, neither did we notice it in 

PDL tension sites. However, after 7 and 14 d of 

force application to teeth, intense staining for IL- 

2 was observed in clusters of mononucleated cells 

in PDL compression sites in close proximity to 

osteoclasts and near the necrotic hyalinized zone. 

The timing of the appearance of IL-2 immunoreactivity 

in the compressed PDL may coincide 

with the localization of osteoclasts in Howship's 

lacunae in the alveolar bone. Similar timing was 

reported by Nieto-Sampedro and Chandy262 to 

occur in injured rat brain, where IL-2 activity in 

the tissue immediately adjacent to the injury 

reached a peak 10 d postlesion. This peak coincided 

with axonal sprouting and astrocyte division. 

In tooth movement, IL-2 may thus be 

associated with attraction and/or proliferation of 

osteoclast progenitors, as well as with a stimulation 

of acid production by active osteoclasts. 

TNF-a is another cytokine with well-documented 

potential as a stimulator of bone resorption. 

This 17-kDa molecule is synthesized mainly 

by monocytes and macrophages, and is an important 

component of the inflammatory process. 

It stimulates endothelial cells to secrete IL-1263 

and increases their adherence to leukocytes.264 In 

fetal mouse calvaria TNF-a increased the pro- 

436 

duction of procollagenase and the activity of collagenase.266 

In human osteoblast-like cells TNFa 

inhibited proliferation and alkaline phosphatase 

activity.267 This effect was not mediated by 

cAMP, and, moreover, TNF-a reduced the cAMP 

elevation caused by PTH.268 In terms of bone 

resorbing potency, IL-1 is much stronger than 

TNF-a; however, Stashenko et al.269 reported that 

suboptimal concentrations of IL-1 3 or IL-la, in 

combination with suboptimal concentrations of 

TNF-a, stimulate the formation of osteoclast-like 

cells in vitro from human marrow cells,270 and 

this effect is synergistic when both agents are 

added simultaneously to the cell cultures. 

In living animals, TNF-a administration has 

an inhibitory effect on bone fracture healing271 

and it induces hypercalcemia.272 Using monoclonal 

antibodies, Hopkins and Meager273 detected 

low levels of TNF-a and INF-y in synovial 

fluids of patients with rheumatoid arthritis, while 

Maury and Teppo274 measured elevated levels of 

TNF-a in the circulation of 46% of patients with 

rheumatoid arthritis and 29% of patients with 

lupus erythematosus. Using immunohistochemical 

staining, Yocum et al.275 localized TNF-a in 

mononuclear cells of the joint lining layer, sublining, 

and perivascular areas. During tooth 

movement in cats, intense cellular staining for 

TNF-a was observed in the compressed PDL after 

7 and 14 d of force application, particularly in 

fibroblasts near alveolar bone osteoclasts. However, 

PDL cells in tension sites also contained 

positive TNF-a immunoreactivity. 

While IL-1, IL-2, and TNF-a have been found 

to stimulate and enhance bone resorption in vivo 

and in vitro, IFN-y was discovered to interfere 

with resorptive processes. This cytokine is a lymphocytic 

product, which antagonizes the effects 

of a number of growth factors. Gowen et al.276 

reported that IFN-y completely abolished the resorptive 

effects of IL-1 and TNF-a in mouse 

calvaria, but that it did not inhibit the resorptive 

effects of PTH and vitamin D3. Direct effects of 

IFN-y and TNF-a on human osteoblast-like cells 

were demonstrated by Gowen et al.277 Cellular 

proliferation and PGE2 production were stimulated 

by TNF-a, while alkaline phosphatase activity 

and osteocalcin release were inhibited; 

however, the effects IFN-y had on bone cell activities 

were the polar opposite. In osteosarcoma 



cells with an osteoblastic phenotype, both TNFa 

and IFN-y inhibited DNA synthesis, and their 

inhibitory effect was even greater when their 

administration was simultaneous.278 A similar inhibitory 

pattern on collagen synthesis was also 

noted. In mice who received bone particle implants 

near their tibiae and seven daily injections 

of IFN-y, macrophages that fused to form osteoclast-like 

cells increased in number, although IFNy 

inhibited the fusion of alveolar macrophages 

in vitro, suggesting that IFN-y limits inflammation 

and favors tissue repair.279 

In dental tissues, IFN-y was found by Melin 

et al.280 to stimulate proliferation of human dental 

pulp fibroblasts and to inhibit the synthesis of 

type I and III collagen and of fibronectin. We 

stained cat jaw sections for IFN-y,281 and found 

practically no immunoreactivity in the unstressed 

PDL or in PDL tension sites in moving teeth. 

However, after 7 and 14 d of tooth movement, 

numerous cells in PDL compression sites displayed 

positive staining for IFN--y. These cells 

were located primarily near alveolar bone osteoclasts, 

suggesting that IFN-y is involved in the 

regulation of bone resorption in vivo. 

E. Neurotransmitters, Cytokines, and 


The above review suggests strongly that a 

number of neurotransmitters and cytokines play 

regulatory roles in the remodeling of mineralized 

and nonmineralized connective tissues. Results 

from experiments in the author's laboratory have 

demonstrated that most of these signal molecules 

can be localized immunohistochemically in cat 

PDL and alveolar bone cells, and that their distribution 

and relative concentration are modified 

when the tissues are stressed mechanically. Taken 

together, these results imply that interactions between 

neurotransmitters, cytokines, and target 

cells occur in paradental tissues during tooth 

movement. However, localization of these molecules 

does not prove that some or all play active 

roles in stimulating or inhibiting cells in mechanically 

stressed paradental tissues. To determine 

whether these signal molecules can evoke 

a biochemical response by PDL fibroblasts, and 

whether such stimulation can affect the activity 

of bone-resorbing and bone-forming cells, we 

resorted to cell and tissue culture experiments, 

which are summarized briefly below. 

VI. THE EFFECTS OF 

NEUROTRANSMITTERS AND 

CYTOKINES ON BONE AND PDL 

FIBROBLASTS IN VITRO 

In these experiments, (1) neonatal mouse calvaria 

were subjected to increasing concentrations 

of neurotransmitters and cytokines to determine 

the effect of 45C release or [3H] proline incorporation; 

(2) human PDL fibroblasts were incubated 

with neurotransmitters or cytokines to determine 

whether this treatment altered the 

concentrations of cellular cAMP and PGE2; and 

(3) conditioned media from cytokine-treated or 

mechanically stressed PDL fibroblasts were tested 

to determine whether they would enhance bone 

resorption in vitro. 

A. The Effects of Neurotransmitters and 

Cytokines on Bone Resorption and 

Formation In Vitro 

In an effort to determine whether bone cells 

can respond to a direct application of neurotransmitters 

or cytokines, we282 administered these 

agents to neonatal mouse calvaria in vitro for a 

24 h incubation period. IL-la and IL-1iI stimulated 

bone resorption more potently than other 

cytokines. Bone formation was inhibited by PTH, 

IL-la, IL-1p, and TNF-a, but not by IFN-y. 

None of the neurotransmitters had any effect on 

the rate of bone formation. These data demonstrate 

that cytokines and neurotransmitters that 

have been found in the PDL during tooth movement 

can directly affect bone remodeling in vitro. 

B. Effects of SP and Cytokines on PGE 

and cAMP in PDL Fibroblasts 

PDL fibroblasts, obtained from extracted 

premolars of young orthodontic patients, were 

removed enzymatically and incubated with SP (1 

x 10-6 M) for 1 to 120 min.219 Significant in- 



437

creases in the levels of cellular cAMP and PGE 

in the medium occurred within 1 min, and persisted 

throughout the incubation period. Likewise, 

human PDL fibroblasts were incubated with 

graded doses of IL-la, IL- 13, TNF-a, and IFNy283 

for 15, 30, and 60 min, or 2, 4, 24, 48, and 

72 h. The cells responded to all the cytokines 

with dose- and time-related increases in the levels 

of cAMP and PGE. These increases were inhibited 

by indomethacin. In another experiment284 

cytokines were added to cell-containing media, 

alone or in combination. The interactions between 

these cytokines varied in degree, depending 

on the particular combinations of cytokines. 

Moreover, the administration of cytokine combinations 

was found to be additive, synergistic, 

subtractive, or inhibitory on the production of 

PGE, depending on the length of incubation time. 

For instance, 24 h after the simultaneous administration 

of IL-1 p and TNF-a, the level of PGE 

in the medium increased synergistically, but at 

72 h it was lower than that produced by control 

cells. In most of the time periods the addition of 

IFN-y in the presence of IL-1 or TNF-a caused 

a reduction in the level of PGE in the medium. 

C. Bone Resorbing Activity Produced by 

Human PDL Fibroblasts 

To determine whether cytokine-stimulated 

PDL cells can enhance bone resorption, we285 

administered IL- la, IL- 1 3, TNF-a, or IFN-y to 

human PDL fibroblasts for a 1-h incubation. The 

medium was then replaced with a fresh medium 

for an additional 24 h incubation period, and the 

latter medium (conditioned medium, CM) was 

added to mouse calvaria containing media. CM 

derived from unstimulated PDL fibroblasts increased 

the rate of bone resorption (45Ca release) 

by 2.5-fold, as compared to control (no CM) 

resorptive rate. CM derived from IL-lp-stimulated 

PDL fibroblasts caused a rate of resorption 

3.5 times greater than control. Resorption in the 

presence of CM obtained from cells stimulated 

by IFN-y or IL- 13 + IFN--y was at the same 

level as that caused by CM of unstimulated cells. 

Indomethacin administration to the calvaria-containing 

medium abolished the synthesis of PGE, 

but reduced only partially the enhanced resorp- 

438 

tion rate caused by the IL-1 CM. These results 

suggest that PDL cells produce nonprostaglandin 

bone-resorbing factor(s), in addition to PGE2. 

The addition of anti-IL-lp monoclonal antibodies 

to the CM failed to inhibit the resorptive effect 

on the calvaria, suggesting that the resorptive 

factor produced by PDL fibroblasts was not IL- 

1 3 (unpublished results). In another recent 

study286 we found that the resorptive effects of 

CM derived from PDL fibroblasts stimulated by 

either IL-la, IL-IL, or TNF-a could be augmented 

by the addition of PTH (1 U/ml) to the 

CM. This observation suggests that extensive resorption 

of bone may result from products of 

cytokine-stimulated PDL fibroblasts, when a systemic 

hormone such as PTH is also present in 

the vicinity of the bone target cells. 

