lec.6 Enamel

It s the hardest tissue in the body, epithelially
derived from the ectoderm layer, it is protective
layer of the tooth. Ameloblasts are the cells
which are responsible for it's formation, derived
from the inner enamel epithelial cells,
ameloblasts are lost as the tooth erupted in to
the oral cavity & hence the enamel is non vital
so it can't renew itself.

Physical properties of E :

1.E is the hardest tissue in the human body
, it's also brittle & subject to fracture
specially if the underlying dentin is
carious, causing a weak foundation.

2. E is very hard because it compose of 96%
mineral (inorganic) in a form of
hydroxyapitite crystal (which is crystalline
calcium phosphate Ca10(Po)6(OH)2 &
4%organic & water. The organic matrix of
E is made from noncollagenous proteins &
contain several protein & enzymes . 90%
of enamel protein is amelogenins & the
remaining 10% consist of enamelin &
ameloblastin

3. E is white to grayish white but it's
appear slightly yellow because it is
translucent & the underlying dentin is
yellowish, give the yellow color to E.
4. E is ranges in thickness from a knife edge
at it's cervical margin to about 2-5mm 2
maximum thickness over the occlusal or
incisal surface

5.E is semipermeable, , it can permit exchange ions
& molecules specially if it is newly erupted tooth.
Florid can penetrate enamel easily in newly
formed & be less permeability in adult one.
6.E may be altered by etching with dilute acids
such as citric acid (E has a property of acid
solubility) etching of the E can be used in the
anterior filling & to allow adherence of a plastic
sealant.

Structure of enamel

The basic units of E are the rods (prisms) & interrod E
(interprismatic
substance).

The number of E rods range from 5 million in
lower lateral incisors to 12 million in upper first
molar.
The length of the rods is greater than the
thickness of E because of the oblique direction &
wavy course of rods.
Rods located in the cusps are longer than that in
the cervical areas.
Diameter of rods increase from the DEJ
(dentinoenamel
junction ) toward the surface of
the enamel at ratio about 1:2.

Rods in cross section appear hexagonal
sometimes they appear round or oval or fish
scales due to different angulations during the
section.

A common pattern of prism is a key hole
in longitudinal section the bodies or heads
of one row of rods & tails of adjacent row
represent the interrod substance , the
bodies are nearer the occlusal & incisal
surface, whereas the tails point cervically.
Each rod formed by 4 ameloblast cells.

Rod sheath :
It is represent the organic matrix of the E & it is incomplete
envelop surrounding the rod.
Direction of rods
In general rods run in a perpendicular direction to the
surface of the dentin with slight inclination toward the
cusps, near the cusp tip they run more vertically & in
the cervical enamel run horizontally.
In the cusps of teeth the rods appear twisted around each
other in a complex arrangement called gnarled enamel.

Hunter Schreger bands:
These are an optical phenomena produce by
changes in the direction of rods. These
bands appear as dark & light alternating
zones clearly seen in longitudinal ground
section & they are found in the inner two
third of the E.

All the E rods are deposited in daily appositional
rate or incremental form of 4micrometer such
increments are noticeable as a brownish bands
in ground section of E. in transverse ground
section these lines appear as concentric circles.
In longitudinal section they appear as dark lines
surround the crown they run obliquely. The
growth lines also appear in the surface of the E
as aridges (wave like grooves) as external
manifestation of stria of Retzius known as
perikymata.

Neonatal lines : is an enlarged stria of Retzius
reflected the great physiological changes
occurring before & after birth.
E lamellae: They are visible cracks of varying
depths extend from the surface of E toward the
DEJ they consist of linear, longitudinally oriented
defects filled with organic material. best
demonstrated in ground section. E lamella
represent a pathway for initiating caries in E.

E tufts:
These are tuft like contain organic material
arise from DEJ up to (1/5
1/3) of E
thickness. They contain greater
concentration of E protein's than the rest
of E & they believed to occur
developmentally due to changes in the
direction of rods.

E spindles
They are only the mesenchymal structure in
the E extending from the odontoblast
process through the E . they appear dark
color in ground section.

Surface structures of E

1.perikymata : it is a shallow grooves on E surface believed
to be manifestation of striae of Retzius.
2. pellicle: as the tooth erupted, it is covered by pellicle
consisting of debris from the E organ that is lost rapidly,
then the salivary pellicle deposit on the tooth surface
which can removed by mechanical cleaning.
3.rods ends: they appear as pits or depression on the
surface of the tooth & these disappear by the
mastication.

4. rodless E: E without prism (rod) , the
mineralization of this type E is higher than the
remaining E due to the lose of rod so the crystal
are roughly arranged. It is 30 micrometer in
thickness occur in 70% of permanent teeth & in
all deciduous teeth in cusp tip & cervical area.
5. Primary E cuticle : It is delicate membrane
cover the entire crown of newly erupted tooth
secreted by ameloblasts & soon removed by
mastication .

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