INTRODUCTION:
Dentin is the first formed
dental hard tissue which provides the bulk and general form of the tooth. Since
it is formed before the enamel, it is responsible for determining the shape of
the crown, the number and size of the roots [1]. This living tissue
is laid down by specialized cells called odontoblasts which have their
cytoplasmic processes inside closely packed dentinal tubules which traverse the
entire thickness of dentin [2].
Dentin has a bone-like matrix but is found
to be harder than bone and softer than enamel. This difference can be readily
distinguished on radiographs were dentin appears more radiolucent than enamel
and more radiopaque than dentin. The hardness varies slightly between tooth
types and between coronal and radicular dentin in such a way that it is
slightly harder in the central part than the periphery. Dentin is usually light
yellowish in color in teeth of younger individuals and darkens with age.
It is brittle, viscoelastic,
provides flexibility and prevents fracture of the overlying enamel [1].
Mature dentin is composed of
approximately 70% inorganic material, 20% organic matter and 10% of water.
Substituted hydroxyapatite crystals arranged in the form of plates forms the
inorganic component of this ectomesenchymally derived tissue. The organic phase
is 90% collagen (mainly type I with small amounts of type III and V) with small
amounts of non collagenous matrix proteins and lipids. Some of these proteins
are dentin phosphoprotein (DPP), dentin sialoprotein (DSP) and dentin
glycoprotein (DGP) [3].
Dentinogenesis, the sequential
process of dentin formation which occurs when specialized cells called
odontoblasts differentiate from the ectomesenchymal cells of the dental
papilla. Dentin is first deposited as a layer of unmineralised matrix called
predentin of varying thickness (10-50 mm) which later mineralises when the non
collagenous proteins get incorporated at the mineralization front. Two patterns
of dentin mineralization have been identified histologically- globular and
linear calcification [3].
The aim of this study was to
understand the significant differences in the histological structure of dentin
in primary and permanent teeth. For this purpose, the following structures were
observed while viewing the dentin under the microscope:
1. Dentinal tubules
2. Peritubular dentin
3. Intertubular dentin
4. Interglobular dentin
5. Circumpulpal dentin
6. Incremental growth lines of
dentin
7. Dead tracts
8. Tomes granular layer
As dentin is the fundamental
substrate of restorative dentistry, its properties and characteristics are
essential determinants all restorative, preventive and disease processes of the
teeth. This study will help in providing an improved understanding of the
dentin structure which will result in better outcomes in various dental
procedures.
MATERIALS
AND METHODS:
A total of 9 (5 permanent and 4
primary) unrestored and non carious molars were collected, manually cleaned and
stored in a diluted solution of hydrogen peroxide for future use in the study.
Preparation of ground section:
A longitudinal section of all
teeth was obtained using a carborundum disc such that both the crown and root
portion of the teeth were included. The cut sections were reduced to uniform
thickness of less than 100µm by grinding the teeth on the Arkansas stone.
Figure 1: Carborundum
Figure 2: Arkansas Stone
disc
Mounting of the sections:
The section was mounted on a
microscopic glass slide using mounting media, Dystrene Polystren Xylene and
covered with a cover slip. The slides were labelled to differentiate between
the primary and permanent teeth. These sections were then observed under
compound light microscope.
The following histological
structures were observed:
1. Dentinal tubules
2. Peritubular dentin
3. Intertubular dentin
4. Interglobular dentin
5. Circumpulpal dentin
6. Incremental growth lines of
dentin
7. Dead tracts
8. Tomes granular layer
RESULT:
Examination of ground sections
of 4 primary teeth under a compound light microscope showed the following
characteristic features.
The dentinal tubules were
prominently larger with somewhat straight course, more numerous and divergent.
The divergence near the root furcation was more prominent in primary teeth.
Peritubular dentin, intertubular dentin, dead tracts and granular layer of
Tomes were also seen in the longitudinal ground sections. However interglobular
dentin and incremental growth lines could not be identified.