D. Interactive Effects of IL-1, and 

Mechanical Stress in PDL Fibroblasts 

Human PDL fibroblasts were stretched in vitro 

by the use of convex templates for 2 to 60 

min.'59 The stretching caused significant elevations 

in PGE and cAMP within 15 min; however, 

the addition of IL-Il caused synergistic, additive, 

or even subtractive effects, depending on 

the duration of the incubation time. In another 

recent study287 CM derived from PDL cells stimulated 

by mechanical stress in the presence of 

IL-1 caused significantly more bone resorption 

than CM obtained from cells that had been treated 

by either factor alone. Again, indomethacin inhibited 

PGE synthesis, but reduced bone resorption 

only partially. 

Taken together, the results of the experiments 

summarized in this section demonstrate that human 

PDL fibroblasts are sensitive to mechanical 

stress, as well as to a variety of chemical signals 

derived from the nervous, immune, and endocrine 

systems. If levels of cellular cAMP or PGE 

in the medium are used as indicators, or if the 

resorptive activity ofCM derived from stimulated 

or unstimulated PDL cells is used as such, it 

becomes clear that when two or more of these 

signals, mechanical or chemical, are presented 

to the PDL cell simultaneously, their response 

would usually differ from that seen following the 

application of one signal alone. Thus, it may be 



possible to observe synergistic, additive, or subtractive 

effects. In the in vivo situation there are 

usually several factors present simultaneously in 

the cell's environment. Therefore, it may be reasonable 

to assume that in tooth movement PDL 

fibroblasts do not respond merely to tension or 

compression, but also to other signals; products 

of neighboring cells; members of other systems, 

such as endothelial cells, monocytes, and nerve 

cells; and cells of the epithelial rests of Malassez. 

Moreover, it seems likely that prostaglandins are 

not the sole link between PDL fibroblasts and 

bone cells; however, the identity of the other 

connecting molecules is unknown at the present 

time, and deserves further probing. 

VII. TOOTH MOVEMENT: CONCLUSIONS 

AND DIRECTIONS FOR FUTURE 

RESEARCH 

For at least 2000 years it has been known 

that a tooth can be moved gradually from one 

spot in the oral cavity to a more desirable one 

by the application of mechanical forces to the 

tooth's crown. In the middle of the 18th century, 

John Hunter explained that this movement is an 

outcome of the habit of the bone "to move out 

of the way of pressure". About 100 years later, 

John Farrar attributed force-induced tooth movement 

to "decalcification of the root socket and 

bending of the alveolar bone". In the years since, 

extensive investigations have confirmed that indeed 

physical and chemical alterations occur concomitantly 

in the paradental tissues, resulting in 

cellular activities that culminate in tissue remodeling 

and tooth movement. 

Histologic studies made it clear that tissues 

can be remodeled only by the action of cells. In 

the case of the PDL, the cells that form and 

degrade the periodontal extracellular matrix are 

primarily the fibroblasts. In the case of the alveolar 

bone, the cells that remodel it are the osteoblasts, 

osteoclasts, and osteocytes. When the 

PDL and alveolar bone are stressed by continuously 

applied forces, both cells and matrix are 

distorted and extracellular fluids are mobilized. 

Bone matrix distortion in vivo is associated with 

reorientation of its proteoglycans, a phenomenon 

that may serve as "strain memory", as the dis- 

torted molecules return slowly to their original 

configuration. This strain-related distortion is also 

associated with the appearance of piezoelectric 

spikes, while fluid flow leads to slow-dissipating 

streaming potentials. These bioelectric phenomena 

can cause alterations in the polarity of the 

plasma membranes of cells, leading to activation 

of membrane enzymes and cell-matrix interactions. 

Stress-caused changes in the shape of cells 

can result in the crystallization of cytoskeletal 

filaments and opening or closing of stress-related 

membrane ion channels. Thus, fibroblasts and 

bone cells can respond, in vivo and in vitro, to 

the distortive effects of mechanical stress, which 

are expressed both in the cells and their surrounding 

matrix. Significantly, bone cells appear to be 

sensitive to short-duration exposures to mechanical 

loads whose distribution in the bone matrix 

deviates from their regular dispersion pattern. This 

finding may suggest that properly designed force 

systems may obviate the need for prolonged periods 

of force application, as is customary in most 

of the prevalent orthodontic techniques. However, 

while such brief loads are capable of evoking 

osteoblastic activity, it is not evident that 

resorptive functions can also be stimulated by 

these strains. The latter may perhaps necessitate 

the application of prolonged strains, particularly 

those that cause periodontal injury. Thus, from 

the orthodontic standpoint, a clinically effective 

force should be somewhat biodisruptive, i.e., be 

capable of causing an inflammatory/reparative 

reaction in the PDL and alveolar bone. Osteoclastic 

activity is quite intense in such a case, 

and it is the activity of these multinucleated cells 

that removes the alveolar bone that stands in the 

way of the moving teeth. 

The applied mechanical stress causes gradual 

fluid shifts within the PDL, resulting in nerve 

fiber distortion, which leads to the release of 

neuropeptides from nerve endings. Some of these 

neuropeptides have vasoactive properties, and 

their interaction with PDL capillaries causes vasodilation, 

plasma extravasation, and migration 

of leukocytes from the capillaries into the extravascular 

spaces of the PDL. These migratory leukocytes 

synthesize and secrete a variety of cytokines 

capable of stimulating fibroblasts, 

endothelial cells, and alveolar bone cells. The 

PDL fibroblasts are responsive to the neuropep- 



439

tides, cytokines, and several growth factors produced 

by endothelial cells and bone cells. These 

fibroblasts, in areas of tension, proliferate and 

synthesize new matrix components, and in areas 

of compression they degrade the necrotic PDL. 

However, in both sites of tension and compression, 

the fibroblasts seem to produce factors that 

activate neighboring bone cells. Recent evidence 

suggests that osteoclasts are regulated by factors 

derived from adjacent osteoblasts, and PGE2 was 

proposed as being a major part of this bridge. 

However, in vitro experiments with PDL fibroblasts 

and mouse calvaria have demonstrated that 

factors produced by unstimulated or stimulated 

(mechanically or chemically) PDL fibroblasts can 

markedly enhance the rate of bone resorption. 

Thus, in tooth movement, PDL fibroblasts may 

not only be responsible for the remodeling of the 

periodontal matrix, but may also be actively involved 

in the regulation of the activity of the cells 

that remodel the alveolar bone. Osteocytes also 

seem to be sensitive to applied loads, and it was 

suggested that these cells, which are capable of 

recognizing and responding to molecular reorientation 

in their surrounding matrix, communicate 

these alterations to bone surface cells (primarily 

osteoblasts), providing them with an 

osteogenic stimulus. 

Orthodontic tooth movement is based predominantly 

on the application of mechanical 

forces to teeth in a judicious fashion supported 

by biomechanical principles. The rationale for 

investigating associated biological phenomena is 

derived from the desire to gain a thorough insight 

into these events in order to determine whether 

our clinical means to move teeth are effective 

and unharmful. Furthermore, this rationale is derived 

from our everlasting quest to improve our 

clinical approaches and from the realization that 

such progress can be derived from increased 

knowledge of biological principles on the tissue, 

cellular, and molecular levels. 

Many questions remain, however, with the 

biochemical and physical systems, only partly 

understood, but certainly deserving much further 

investigation. Among these issues are the effects 

of mechanical stresses on cells of the epithelial 

rests of Malassez, and on alveolar bone marrow 

spaces and the interaction of these cells with cells 

of the PDL. Another poorly understood phenom- 

440 

enon is the tooth movement-related dental root 

resorption. Above all remains our inability to 

predict the nature and pattern of the individual 

biological response to tooth moving forces. It 

seems likely that new information on these issues 

will be derived from experiments that correlate 

and integrate in vivo and in vitro systems and 

findings. The ability to maintain and challenge 

human PDL fibroblasts in vitro offers an opportunity 

to thoroughly examine one of the major 

cell types that is directly affected by orthodontic 

forces. Similar approaches can be applied to other 

PDL cell types, i.e., endothelial and epithelial 

cells. Interactions between these cell types and 

bone cells remain an intriguing issue within the 

framework of force-induced cellular activation 

and tissue remodeling. Ultimately, further explorations 

in this area should elucidate the cellular/molecular 

basis of tooth movement, and enable 

clinicians to include biological considerations 

in planning treatment for individual patients. 

ACKNOWLEDGMENT 

Supported by grant number DE-08428 from 

the National Institute of Dental Research. 

REFERENCES 

1. Weinberger, B. W., Orthodontics, An Historical 

Review of its Origin and Evolution, C. V. Mosby, 

St. Louis, 1926, 54. 

2. Weinberger, B. W., Orthodontics, An Historical 

Review of its Origin and Evolution, C. V. Mosby, 

St. Louis, 1926, 138. 

3. Hunter, J., The Natural History of the Human Teeth, 

J. Johnson, London, 1778, 99. 

4. Delabarre, C. F., Odontologie ou Observations sans 

les Dents Humaines, Paris, 1815. 

5. Farrar,J. N., Treatise on Irregularities of the Teeth 

and Their Correction, Vol. 2, DeVinne Press, New 

York, 1888, 758. 

6. Wolff, J. D., Das Gesetz der Transformation der 

Knochen, Hirshwald, Berlin, 1982. 

7. Sandstedt, C., Einige beitrage zur theori der zahnregulierung, 

Nord. Tandlaeg, Tidskr., 5, 236, 1904. 

8. Sandstedt, C., Einige beitrage zur theori der zahnregulierung, 

Nord. Tandlaeg, Tidskr., 6, 1, 1905. 

9. Oppenheim, A., Tissue changes, particularly of the 

bone, incident to tooth movement, Trans. Eur. Orthodont. 

Soc., 8, 11, 1911. 



10. Schwarz, A. M., Tissue changes incidental to orthodontic 

tooth movement, Int.J. Orthodont., 18, 331, 

1932. 

11. Fukada, E. and Yasuda, I., On piezoelectric effect 

of bone, J. Phys. Soc. Jpn., 12, 1158, 1957. 

12. Gillooly, C. J., Hosley, R. T., Mathews, J. R., 

and Jewett, D. L., Electric potentials recorded from 

mandibular alveolar bone as a result of forces applied 

to the tooth, Am. J. Orthodont., 54, 649, 1968. 

13. Baumrind, S., A reconsideration of the propriety of 

the "pressure-tension" hypothesis, Am. J. Orthodont., 

55, 12, 1969. 

