Dental Pulp and Innervation

COURSES > HUMAN ANATOMY II, DDS09, AUT06 > CONCISE ORAL HISTOLOGY > DENTAL PULP
Dental Pulp
Dental Pulp and Innervation
Dental pulp is an unmineralized oral tissue composed of soft connective tissue, vascular, lymphatic and
nervous elements that occupies the central pulp cavity of each tooth. Pulp has a soft, gelatinous
consistency. By either weight or volume, the majority of pulp (75-80%) is water. Aside from the presence
of pulp stones, there is little or no inorganic component in normal dental pulp.
The pulp cavity extends down through the root of the tooth as the root canal which opens into the
periodontium via the apical foramen. Blood vessels, nerves etc. of dental pulp enter and leave the tooth
through this foramen. This sets up a form of communication between the pulp and surrounding tissue,
and is clinically important in the spread of inflammation from the pulp out into periodontium.
Developmentally and functionally, pulp and dentin are closely related. Both are products of the neural
crest-derived connective tissue that formed the dental papilla.
Functions of Dental Pulp
Dental pulp also has several other functions:
Inductive -- Very early in development the dental papilla interacts with surrounding tissues to
initiate tooth formation.
Formative -- The odontoblasts of the outer layer of the pulp organ form the dentin (primary and
secondary).
Protective -- A direct response to cutting procedures, caries, extreme pressure, etc., involves the
formation of tertiary dentin (reactive or repairative). Formation of sclerotic dentin, in the process
of obliterating the dentinal tubules, may also be protective to the pulp.
Sensory -- Rich innervation that alerts the individual when injury occurs.
The primary function of dental pulp is providing vitality to the tooth. Loss of the pulp, however, does not
mean the tooth will be lost.
Histology of Dental Pulp
Dental pulp is a loose connective tissue, and it contains the major constituents common to all
connective tissues:
Cells: fibroblasts and undifferentiated mesenchymal cells as well as other cell types
(macrophages, lymphocytes, etc.) required for the maintenance and defense of the tissue.
Fibrous matrix: collagen fibers, type I and III, are present in an unbundled and randomly
dispersed fashion, higher in density around blood vessels and nerves.
Ground substance: the environment that surrounds both cells and fibers of the pulp is rich in
proteoglycans, glycoproteins and large amounts of water.
The large number of undifferentiated mesenchymal cells facilitates the recruitment of newly
differentiating cells to replace others when they are lost - specifically odontoblasts. Odontoblasts form
the outermost region/layer of the pulp, immediately adjacent to the dentin component of the tooth.
These cells are responsible for the secretion of dentin and the formation of dentinal tubules in the crown
and root.
The fact that the pulp is surrounded by mineralized dentin makes relatively minor pathologic events like
inflammation, that cause swelling elsewhere, lead to a compression of the pulp leading to intense pain.
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Architecture of the Pulp
The peripheral aspect of dental pulp, referred to as the odontogenic zone (1), differentiates into a
layer of dentin-forming odontoblasts (A).
Immediately subjacent to the odontoblast layer is the cell-free zone (of Weil). This region (2)
contains numerous bundles of reticular fibers (B). These fibers pass from the central pulp region,
across the cell-free zone and between the odontoblasts, their distal ends incorporated into the
matrix of the dentin layer. Numerous capillaries (C) and nerves (D) are also found in this zone.
Just under the cell-free zone is the cell-rich zone (3) containing numerous fibroblasts (E) - the
predominant cell type of pulp. Undifferentiated mesenchymal cells are also present in the pulp
and can give rise to odontoblasts, fibroblasts or macrophages. Since odontoblasts themselves
are incapable of cell division, any dental procedure that relies on the formation of new dentin (F)
after destruction of odontoblasts, depends on the differentiation of new odontoblasts from these
multipotential cells of the pulp. Lymphocytes, plasma cells and eosinophils are other cell types
also common in dental pulp.
Medial to the cell-rich zone is the deep pulp cavity (4), and adjacent to, or within, the cell-rich
zone is the subodontoblastic plexus (of Raschkow) of nerves (G).
Vascular Supply to the Pulp
One or more small arterioles enter the pulp via the apical
foramen and once they reach the pulp chamber in the
crown they branch out to form a dense capillary network
immediately under and extending up into the odontoblast
layer. Small venules drain the capillary bed and
eventually leave as veins via the apical foramen. Only a
few thin-walled, irregularly shaped lymphatic vessels are
present in the dental pulp. Of considerable clinical
significance is the fact that following injury vascular
congestion may occur, and this may lead to necrosis of
the pulp.
Innervation of the Pulp
Several large nerves enter the apical foramen of each
molar and premolar with single ones entering the anterior
teeth.
Autonomic nerve fibers (sympathetic only) -- They
innervate the smooth muscle cells of the
arterioles and therefore function in regulation of
blood flow in the capillary network.
Afferent
(sensory)
fibers --
These arise
from the
maxillary and mandibular branches of the fifth cranial nerve
(trigeminal). They may terminate in the central pulp, but
many send out individual fibers that form the
subodontoblastic plexus. From this plexus, fibers extend
between the odontoblasts and in the coronal dentin
(particularly the pulp horns) they travel for short distances
into the dentinal tubules. All sensory nerve endings in the
pulp function in transmitting nociceptive information, whether
induced by heat, cold or pressure. Nerve fibers are more
predominant in pulp horns and lateral walls of the coronal
pulp, and less extensive in the root.
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The figure illustrates nerve endings (F) arising from the subodontoblastic plexus (E) and passing up
between odontoblasts (A) to enter the dentinal tubule where they terminate (G) near an odontoblast
process (D). B = predentin, C = dentin
Dentin sensitivity is explained by the 'hydrodynamic theory'. Movement of fluid in the dentinal tubules
results in the activation of small, myelinated Aδ fibers causing sharp pain. Injury and inflammation later
lead to the activation of unmyelinated C fibers (dull ache sensation), which are found in the
subodontoblastic layer and the deep pulp. Dental pain is often very hard to localize, and referred pain
(either to the teeth or from the teeth) can complicate diagnosis.
Age-Related and Pathologic Changes in the Pulp
Specific changes occur in dental pulp with age. Cell death results in a decreased number of cells, and
the surviving fibroblasts respond by producing more fibrous matrix. However, the most significant
change is a reduction in the size of the pulp chamber due to the continued deposition of secondary
dentin.
Calcified bodies also become more prominent in older teeth:
Pulp stones (or denticles) are small calcified bodies that can be found in up to 90% of the pulp of
older teeth. These calcified bodies are generally found loose within the pulp, but may eventually
grow large enough to encroach on adjacent dentin and become attached. A Pulp stone
containing tubular dentin is referred to as a "true pulp stone", whereas irregularly calcified tissue
is called a "false pulp stone".
Diffuse calcification occurs when an injured pulp undergoes calcification in a number of locations.
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