14. Reitan, K., The initial tissue reaction incident to 

orthodontic tooth movement as related to the influence 

of function, Acta Odont. Scand., Suppl. 6, 1951. 

15. Reitan, K., Tissue reaction as related to the age 

factor, Dent. Rec., 74, 271, 1954. 

16. Reitan, K., Experiments of rotation of teeth and their 

subsequent retention, Trans. Eur. Orthodont. Soc., 

34, 124, 1958. 

17. Reitan, K., Behavior of Malassez' epithelial rests 

during orthodontic tooth movement, Acta Odont. 

Scand., 19, 425, 1961. 

18. Reitan, K., Effects of force magnitude and direction 

of tooth movement on different alveolar bone types, 

Angle Orthodont., 34, 244, 1964. 

19. Reitan, K., Initial tissue response behavior during 

apical tooth resorption, Angle Orthodont., 44, 68, 

1974. 

20. Reitan, K. and Kvam, E., Comparative behavior 

of human and animal tissue during experimental tooth 

movement, Angle Orthodont., 11, 1, 1971. 

21. Storey, E. and Smith, R., Force in orthodontics and 

its relation to tooth movement, Aust. J. Dent., 56, 

11, 1952. 

22. Smith, R. and Storey, E., The importance of force 

in orthodontics, Aust. J. Dent., 56, 291, 1952. 

23. Storey, E., Bone changes associated with tooth 

movement: the influence of the menstrual cycle on 

the rate of tooth movemement, Aust. J. Dent., 58, 

80, 1954. 


movement: a histological study of the effect of force 

in the rabbit, guinea pig and rat, Aust. J. Dent., 59, 

147, 1955. 


movement: a histological study of the effect of force 

for varying durations in the rabbit, guinea pig and 

rat, Aust. J. Dent., 59, 209, 1955. 


movement: a histological study of the effect of age 

and sex in the rabbit and guinea pig, Aust. J. Dent., 

59, 220, 1955. 

27. Storey, E., The nature of tooth movement, Am. J. 

Orthodont., 63, 292, 1973. 

28. Deguchi, T. and Mori, M., Histochemical observations 

on oxidative enzymes in periodontal tissue 

during experimental tooth movement in rat, Arch. 

Oral Biol., 13, 49, 1968. 

29. Takimoto, K., Deguchi, T., and Mori, M., Histochemical 

detection of succinic dehydrogenase in 

osteoclasts following experimental tooth movement, 

J. Dent. Res., 45, 1473, 1966. 

30. Takimoto, K., Deguchi, T., and Mori, M., Histochemical 

detection of acid and alkaline phosphatase 

in periodontal tissue following experimental tooth 

movement, J. Dent. Res., 47, 340, 1968. 

31. Waldo, C. M. and Rothblatt, J. M., Histologic 

response to tooth movement in the laboratory rat: 

procedure and preliminary observation, J. Dent. Res., 

33, 481, 1954. 

32. Lilja, E., Lindskog, S., and Hammarstrom, L., 

Histochemistry of enzymes associated with tissue 

degradation incident to orthodontic tooth movement, 

Am. J. Orthodont., 83, 62, 1983. 

33. Lilja, E., Bjornestedt, T., and Lindskog, S., Cellular 

enzyme activity associated with tissue degradation 

following orthodontic tooth movement in man, 

Scand. J. Dent. Res., 91, 381, 1983. 

34. Kurihara, S. and Enlow, D. H., A histochemical 

and electron microscopic study of an adhesive type 

of collagen attachment on resorptive surfaces of alveolar 

bone, Am. J. Orthodont., 77, 532, 1980. 

35. Martinez, R. H. and Johnson, R. B., Effects of 

orthodontic tooth movement on the alcian blue staining 

patterns of rat alveolar bone: a histochemical study, 

Histol. Histopathol., 3, 39, 1988. 

36. Noda, K., Localization of cytochrome c oxidase activity 

in osteoclasts treated with calcitonin during experimental 

tooth movement, Acta Histochem. Cytochem., 

21, 301, 1988. 

37. Rygh, P. and Selvig, K. A., Erythrocytic crystallization 

in rat molar periodontium incident to tooth 

movement, Scand. J. Dent. Res., 81, 62, 1973. 

38. Ten Cate, A. R., Deporter, D. A., and Freeman, 

E., The role of the fibroblasts in the remodeling of 

periodontal ligament during physiological tooth 

movement, Am. J. Orthodont., 69, 155, 1976. 

39. Ten Cate, A. R., Freeman, E., and Dickinson, 

J. B., Sutural development: structure and its response 

to rapid expansion, Am. J. Orthodont., 71,622, 1977. 

40. Rygh, P., Elimination of hyalinized periodontal tissues 

associated with orthodontic tooth movement, 

Scand. J. Dent. Res., 82, 57, 1974. 

41. Rygh, P., Ultrastructural changes in tension zone of 

rat molar periodontium incident to orthodontic tooth 

movement, Am. J. Orthodont., 70, 269, 1976. 

42. Edwards, J. E., A study of the periodontium during 

orthodontic rotation of teeth, Am. J. Orthodont., 54, 

441, 1968. 

43. Martinez, R. H. and Johnson, R. B., Effects of 

orthodontic forces on the morphology and diameter 

of Sharpey fibers on the alveolar bone of the rat, 

Anat. Rec., 219, 10, 1987. 

44. Kurihara, S. and Enlow, D. H., An electron microscopic 

study of attachments between periodontal 

fibers and bone during alveolar remodeling, Am. J. 

Orthodont., 77, 516, 1980. 



441

45. Kvam, E., Scanning electron microscopy of tissue 

changes on the pressure surface of human premolars 

following tooth movement, Scand. J. Dent. Res., 80, 

357, 1972. 

46. Barber, A. F. and Sims, M. R., Rapid maxillary 

expansion and external root resorption in man: a scanning 

electron microscope study, Am. J. Orthodont., 

79, 630, 1981. 

47. Langford, S. R. and Sims, M. R., Root surface 

resorption, repair, and periodontal attachment following 

rapid maxillary expansion in man, Am. J. 

Orthodont., 81, 108, 1982. 

48. Rygh, P., Orthodontic root resorption studied by 

electron microscopy, Angle Orthodont., 47, 1, 1977. 

49. Nakane, S. and Kameyama, Y., Root resorption 

caused by mechanical injury of the periodontal soft 

tissue in rats, J. Periodont Res., 22, 390, 1987. 

50. Goldie, R. S. and King, G. J., Root resorption and 

tooth movement in orthodontically treated, calciumdeficient, 

and lactating rats, Am. J. Orthodont., 85, 

424, 1984. 

51. Engstrom, C., Granstrom, G., and Thilander, B., 

Effect of orthodontic force on peridontal tissue metabolism, 

Am. J. Orthodont. Dentofacial Orthoped., 

93, 486, 1988. 

52. Garant, P. R. and Cho, M. I., Cytoplasmic polarization 

of periodontal ligament fibroblasts. Implications 

for cell migration and collagen secretion, J. 

Periodont. Res., 14, 95, 1979. 

53. Garant, P. R., Cho, M. I., and Cullen, M. R., 

Attachment of periodontal ligament fibroblasts to the 

extracellular matrix in the squirrel monkey, J. Periodont. 

Res., 17, 70, 1982. 

54. Cho, M. I. and Garant, P. R., Mirror symmetry 

of newly divided rat periodontal ligament fibroblasts 

in situ and its relationship to cell migration, J. Periodont. 

Res., 20, 185, 1985. 

55. Garant, P. R. and Cho, M. I., Fibroblast migration, 

cytoplasmic polarity, and matrix secretion in the periodontal 

ligament, in The Biology of Tooth Movement, 

Norton, L. A. and Burstone, C. J., Eds., CRC 

Press, Boca Raton, FL, 1989, 29. 

56. Willingham, M. C., Yamada, K. M., Yamada, 

S. S., Pouyssegur, J., and Pastan, I., Microfilament 

bundles and cell shape are related to adhesiveness 

to substratum and are dissociable from growth 

control in cultured fibroblasts, Cell, 10, 375, 1977. 

57. McCulloch, C. A. G., Barghava, U., and Melcher, 

A. H., Cell death and the regulation of populations 

of cells in the periodontal ligament, Cell Tissue Res., 

255, 129, 1989. 

58. Kvam, E., Cellular dynamics on the pressure side 

of the rat periodontium following experimental tooth 

movement, Scand. J. Dent. Res., 80, 369, 1972. 

59. Roberts, W. E. and Jee, W. S. S., Cell kinetics of 

orthodontically-stimulated periodontal ligament in the 

rat, Arch. Oral Biol., 19, 17, 1974. 

60. Roberts, W. E., Chase, D. C., and Jee, W. S. S., 

Counts of labeled mitoses in the orthodontically stim- 

442 

ulated periodontal ligament of the rat, Arch. Oral 

Biol., 19, 665, 1974. 

61. Roberts, W. E., Cell kinetic nature and diurnal periodicity 

of the rat periodontal ligament, Arch. Oral 

Biol., 20, 456, 1975. 

62. Roberts, W. E., Aubert, M. M., Sparaga, J. A., 

and Smith, R. K., Circadian periodicity of cell kinetics 

of rat molar periodontal ligament, Am. J. Orthodont., 

76, 316, 1979. 

63. Smith, R. K. and Roberts, W. E., Cell kinetics of 

the initial response of orthodontically induced osteogenesis 

in rat molar periodontal ligament, Calcified 

Tissue Int., 30, 51, 1980. 

64. Roberts, W. E. and Chase, D. C., Kinetics of cell 

proliferation and migration associated with orthodontically-induced 

osteogenesis, J. Dent. Res., 60, 

174, 1981. 

65. Roberts, W. E., Klinger, E., and Mozsary, P. G., 

Circadian rhythm of mechanically mediated differentiation 

of osteoblasts, Calcified Tissue Int., 36, 

S63, 1984. 

66. Roberts, W. E. and Ferguson, D. J., Cell kinetics 

of the periodontal ligament, in The Biology of Tooth 

Movement, Norton, L. A. and Burstone, C. J., Eds., 

CRC Press, Boca Raton, FL, 1989, 55. 

67. Yee, J. A., Kimmel, D. B., and Jee, W. S. S., 

Periodontal ligament cell kinetics following orthodontic 

tooth movement, Cell Tissue Kinet., 9, 293, 

1976. 

68. Yee, J. A., Response of periodontal ligament cells 

to orthodontic force: ultrastructure identification of 

proliferating fibroblasts, Anat. Rec., 194, 603, 1979. 

69. Roberts, W. E. and Cox, W. B., Morphometric 

quantification of nuclear volume changes during osteoblast 

histogenesis, in Bone Histomorphometry, 

Meunier, P. J., Ed., Armour Montagu, Paris, 1977, 

267. 

70. McCulloch, C. A. G., Nemeth, E., Lowenberg, 

B., and Melcher, A. H., Paravascular cells in endosteal 

spaces of alveolar bone contribute to periodontal 

ligament cell populations, Anat. Rec., 219, 

233, 1987. 

71. McCulloch, C. A. G. and Heersche, J. N. M., 

Lifetime of the osteoblast in mouse periodontium, 

Anat. Rec., 222, 129, 1988. 

72. Morris, C. E., Mechanosensitive ion channels, J. 

Memb. Biol., 113, 93, 1990. 

73. Brezden, B. L., Gardner, D. R., and Morris, 

C. E., A potassium-selective channel in isolated 

Lymnaea stagnalis heart muscle cells, J. Exp. Biol., 

123, 175, 1986. 

74. Moody, W. J. and Bosma, M. M., A nonselective 

cation channel activated by membrane deformation 

in oocytes of the ascidian Boltenia Villosa, J. Membr. 

Biol., 107, 179, 1989. 

75. Sachs, F., Mechanical transduction in biological systems, 

Crit. Rev. Biomed. Eng., 16, 141, 1988. 

76. Christensen, O., Mediation of cell volume regula- 



tion by Ca2+ influx through stretch-activated channels, 

Nature, 330, 66, 1987. 

77. Cooper, K. E., Tang, J. M., Rae, J. L., and 

Eisenberg, R. S., A cation channel in frog lens epithelia 

responsive to pressure and calcium, J. Membr. 

Biol., 93, 259, 1986. 

78. Lansman, J. B., Hallam, T. J., and Rink, T. J., 

Single stretch-activated ion channels in vascular endothelial 

cells as mechanotransducers, Nature, 325, 

811, 1987. 

79. Morris, C. E. and Sigurdson, W. J., Stretch-inactivated 

ion channels coexist with stretch-activated 

ion channels, Science, 243, 807, 1989. 

80. Harris, A. K., Wild, P., and Stopak, D., Silicone 

rubber substrata: a new wrinkle in the study of cell 

locomotion, Science, 208, 177, 1980. 

81. Ingber, D. E. and Folkman, J., Tension and 

compression as basic determinants of cell form and 

function: utilization of a cellular tensegrity mechanism, 

in Cell Shape: Determinants, Regulation, and 

Regulatory Role, Stein, W. D. and Bronner, F., Eds., 

Academic Press, San Diego, CA, 1989, 3. 

82. Vanderburgh, H. H. and Kaufman, S., Stretchinduced 

growth of skeletal myotubes correlates with 

activation of the sodium pump, J. Cell Physiol., 109, 

205, 1981. 

83. Cogoli, A., Valluchi-Morf, M., Mueller, M., and 

Briegleb, W., Effect of hypogravity on human lymphocyte 

activation, Aviat. Space Environ. Med., 51, 

29, 1980. 

84. Leung, D. Y. M., Glagov, S., and Mathews, M. B., 

Cyclic stretching simulates synthesis of matrix components 

by arterial smooth muscle cells in vitro, Science, 

191, 475, 1976. 

85. Quinn, R. S. and Rodan, G. A., Enhancement of 

ornithine decarboxylase and Na+, K+ ATPase in osteosarcoma 

cells by intermittent compression, 

Biochem. Biophys. Res. Commun., 100, 1696, 1981. 

86. Ingber, D. E. and Jamieson, J. D., Cells as tensegrity 

structures: architectural regulation of histodifferentiation 

by physical forces transduced over 

basement membrane, in Gene Expression During 

Normal and Malignant Differentiation, Andersson, 

L. C., Gahmberg, C. G., and Ekblom, P., Eds., 

Academic Press, Orlando, FL, 1985, 13. 

87. Joshi, H. C., Chu, D., Buxbaum, R. E., and 

Heidermann, S. R., Tension and compression in the 

cytoskeleton of PC 12 neurites, J. Cell Biol., 101, 

697, 1985. 

88. Herman, B. and Pledger, W. J., Platelet-derived 

growth factor-induced alterations in vinculin and actin 

distribution in BALB/c-3T3 cells, J. Cell Biol., 

100, 1031, 1985. 

89. Berezney, R. and Coffey, D. S., Nuclear protein 

matrix: association with newly synthesized DNA, 

Science, 189, 291, 1975. 

90. Pardoll, D. M., Vogelstein, B., and Coffey, D. S., 

A fixed site of DNA replication in eucaryotic cells, 

Cell, 19, 527, 1980. 

91. Barrak, E. R., and Coffey, D. S., Biological properties 

of the nuclear matrix: steroid hormone binding, 

Recent Prog. Hormone Res., 38, 133, 1982. 

92. Nicolini, C., Belmont, A. S., and Martelli, A., 

Critical nuclear DNA size and distribution associated 

with S phase initiation, Cell Biophys., 8, 103, 1986. 

93. Jiang, L.-W. and Schindler, M., Nuclear transport 

in 3T3 fibroblasts: effects of growth factors, transformation 

and cell shape, J. Cell Biol., 106, 13, 

1988. 

94. Emerman, J. T. and Pitelka, D. R., Maintenance 

and induction of morphological differentiation in associated 

mammary epithelium on floating collagen 

membranes, In Vitro, 13, 316, 1977. 

95. Lanyon, L. E., Goodship, A. E., Pye, C. J., and 

MacFie, J. H., Mechanically adaptive bone remodeling, 

J. Biomech., 15, 767, 1982. 

96. Rubin, C. T. and Lanyon, L. E., Regulation of 

bone formation by applied dynamic loads, J. Bone 

Jt. Surg., 66-A, 397, 1984. 

97. Lanyon, L. E. and Rubin, C. T., Static vs. dynamic 

loads as an influence on bone remodeling, J. Biomech., 

17, 897, 1984. 

98. Rubin, C. T. and Lanyon, L. E., Regulation of 

bone mass by mechanical strain magnitude, Calcified 

Tissue Int., 37, 411, 1985. 

99. Lanyon, L. E., Rubin, C. T., and Baust, G., Modulation 

of bone loss during calcium insufficiency by 

controlled dynamic load, Calcified Tissue Int., 38, 

209, 1986. 

100. Rubin, C. T. and Lanyon, L. E., Osteoregulatory 

nature of mechanical stimuli: function as a determinant 

for adaptive remodeling in bone, J. Orthoped. 

Res., 5, 300, 1987. 

101. Lanyon, L. E., Functional strain in bone tissue as 

an objective, and controlling stimulus for adaptive 

bone remodeling, J. Biomech., 20, 1083, 1987. 

102. Pead, M. J., Suswillo, R., Skerry, T. M., Vedi, 

S., and Lanyon, L. E., Increased 3H-uridine levels 

in osteocytes following a single short period of dynamic 

bone loading in vivo, Calcified Tissue Int., 43, 

92, 1988. 

103. Skerry, T. M., Bitensky, L., Chayen, J., and 

Lanyon, L. E., Loading-related reorientation of bone 

proteoglycan in vivo. Strain memory in bone tissue?, 

J. Orthoped. Res., 6, 547, 1988. 

104. Skerry, T. M., Suswillo, R., El Haj, A. J., Ali, 

N. N., Dodds, R. A., and Lanyon, L. E., Loadinduced 

proteoglycan orientation in bone tissue in 

vivo and in vitro, Calcified Tissue Int., 46, 318, 1990. 

105. Smith, E., Reddan, W., and Smith, P., Physical 

activity and calcium modalities for bone mineral increased 

in aged women, Med. Sci. Sports Exer., 13, 

60, 1980. 

106. Smith, E. L., Smith, P. E., Ensign, C. J., and 

Shea, M. M., Bone involution decrease in exercising 

middle-aged women, Calcified Tissue Int., 36, S 129, 

1984. 

107. Mack, P. B., LaChance, P. A., Vose, G. P., and 



443

Vogt, F., Bone demineralization of foot and hand of 

Gemini-Titan IV, V, and VII astronauts during orbital 

flight, Am. J. Roentgenol., 100, 503, 1967. 

108. Cann, C. E. and Adachi, R. R., Bone resorption 

and mineral excretion in rats during space flight, Am. 

J. Physiol., 244, R327, 1983. 

109. Rambaut, P. C. and Goode, A. W., Skeletal changes 

during space flight, Lancet, p. 1050, 9, November 

1985. 

110. Bikle, D. D., Halloran, B. P., Cone, C. M., Globus, 

R. K., and Morey-Holton, E., The effect of simulated 

weightlessness on bone maturation, Endocrinology, 

120, 678, 1987. 

111. Yamaguchi, M., Ozaki, K., and Hoshi, T., Simulated 

weightlessness and bone metabolism: decrease 

of protein and DNA synthesis in the femoral diaphysis 

of rats, Res. Exp. Med., 189, 331, 1989. 

112. Glucksmann, A., The role of mechanical stresses in 

bone formation in vitro, J. Anat., 76, 231, 1942. 

113. Functional Adaptation in Bone Tissue, A Kroc Foundation 

Conference, July 1983, Cowin, S. C., Lanyon, 

L. E., and Rodan, G., Eds., Calcified Tissue 

Int., 36(Suppl. 1), 1, 1984. 

114. Pienkowski, D. and Pollack, S. R., The origin of 

stress-generated potentials in fluid saturated bone, J. 

Orthoped. Res., 1, 30, 1983. 

115. Pollack, S. R., Salzstein, R., and Pienkowski, D., 

The electric double layer in bone and its influence 

on stress-generated potentials, Calcified Tissue Int., 

36, S77, 1984. 

116. Marino, A. A., Spadaro, J. A., Fukada, E., Kah, 

L. D., and Becker, R. O., Piezoelectricity in collagen 

films, Calcified Tissue Int., 31, 257, 1970. 

117. Fukada, E. and Yasuda, I., Piezoelectric effects in 

collagen, Jpn. J. Appl. Phys., 3, 117, 1964. 

118. Marino, A. A. and Becker, R. O., Piezoelectricity 

in hydrated frozen bone and tendon, Nature, 253, 

627, 1975. 

119. Marino, A. A. and Becker, R. O., The origin of 

the piezoelectric effect in bone, Calcified Tissue Int., 

8, 177, 1971. 

120. Bassett, C. A. L. and Becker, R. O., Generation 

of electric potentials by bone in response to mechanical 

stress, Science, 137, 1063, 1962. 

121. Cochran, G. V. B., Pawluk, R. J., and Bassett, 

C. A. L., Stress generated electric potentials in the 

mandible and teeth, Arch. Oral Biol., 12, 917, 1967. 

122. Zengo, A. N., Pawluk, R. J., and Bassett, C. A. L., 

Stress-induced bioelectric potentials in the dentoalveolar 

complex, Am. J. Orthodont., 64, 17, 1973. 

123. Zengo, A. N., Bassett, C. A. L., Pawluk, R. J., 

and Prountzos, G., In vivo bioelectric potentials in 

the dentoalveolar complex, Am. J. Orthodont., 66, 

130, 1974. 

124. Marino, A. A. and Gross, B. D., Piezoelectricity 

in cementum, dentine and bone, Arch. Oral Biol., 

34, 507, 1989. 

125. Grimm, F. M., Bone bending, a feature of ortho- 

444 

dontic tooth movement, Am. J. Orthodont., 62, 384, 

1972. 

126. DeAngelis, V., Observations on the response of alveolar 

bone to orthodontic force, Am. J. Orthodont., 

58, 284, 1970. 

127. Borgens, R. B., Endogenous ionic currents traverse 

intact and damaged bone, Science, 225, 478, 1984. 

128. Otter, M., Shoenung, J., and Williams, W. S., 

Evidence for different sources of stress-generated potentials 

in wet and dry bone, J. Orthoped. Res., 3, 

321, 1985. 

129. Friedenberg, Z. B. and Kohanim, M., The effect 

of direct current on bone, Surg. Gynecol. Obstet., 

127, 97, 1968. 

130. Friedenberg, Z. B., Harlow, M. C., and Brighton, 

C. T., Healing of nonunion of the medial malleolus 

by means of direct current: a case report, J. Trauma, 

11, 863, 1971. 

131. Bassett, C. A. L., Pilla, A. A., and Pawluk, R. J., 

Non-operative salvage of surgically resistant pseudoarthroses 

and non-unions by pulsing electromagnetic 

fields, Clin. Orthoped., 124, 128, 1977. 

132. Friedenberg, Z. B. and Brighton, C. T., Bioelectricity 

and fracture healing, Plastic Reconstr. Surg., 

68, 435, 1981. 

133. Marino, A. A., Electrical stimulation in orthopaedics: 

past, present and future, J. Bioelect., 3, 235, 

1984. 

134. Beeson, D. C., Johnston, L. E., and Wisotzky, J., 

Effect of constant currents on orthodontic tooth 

movement in the cat, J. Dent. Res., 54, 251, 1975. 

135. Davidovitch, Z., Finkelson, M. D., Steigman, S., 

Shanfeld, J. L., Montgomery, P. C., and Korostoff, 

E., Electric currents, bone remodeling, and orthodontic 

tooth movement. I. The effect of electric currents 

on periodontal cyclic nucleotides, Am. J. Orthodont., 

77, 14, 1980. 

136. Davidovitch, Z., Finkelson, M. D., Steigman, S., 

Shanfeld, J. L., Montgomery, P. C., and Korostoff, 

E., Electric currents, bone remodeling, and orthodontic 

tooth movement. II. Increase in rate of tooth 

movement and periodontal cyclic nucleotide levels 

by combined force and electric current, Am. J. Orthodont., 

77, 33, 1980. 

137. Davidovitch, Z., Montgomery, P. C., and 

Shanfeld, J. L., Cellular localization and concentration 

of bone cyclic nucleotides in response to acute 

PTE administration, Calcified Tissue Int., 24, 81, 

1977. 

138. Murad, F., Brewer, B., Jr., and Vaughn, M., 

Effect of thyrocalcitonin on adenosine, 3',5'-cyclic 

phosphate formation by rat kidney and bone, Proc. 

Natl. Acad. Sci. U.S.A., 65, 446, 1970. 

139. Rasmussen, H. and Bordier, P., Vitamin D and 

bone, Metab. Bone Dis. Relat. Res., 1, 7, 1978. 

140. Kornstein, R., Somjen, D., Danon, A., Fisher, 

H., and Binderman, I., Pulsed capacitive electric 

induction of cyclic AMP changes, Ca45 uptake and 



DNA synthesis in bone cells, Trans. Bioelectr. Repair 

Growth, 1, 34, 1981. 

141. Rodan, G. A., Bourret, L. A., Harvey, A., and 

Mensi, T., Cyclic AMP and cyclic GMP: mediators 

of the mechanical effects on bone remodeling, Science, 

189, 467, 1975. 

142. Tashjian, A. H., Jr., Voelkel, E. F., Levine, L., 

and Goldhaber, P., Evidence that the bone resorption-stimulating 

factor produced by mouse fibrosarcoma 

cells is prostaglandin E2, J. Exp. Med., 136, 

1329, 172. 

143. Gomes, B. C., Hausmann, E., Weinfeld, N., and 

DeLuca, C., Prostaglandins: bone resorption stimulating 

factors released from monkey gingiva, Calcified 

Tissue Int., 19, 285, 1976. 

144. Shiozawa, S., Williams, R. C., Jr., and Ziff, M., 

Immunoelectron microscopic demonstration of prostaglandin 

E in rheumatoid synovium, Arthr. Rheum., 

25, 685, 1982. 

145. Dekel, S., Lenthal, G., and Francis, M. J. 0., 

Release of prostaglandins from bone and muscle after 

tibial fracture, Bone Jt. Surg., 63B, 185, 1981. 

146. Uchida, A., Yamashita, K., Hashimoto, K., and 

Shimomura, Y., The effects of mechanical stress on 

cultured growth cartilage cells, Connect. Tissue Res., 

17, 305, 1988. 

147. Somjen, D., Binderman, I., Berger, E., and Harell, 

A., Bone remodeling induced by physical stress is 

prostaglandin E2 mediated, Biochim. Biophys. Acta, 

627, 91, 1980. 

148. Shen, V., Rifas, L., Kohler, G., and Peck, W. A., 

Prostaglandins change cell shape and increase intercellular 

gap junctions in osteoblasts cultured frdm rat 

fetal calvaria, J. Bone Min. Res., 1, 243, 1986. 

149. Hasegawa, S., Sato, S., Saito, S., Suzuki, Y., and 

Brunette, D. M., Mechanical stretching increases 

the number of cultured bone cells synthesizing DNA 

and alters their pattern of protein synthesis, Calcified 

Tissue Int., 37, 431, 1985. 

150. Buckley, M. J., Banes, A. J., Levin, L. G., Sumpio, 

B. E., Sato, M., Jordan, R., Gilbert, J., Link, 

G. W., and Tran Son Tay, R., Osteoblasts increase 

their rate of division and align in response to cyclic, 

mechanical tension in vitro, Bone Min., 4, 225, 1988. 

151. Klein-Nutlend, J., Velhuijzen, J. P., de Jong, M., 

and Burger, E. H., Increased bone formation and 

decreased bone resorption in fetal mouse calvaria as 

a result of intermittent compressive force, in vitro, 

Bone Min., 2, 441, 1987. 

152. Burger, E. H., Velhuijzen, J. P., Klein-Nulend, 

J., and Van Loon, J. J. W. A., Osteoclastic invasion 

and mineral resorption of fetal mouse long 

bone rudiments are inhibitied by culture under intermittent 

compressive force, Connect. Tissue Res., 20, 

131, 1989. 

153. Ozawa, H., Imamura, K., Etsuko, A., Takahashi, 

N., Hiraide, T., Shibasaki, Y., Fukuhara, T., and 

Suda, T., Effect of continuously applied compressive 

pressure on mouse osteoblast-like cells (MC3T3-E1) 

in vitro, J. Cell Physiol., 142, 177, 1990. 

154. Duncan, G. W., Yen, E. H. K., Pritchard, E. T., 

and Suga, D. M., Collagen and prostaglandin synthesis 

in force-stressed periodontal ligament, in vitro, 

J. Dent. Res., 63, 665, 1984. 

155. Meikle, M. C., Heath, J. K., Atkinson, S. J., 

Hembry, R. M., and Reynolds, J. J., Molecular 

biology of stressed connective tissues at sutures and 

hard tissues in vitro, in The Biology of Tooth Movement, 

Norton, L. A. and Burstone, C. J., Eds., CRC 

Press, Boca Raton, FL, 1989, 71. 

156. Curtis, A. S. G. and Sechar, G. M., The control 

of cell division by tension or diffusion, Nature, 274, 

52, 1978. 

157. Norton, L. A., Anderson, K. L., Melsen, B., 

Arenholt Bindslev, D., and Celis, J. E., Buccal 

mucosa fibroblasts and periodontal ligament cells 

perturbed by tensile stimuli in vitro, Scand, J. Dent. 

Res., 98, 36, 1990. 

158. Ngan, P. W., Crock, B., Varghese, J., Lanese, 

R., Shanfeld, J. L., and Davidovitch, Z., Immunohistochemical 

assessment of the effect of chemical 

and mechanical stimuli on cAMP and prostaglandin 

E levels in human gingival fibroblasts, in vitro, Arch. 


159. Ngan, P., Saito, S., Saito, M., Shanfeld, J., and 

Davidovitch, Z., The interactive effects of mechanical 

stress and interleukin 1 on prostaglandin E and 

cyclic AMP production in human periodontal ligament 

fiborblasts in vitro: comparison with cloned 

osteoblastic cells of mouse (MC3T3-E1), Arch. Oral 

Biol., 35, 717, 1990. 

160. Ragnarsson, B., Carr, G., and Daniel, J. C., Isolation 

and growth of human periodontal ligament cells 

in vitro, J. Dent. Res., 64, 1026, 1985. 

161. Brunette, D. M., Kanoza, R. J., Marmary, Y., 

Chan, J., and Melcher, A. H., Interactions between 

epithelial and fibroblast-like cells in cultures derived 

from monkey periodontal ligament, J. Cell Sci., 27, 

127, 1977. 

162. Brunette, D. M., Heersche, J. N. M., Purdon, 

A. D., Sodek, J., Moe, H. K., and Assuras, J. N., 

In vitro cultured parameters and protein and prostaglandin 

secretion of epithelial cells derived from porcine 

rests of Malassez, Arch. Oral Biol., 24, 199, 

1979. 

163. Merrilees, M. J., Sodek, J., and Aubin, J. E., 

Effect of cells of epithelial rests of Malassez and 

endothelial cells on synthesis of glycosaminoglycans 

by periodontal ligament fibroblasts in vitro, Dev. Biol., 

97, 146, 1983. 

164. Birek, C., Heersche, J. N. M., Jez, D., and 

Brunette, D. M., Secretion of a bone resorbing factor 

by epithelial cells cultured from porcine rests of 

Malassez, J. Periodont. Res., 18, 75, 1983. 

165. Brunette, D. M., Mechanical stretching increases 



445

the number of epithelial cells synthesizing DNA in 

culture, J. Cell Sci., 69, 35, 1984. 

166. Klein, D. C. and Raisz, L. G., Prostaglandins: stimulation 

of bone resorption in tissue culture, Endocrinology, 

86, 1436, 1970. 

167. Rodan, G. A. and Martin, T. J., Role of osteoblasts 

in hormonal control of bone resorption 

a hypoth- 

esis, Calcified Tissue Int., 33, 349, 1981. 

168. Bindeman, I., Zor, U., Kaye, A. M., Shimshoni, 

Z., Harell, A., and Somjen, D., The transduction 

of mechanical force into biochemical events in bone 

cells may involve activation of phospholipase A2, 

Calcified Tissue Int., 42, 261, 1988. 

169. Davidovitch, Z., Shanfeld, J. L., Montgomery, 

P. C., Lally, E., Laster, L., Furst, L., and 

Korostoff, E., Biochemical mediators of the effects 

of mechanical forces and electric currents on mineralized 

tissues, Calcified Tissue Int., 36, S86, 1984. 

170. Davidovitch, Z., Nicolay, O., Alley, K., Zwilling, 

B., Lanese, R., and Shanfeld, J. L., First and second 

messenger interactions in stressed connective tissues 

in vivo, in The Biology of Tooth Movement, 

Norton, L. A. and Burstone, C. J., Eds., CRC Press, 

Boca Raton, FL, 1989, 97. 

171. Yamasaki, K., Miura, F., and Suda, T., Prostaglandin 

as a mediator of bone resorption induced by 

experimental tooth movement in rats, J. Dent. Res., 

59, 1635, 1980. 

172. Yamasaki, K., Shibata, Y., and Fukuhara, T., 

The effects of prostaglandins on experimental tooth 

movement in monkeys (Macaca fuscata), J. Dent. 

Res., 61, 1444, 1982. 

173. Yamasaki, K., Shibata, Y., Imai, S., Tani, Y., 

Shibasaki, Y., and Fukuhara, T., Clinical application 

of prostaglandin E, (PGE,) upon orthodontic 

tooth movement, Am. J. Orthodont., 85, 508, 1984. 

174. Yamasaki, K., Pharmacological control of tooth 

movement, in The Biology of Tooth Movement, Norton, 

L. A. and Burstone, C. J., Eds., CRC Press, 

Boca Raton, FL, 1989, 287. 

175. Jee, W. S. S., Ueno, K., Deng. Y. P., and 

Woodbury, D. M., The effect of prostaglandin E2 

in growing rats: increased metaphyseal hard tissue 

and cortico-endosteal bone formation, Calcified Tissue 

Int., 37, 148, 1985. 

176. Marks, S. C., Jr. and Miller, S. C., Local infusion 

of prostaglandin E, stimulates mandibular bone formation 

in vivo, J. Oral Pathol., 17, 500, 1988. 

177. Nefussi, J.-R. and Baron, R., PGE2 stimulates both 

resorption and formation of bone in vitro: differential 

responses of the periosteum and the endosteum in 

fetal rat long bone cultures, Anat. Rec., 211,9, 1985. 

178. Lee, W., Experimental study of the effect of prostaglandin 

administration on tooth movement with 

particular emphasis on the relationship to the method 

of PGE, administration, Am. J. Orthodont. Dentofacial 

Orthoped., 98, 231, 1990. 

179. Decker, J. L., Malone, D. G., Haroui, B., Wahl, 

S., Schrieber, L., Klippel, J., Steinberg, A. G., 

446 

and Wilder, R., Rheumatoid arthritis: evolving concepts 

of pathogenesis and treatment, Ann. Intern. 

Med., 101, 810, 1984. 

180.. Brandt, K. D. and Slowman-Kovacs, S., Nonsteroidal 

antiinflammatory drugs in treatment of osteoarthritis, 

Clin. Orthoped., 213, 84, 1986. 

181. Torbinejad, M., Clagett, J., and Engel, D., A cat 

model for the evaluation of mechanisms of bone resorption: 

induction of bone loss by stimulated immune 

complexes and inhibition by indomethacin, 

Calcified Tissue Int., 29, 207, 1979. 

182. Waite, I. M., Saxton, C. A., Young, A., Wagg, 

B. J., and Corbett, M., The periodontal status of 

subjects receiving non-steroidal anti-inflammatory 

drugs, J. Periodont. Res., 16, 100, 1981. 

183. Saffar, J. L. and Lasfargues, J. J., A histometric 

study of the effect of indomethacin and calcitonin on 

bone remodeling in hamster periodontitis, Arch. Oral 

Biol., 29, 555, 1984. 

184. Williams, R. C., Jeffcoat, M. K., Kaplan, M. L., 

Goldhaber, P., Johnson, H. G., and Wechter, 

W. J., Flurbiprofen: a potent inhibitor of alveolar 

bone resorption in beagles, Science, 277, 640, 1985. 

185. Melcher, A. H., An overview of the anatomy and 

physiology of the periodontal ligament, in The Biology 

of Tooth Movement, Norton, L. A. and Burstone, 

C. J., Eds., CRC Press, Boca Raton, FL, 

1989, 1. 

186. van Steenberghe, D., The structure and function of 

periodontal innervations, J. Periodont. Res., 14, 185, 

1979. 

187. Pfaffman, C., Afferent impulses from the teeth due 

to pressure and noxious stimulation, J. Physiol., 97, 

207, 1939. 

188. Cash, R. M. and Linden, R. W. A., The distribution 

of mechanoreceptors in the periodontal ligament 

of the mandibular canine teeth of the cat, J. 

Physiol., 330, 439, 1982. 

189. Jyvisjiirvi, E., Kniffki, K.-D., and Mengel, 

M. K. C., Functional characteristics of afferent C 

fibers from tooth pulp and periodontal ligament, Prog. 

Brain Res., 74, 237, 1988. 

190. Freezer, S. R. and Sims, M. R., A transmission 

electron-microscopy stereological study of the blood 

vessels, oxytalan fibres and nerves of mouse-molar 

periodontal ligament, Arch. Oral Biol., 32, 407, 1987. 

191. Gould, T. R. L., Melcher, A. H., and Brunette, 

D. M., Location of progenitor cells of periodontal 

ligament of mouse molar stimulated by wounding, 

Anat. Rec., 188, 133, 1977. 

192. McCulloch, C. A. G. and Melcher, A. H., Cell 

density and cell generation in the periodontal ligament 

of mice, Am. J. Anat., 167, 43, 1983. 

193. Khouw, F. E. and Goldhaber, P., Changes in vasculature 

of the periodontium associated with tooth 

movement in the rhesus monkey and dog, Arch. Oral 

Biol., 15, 1125, 1970. 

194. Rygh, P., Ultrastructural vascular changes in pressure 

zones of rat molar periodontium incident to or- 



thodontic movement, Scand. J. Dent. Res., 80, 307, 

1972. 

195. Jeffcoat, M. K., Kaplan, M. L., Rumbaugh, C. L., 

and Goldhaber, P., Magnification angiography in 

beagles with periodontal disease, J. Periodont. Res., 

17, 294, 1982. 

196. Bien, S. M., Fluid dynamic mechanisms which regulate 

tooth movement, Adv. Oral Biol., 2, 173, 1966. 

197. Lotz, M., Vaughan, J. H., and Carson, D. A., 

Effect of neuropeptides on production of inflammatory 

cytokines by human monocytes, Science, 241, 

1218, 1988. 

198. Laurenzi, M. A., Persson, M. A. A., Dalsgaard, 

C.-J., and Ringden, O., Stimulation of human B 

lymphocyte differentiation by the neuropeptides substance 

P and neurokinin A, Scand. J. Immunol., 30, 

695, 1989. 

199. Hafstrom, I., Gyllenhammar, H., Palmblad, J., 

and Ringertz, B., Substance P activates and modulates 

neutrophil oxidative metabolism and aggregation, 

J. Rhemuatol., 16, 1033, 1989. 

200. Roscetti, G., Ausiello, C. M., Palma, C., Gulla, 

P., and Roda, G., Enkephalin activity on antigeninduced 

proliferation of human peripheral blood mononucleate 

cells, Int. J. Immunopharm., 10, 819, 1988. 

201. Marotti, T., Sverko, V., and Hrsak, I., Modulation 

of superoxide anion release from human polymorphonuclear 

cells by Met- and Leu-enkephalin, 

Brain, Behav. Immun., 4, 13, 1990. 

202. Weinstock, J. V., Blum, A., Walder, J., and 

Walder, R., Eosinophils from granulomas in Schistosomiasis 

mansoni produce substance P, J. Immunol., 

141, 961, 1988. 

203. Weinstock, J. V. and Blum, A. M., Release of 

substance P by granuloma eosinophils in response to 

secretagogues in murine Schistosomiasis mansoni, Cell 

Immunol., 125, 380, 1990. 

204. Roth, K. A., Lorenz, R. G., Unanue, R. A., and 

Weaver, C. T., Nonopiate active proenkephalin-derived 

peptides are secreted by T helper cells, FASEB 

J., 3, 2402, 1989. 

205. Fagarasan, M. O., Bishop, J. F., Rinaudo, M. S., 

and Axelrod, J., Interleukin 1 induces early protein 

phosphorylation and requires only a short exposure 

for late induced secretion of P-endorphin in a mouse 

pituitary cell line, Proc. Natl. Acad. Sci. U.S.A., 87, 

2555, 1990. 

206. Fagarasan, M. 0. and Axelrod, J., Interleukin-1 

amplifies the action of pituitary secretagogues via 

protein kinases, Int. J. Neurosci., 51, 311, 1990. 

207. Su, T.-P., London, E. D., and Jaffe, J. H., Steroid 

binding at a receptors suggests a link between endocrine, 

nervous, and immune systems, Science, 240, 

219, 1988. 

208. Levine, J. D., Clark, R., Devor, M., Helms, C., 

Moskowitz, M. A., and Basbaum, A. I., Intraneuronal 

substance P contributes to the severity of experimental 

arthritis, Science, 226, 547, 1984. 

209. Lotz, M., Carson, D. A., and Vaughan, J. H., 

Substance P activation of rheumatoid synoviocytes: 

neural pathway in pathogenisis of arthritis, Science, 

235, 893, 1987. 

210. Yokoyama, M. M. and Fujimoto, K., Role of lymphocyte 

activation by substance P in rheumatoid arthritis, 

Int. J. Tissue React., 12, 1, 1990. 

211. O'Bryne, E. M., Blancuzzi, V. J., Wilson, D. E., 

Wong, M., Peppard, J., Simke, J. P., and Jeng, 

A. Y., Increased intra-articular substance P and prostaglandin 

E2 following injection of interleukin-1 in 

rabbits, Int. J. Tissue React., 12, 11, 1990. 

212. Lam, F. Y. and Ferrell, W. R., Inhibition of carrageenan-induced 

joint inflammation by substnace P 

antagonist, Ann. Rheum. Dis., 48, 928, 1989. 

213. Lam, F. Y. and Ferrell, W. R., Capsaicin suppresses 

substance P-induced joint inflammation in the 

rat, Neurosci. Lett., 105, 155, 1989. 

214. Olgart, L., Gazelius, B., Brodin, E., and Pernow, 

B., Localization of substance P-like immunoreactivity 

in nerves in the tooth pulp, Pain, 4, 153, 1977. 

215. Olgart, L., Gazelius, B., Brodin, E., and Nilsson, 

G., Release of substance P-like immunoreactivity from 

the dental pulp, Acta Physiol. Scand., 101, 510, 1977. 

216. Wakisaka, S., Nishikawa, S., Ichikawa,H., 

Matsuo, S., Takano, Y., and Akai, M., The distribution 

and origin of substance P-like immunoreactivity 

in the rat molar pulp and periodontal tissues, 

Arch. Oral Biol., 30, 813, 1985. 

217. Wakisaka, S., Ichikawa, H., Nishikawa, S., 

Uchiyama, T., Matsuo, S., Takano, Y., and Akai, 

M., Immunohistochemical study on regeneration of 

substance P-like immunoreactivity in rat molar pulp 

and periodontal ligament following resection of the 

inferior alveolar nerve, Arch. Oral Biol., 32, 225, 

1987. 


Matsuo, Y., Takano, Y., and Akai, M., Immunohistochemical 

observation on the correlation between 

substance P- and vasoactive intestinal polypeptide-like 

immunoreactivities in the feline dental 

pulp, Arch. Oral Biol., 32, 449, 1987. 

219. Davidovitch, Z., Nicolay, O. F., Ngan, P. W., and 

Shanfeld, J. L., Neurotransmitters, cytokines, and 

the control of alveolar bone remodeling in orthodontics, 

Dent. Clin. N. Am., 31, 411, 1988. 

220. Said, S. I. and Mutt, V., Polypeptide with broad 

biological activity: isolation from small intestine, Science, 

169, 1217, 1970. 

221. Hohmann, E., Levine, L., and Tashjian, A. H., 

Jr., Vasoactive intestinal peptide stimulates bone resorption 

via a cyclic adenosine 3',5'-monophosphatedependent 

mechanism, Endocrinology, 112, 1233, 

1983. 

222. Hohmann, E. L. and Tashjian, A. H., Jr., Functional 

receptors for vasoactive intestinal peptide of 

human osteosarcoma cells, Endocrinology, 114, 1321, 

1984. 

223. Hohmann, E. L., Elde, R. P., Rysavy, J. A., 

Einzig, S., and Gebhard, R. L., Innervation of 



447

periosteum and bone by sympathetic vasoactive intestinal 

peptide-containing nerve fibers, Science, 232, 

868, 1986. 

224. Herness, M. S., Vasoactive intestinal peptide-like 

immunoreactivity in rodent taste cells, Neuroscience, 

33, 411, 1989. 

225. Motakef, M., Shanfeld, J., and Davidovitch, Z., 

Localization of VIP at bone resorption sites in vivo, 

J. Dent. Res., 69, 253, 1990. 

226. Rosenfeld, M. G., Mermod, J.-J., Amara, S. G., 

Swanson, L. W., Sawchenko, P. E., Rivier, J., 

Vale, W. W., and Evans, R. M., Production of a 

novel neuropeptide encoded by the calcitonin gene 

via tissue-specific RNA processing, Nature, 304, 129, 

1983. 

227. Stjarne, P., Lundblad, L., Angard, A., Hokfelt, 

T., and Lundberg, J. M., Tachykinins and calcitonin 

gene-related peptide: co-existence in sensory 

nerves of the nasal mucosa and effects on blood flow, 

Cell Tissue Res., 256, 439, 1989. 

228. Fujimori, A., Saito, A., Kimura, S., Watanabe, 

T., Uchiyama, Y., Kawasaki, H., and Goto, K., 

Neurogenic vasodilation and release of calcitonin generelated 

peptide (CGRP) from perivascular nerves in 

the rat mesenteric artery, Biochem. Biophys. Res. 

Commun., 165, 1391, 1989. 

229. Larsson, J., Ekblom, A., Henriksson, K., 

Lundeberg, T., and Theordorsson, E., Immunoreactive 

tachykinins, calcitonin gene-related peptide 

and neuropeptide Y in human synovial fluid from 

inflamed knee joints, Neurosci. Lett., 100, 326, 1989. 

230. Silverman, J. D. and Kruger, L., Calcitonin-generelated-peptide 

immunoreactive innervation of the rat 

head with emphasis on specialized sensory structures, 

J. Comp. Neurol., 280, 303, 1989. 


Matsuo, S., Takano, Y., and Akai, M., The distribution 

and origin of calcitonin gene-related peptide-containing 

nerve fibers in feline dental pulp, Histochemistry, 

86, 585, 1987. 

232. Taylor, P. E., Byers, M. R., and Redd, P. E., 

Sprouting of CGRP nerve fibers in response to dentin 

injury in rat molars, Brain Res., 461, 371, 1988. 

233. Kata, J., Ichikawa, H., Wakisaka, S., Matsuo, 

S., Sakuda, M., and Akai, M., The distribution of 

vasoactive intestinal polypeptides and calcitonin generelated 

peptide in the periodontal ligament of mouse 

molar teeth, Arch. Oral Biol., 35, 63, 1990. 

234. Hill, E. L. and Elde, R., Calcitonin gene-related 

peptide-immunoreactive nerve fibers in mandibular 

periosteum of rat: evidence for primary afferent origin, 

Neurosci. Lett., 85, 172, 1988. 

235. Gazelius, B., Edwall, B., Olgart, L., Lundberg, 

J. M., Hokfelt, T., and Fischer, J. A., Vasodilatory 

effects and coexistence of calcitonin gene-related 

peptide (CGRP) and substance P in sensory nerves 

of cat dental pulp, Acta Physiol. Scand., 130, 33, 

1987. 

236. Takahashi, O., Traub, R. J., and Ruda, M. A., 

448 

Demonstration of calcitonin gene-related peptide immunoreactive 

axons containing dynorphin A (1-8) 

immunoreactive spinal neurons in a rat model of peripheral 

inflammation and hyperalgesia, Brain Res., 

475, 168, 1988. 

237. Kvinnsland, I. and Kvinnsland, S., Changes in 

CGRP-immunoreactive nerve fibers during experimental 

tooth movement in rats, Eur. J. Orthodont., 

12, 320, 1990. 

238. Okamoto, Y., Davidovitch, Z., and Shanfeld, J., 

CGRP in dental pulp and periodontal ligament during 

tooth movement (abstract), J. Dent. Res., submitted. 

239. Tashiro, T., Takahashi, O., Satoda, T., 

Matsushima, R., and Mizuno, N., Immunohistochemical 

demonstration of coexistence of enkephalinand 

substance P-like immunoreactivities in axonal 

components in the lumbar segments of cat spinal cord, 

Brain Res., 424, 391, 1987. 

240. Jessell, T. M. and Iversen, L. L., Opiate analgesics 

inhibit substance P release in rat trigeminal nucleus, 

Nature, 268, 549, 1977. 

241. Walker, J. A., Tanzer, F. S., Harris, E. F., 

Wakelyn, C., and Desiderio, D. M., The enkephalin 

response in human tooth pulp to orthodontic 

force, Am. J. Orthodont. Dentofacial Orthoped., 92, 

9, 1987. 

242. Tanzer, F. S., Tolun, E., Fridland, G. H., Dass, 

C., Killmar, J., Tinsley, P. W., and Desiderio, 

D. M., Methionine-enkephalin peptides in human 

teeth, Int. J. Peptide Protein Res., 32, 117, 1988. 

243. Robinson, Q. C., Killmar, J. T., Desiderio, D. M., 

Harris, E. F., and Fridland, G., Immunoreactive 

evidence of beta-endorphin and methionine-arg-glyleu 

in human tooth pulp, Life Sci., 45, 987, 1989. 

244. Hanazawa, S., Ohmori, Y., Amano, S., Miyoshi, 

T., Kumegawa, M., and Kitano, S., Spontaneous 

production of interleukin-1-like cytokine from mouse 

osteoblastic cell line (MC3T3-E1), Biochem. Biophys. 

Res. Commun., 131, 774, 1985. 

245. Hanazawa, S., Amano, S., Nakada, K., Ohmori, 

Y., Miyoshi, T., Hirose, K., and Kitano, S., Biological 

characterization of interleukin-1-like cytokine 

produced by cultured bone cells from newborn mouse 

calvaria, Calcified Tissue Int., 41, 31, 1987. 

246. Rath, N. C., Oronsky, A. L., and Kerwar, S. S., 

Synthesis of interleukin-1-like activity by normal rat 

chondrocytes in culture, Clin. Immunol. Immunopathol., 

47, 39, 1988. 

247. Kaplan, E., Dinarello, C. A., and Gelfand, J. A., 

Interleukin-1 and the response to injury, Immunol. 

Res., 8, 118, 1989. 

248. Gowen, M., Wood, D. D., and Russell, G. G., 

Stimulation of the proliferation of human bone cells 

in vitro by human monocyte products with interleukin-1 

activity, J. Clin. Invest., 75, 1223, 1985. 

249. Boyce, B. F., Yates, A. J. P., and Mundy, G. R., 

Bolus injections of recombinant human interleukin- 

1 cause transient hypocalcemia in normal mice, Endocrinology, 

125, 2780, 1989. 



250. Sabatini, M., Boyce, B., Aufdemorte, T., 

Bonewald, L., and Mundy, G. R., Infusions of 

recombinant human interleukins la and 1p cause 

hypercalcemia in normal mice, Proc. Natl. Acad. Sci. 

U.S.A., 85, 5235, 1988. 

251. Feige, U., Karbowski, A., Rordorf-Adam, C., and 

Pataki, A., Arthritis induced by continuous infusion 

of hr-interleukin-la into the rabbit knee joint, Int. J. 

Tissue React., 11, 225, 1989. 

252. Mizel, S. B., Dayer, J. M., Krane, S. M., and 

Mergenhagen, S. E., Stimulation of rheumatoid synovial 

cell collagenase and prostaglandin production 

by partially purified lymphocyte-activating factor (interleukin 

1), Proc. Natl. Acad. Sci. U.S.A., 78, 2474, 

1981. 

253. Rafter, G. W., Interleukin 1 and rheumatoid arthritis, 

Med. Hypoth., 27, 221, 1988. 

254. Johnson, W. J., Breton, J., Newman-Tarr, T., 

Connor, J. R., Meunier, P. C., and Dalton, B. J., 

Interleukin-1 release by rat synovial cells is dependent 

on sequential treatment with r-interferon and 

lipopolysaccharide, Arthr. Rheum., 33, 261, 1990. 

255. Masada, M. P., Persson, R., Kenney, J. S., Lee, 

S. W., Page, R. C., and Allison, A. C., Measurement 

of interleukin-la and - 1 in gingival cervicular 

fluid: implications for the pathogenesis of periodontal 

disease, J. Periodont. Res., 25, 156, 1990. 

256. Thomson, B. M., Saklatvala, J., and Chambers, 

T. J., Osteoblasts mediate interleukin 1 stimulation 

of bone resorption by rat osteoclasts, J. Exp. Med., 

150, 104, 1986. 

257. Dewhirst, F. E., Ago, J. M., Peros, W. J., and 

Stashenko, P., Synergism between parathyroid hormone 

and interleukin 1 in stimulating bone resorption 

in organ culture, J. Bone Min. Res., 2, 127, 1987. 

258. Gowen, M. and Mundy, G. R., Actions of recombinant 

interleukin 1, interleukin 2 and interferongamma 

on bone resorption in vitro, J. Immunol., 136, 

2478, 1986. 

259. Tatakis, D. N., Schneebergy, G., and Dziak, R., 

Recombinant interleukin-l stimulates prostaglandin 

E2 production by osteoblastic cells: synergy with 

parathyroid hormone, Calcified Tissue Int., 42, 358, 

1988. 

260. Ries, W. L., Seeds, M. C., and Key, L. L., Interleukin-2 

stimulates osteoclastic activity: increased 

acid production and radioactive calcium release, J. 

Periodont. Res., 24, 242, 1989. 

261. Davidovitch, Z., Lynch, P., and Shanfeld, J. L., 

Interleukin 2 localization in the mechanically-stressed 

periodontal ligament, J. Dent. Res., 68, 332, 1989. 

262. Nieto-Sampedro, M. and Chandy, K. G., Interleukin-2-like 

activity in injured rat brain, Neurochem. 

Res., 12, 723, 1987. 

263. Bevilacqua, M. P., Pober, J. S., Majeau, G. R., 

Fiers, W., Cotran, R. S., and Gimbrone, Jr., Recombinant 

tumor necrosis factor induces procoagulant 

activity in cultured human vascular endothelium: 

characterization and comparison with the actions of 

interleukin 1, Proc. Natl. Acad. Sci. U.S.A., 83, 

4533, 1986. 

264. Gamble, J. R., Harlan, J. M., Klebanoff, S. J., 

and Vadas, M. A., Stimulation of the adherence of 

neutrophils to umbilical vein endothelium by human 

recombinant tumor necrosis factor, Proc. Natl. Acad. 

Sci. U.S.A., 82, 8667, 1985. 

265. Bertolini, D. R., Nedwin, G. E., Bringman, T. S., 

Smith, D. D., and Mundy, G. R., Stimulation of 

bone resorption and inhibition of bone formation in 

vitro by human tumor necrosis factors, Nature, 319, 

516, 1986. 

266. Delaisse, J.-M., Eeckhout, Y., and Vaes, G., Boneresorbing 

agents affect the production and distribution 

of procollagenase as well as the activity of collagenase 

in bone tissue, Endocrinology, 123, 264, 1988. 

267. Yoshihara, R., Shiozawa, S., Imai, Y., and Fujita, 

T., Tumor necrosis factor a and interferon T inhibit 

proliferation and alkaline phosphatase activity of human 

osteoblastic SaOS-2 cell line, Lymphokine Res., 

9, 59, 1990. 

268. Shapiro, S., Tatakis, D. N., and Dziak, R., Effect 

of tumor necrosis factor alpha on parathyroid hormone-induced 

increase in osteoblastic cell cyclic 

AMP, Calcified Tissue Int., 46, 60, 1990. 

269. Stashenko, P., Dewhirst, F. E., Peros, W. J., Kent, 

R. L., and Ago, J. M., Synergistic interactions between 

interleukin 1, tumor necrosis factor, and lymphotoxin 

in bone resorption, J. Immunol., 138, 1464, 

1987. 

270. Pfeilschifter, J., Chenu, C., Bird, A., Mundy, 

G. R., and Roodman, G. D., Interleukin-1 and tumor 

necrosis factor stimulate the formation of human 

osteoclast-like cells in vitro, J. Bone Min. Res., 4, 

113, 1989. 

271. Hashimoto, J., Yoshikawa, H., Takaoka, K., 

Shimizu, N., Mashuhara, K., Tsuda, T., 

Miyamoto, S., and Ono, K., Inhibitory effects of 

tumor necrosis factor alpha on fracture healing in rats, 

Bone, 10, 453, 1989. 

272. Johnson, R. A., Boyce, B. F., Mundy, G. R., and 

Roodman, G. D., Tumors producing human tumor 

necrosis factor induce hypercalcemia and osteoclastic 

bone resorption in nude mice, Endocrinology, 124, 

1424, 1989. 

273. Hopkins, S. J. and Meager, A., Cytokines in synovial 

fluid. II. The presence of tumor necrosis factor 

and interferon, Clin. Exp. Immunol., 73, 88, 1988. 

274. Maury, C. P. J. and Teppo, A.-M., Cachectin/ 

tumor necrosis factor-a in the circulation of patients 

with rheumatic disease, Int. J. Tissue React., 11, 

189, 1989. 

275. Yocum, D. E., Esparza, L., Dubry, S., Benjamin, 

J. B., Volz, R., and Scuder, P., Characteristics of 

tumor necrosis factor production in rheumatoid arthritis, 

Cell Immunol., 122, 131, 1989. 

276. Gowen, M., Nedwin, G. E., and Mundy, G. R., 

Preferential inhibition of cytokine-stimulated bone re- 



449

sorption by recombinant interferon gamma, J. Bone 

Min. Res., 1, 469, 1986. 

277. Gowen, M., MacDonald, B. R., and Russell, G., 

Actions of recombinant human T-interferon and tumor 

necrosis factor a on the proliferation and osteoblastic 

characteristics of human trabecular bone cells in vitro, 

Arthr. Rheum., 31, 1500, 1988. 

278. Nanes, M. S., McKoy, W. M., and Marx, S. J., 

Inhibitory effects of tumor necrosis factor-a and interferon-T 

on deoxyribonucleic acid and collagen synthesis 

by rat osteosarcoma cells (ROS 17/2.8), Endocrinology, 

124, 339, 1989. 

279. Vignery, A., Niven-Fairchild, T., and Shepard, 

M. H., Recombinant murine interferon-r inhibits the 

fusion of mouse alveolar macrophages in vitro, but 

stimulates the formation of osteoclast-like cells in 

implanted syngeneic bone particles in mice in vivo, 

J. Bone Min. Res., 5, 637, 1990. 

280. Melin, M., Hartmann, D. J., Magloire, H., 

Falcoffe, E., Auriault, C., and Grimaud, J. A., 

Human recombinant gamma-interferon stimulates 

proliferation and inhibits collagen and fibronectin 

production by human dental pulp fibroblasts, Cell 

Mol. Biol., 35, 97, 1989. 

281. Judis, G., Davidovitch, Z., and Shanfeld, J., Identification 

of IFN at in vivo bone resorption sites, J. 

Dent. Res., 69, 244, 1990. 

282. Rosol, T. J., Shanfeld, J. L., and Davidovitch, 

Z., Biochemical aspects of orthodontic tooth movement. 

II. Effects of neurotransmitters and cytokines 

on bone resorption and formation in vitro, J. Dent. 

Res., submitted. 

283. Saito, S., Ngan, P., Saito, M., Kim, K., Lanese, 

R., Shanfeld, J., and Davidovitch, Z., Effects of 

cytokines on prostaglandin E and cAMP levels in 

human periodontal ligament fiborblasts in vitro, Arch. 


284. Saito, S., Ngan, P., Saito, M., Lanese, R., 

Shanfeld, J., and Davidovitch, Z., Interactive effects 

between cytokines on PGE production by human 

periodontal ligament fibroblasts, in vitro, J. Dent. 

Res., 69, 1456, 1990. 

285. Saito, S., Rosol, T. J., Saito, M., Ngan, P. W., 

Shanfeld, J., and Davidovitch, Z., Bone-resorbing 

activity and prostaglandin E produced by human periodontal 

ligament cells in vitro, J. Bone Min. Res., 

5, 1013, 1990. 

286. Saito, S., Saito, M., Ngan, P., Lanese, R., 

Shanfeld, J., and Davidovitch, Z., Effects of parathyroid 

hormone and cytokines on prostaglandin E 

synthesis and bone resorption by human periodontal 

ligament fibroblasts, Arch. Oral Biol., in press. 

287. Saito, S., Saito, M., Ngan, P. W., Shanfeld, J., 

and Davidovitch, Z., Interleukin 1 beta and prostaglandin 

E are involved in the response of periodontal 

cells to mechanical stress in vivo and in vitro, 

Am. J. Orthodont. Dentofacial Orthoped., in press. 

450

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