Examination of ground sections
of 5 permanent teeth under compound light microscope showed the following light
microscopic structures.
A normal course of dentinal
tubules with S-shaped primary curvatures and sinusoidal secondary curvatures
was observed. Fundamental structures such as peritubular dentin, intertubular
dentin, interglobular dentin, circumpulpal dentin were prominently seen in the
permanent teeth. Incremental lines of Von Ebner, dead tracts and Tomes granular
layer were also clearly seen.
Figure 3: Permanent tooth- Hunter Schreger bands Enamel spindles,
Interglobular dentin
Figure 4: Cross sectional view of dentinal tubules.
Figure 5: Dentinal
tubules coursing inwards near the cementum
Figure 6: Primary
teeth-a) numerous dentinal tubules. b) divergence of tubules near the
furcation. c) larger tubules. d) increased tubular density.
Figure 7: Primary teeth-a) numerous dentinal tubules. b)
divergence of tubules near the furcation. c) larger tubules. d) increased
tubular density.
Figure 8: Primary tooth- straight dentinal Tubules, enamel
spindles, enamel cracks, Enamel lamella, dead tracts.
DISCUSSION:
By forming the bulk, providing
the shape and rigidity to the tooth, dentin becomes an impertinent hard tissue
effective during mastication. It is covered by the hardest tissue, the enamel
in the crown portion and cementum in the root. Dentin and pulp are often
considered as a single unit forming the dentin-pulp complex which is the
developmental and functional unit of the tooth [4].
For long it has been assumed
that the morphology, composition and histological structure primary and
permanent teeth are similar. However, some evidence suggests that significant
differences exist between them which leads to difference in the properties of
dentin. Hence, this study was an attempt to compare these differences if
observed [5-7].
Dentinal tubules:
Dentinal tubules are canaliculi
that traverse the thickness of dentin from the dentinoenamel junction to the
mineralization front. Cytoplasmic extensions of odontoblasts are seen within
them. These tubules have a sigmoidal S-shaped course from the outer surface of
dentin to the perimeter of coronal pulp indicating the path taken by
odontoblasts during dentinogenesis. In molar teeth, the number of tubules vary
from 59,000 to 76,000 per square millimeter at the pulpal surface anD
approximately half as many near the enamel. In ground sections, empty tubules
appear black by transmitted light as they entrap air [3].
On microscopic examination, the
structure of dentinal tubules showed a highly significant difference. The
number of dentinal tubules was found to be more in primary than permanent
teeth. They appeared to be more divergent in primary teeth and this divergence
was maximum near the root furcation. The course taken by the dentinal tubules
was found to be different in the two teeth types. Dentinal tubules in permanent
teeth showed the usual S- shaped sigmoidal course however they appeared to be
somewhat straight in primary teeth.
This reason for this difference
could be attributed to the lower difference between the surface area of dentin
near the dentinoenamel junction and that near the pulp. This is explained by
the fact that primary teeth have wider pulps than permanent teeth. This could
result in less crowding of odontobalsts and hence the straight course [8].
Peritubular dentin:
Peritubular dentin is the
hypermineralised structure which immediately surrounds the dentinal tubules and
is 9% more mineralized than intertubular dentin found between the tubules [3].
There was no significant
difference observed in this structure in primary and permanent teeth.
Intertubular dentin:
Intertubular dentin represents
the primary formative product of odontoblasts and mainly consists of type I
collagen fibrils (50-200nm in diameter) between the dentinal tubules. It forms
the main body of dentin and is located between zones of peritubular dentin.
This highly mineralized structure is retained after decalcification [1].
Intertubular dentin also does not show much difference in primary and permanent
teeth.
Interglobular dentin:
The areas of unmineralised or
hypomineralised dentin where globular calcospherites have failed to fuse into a
homogenous mass are called interglobular dentin. It is most commonly seen in
circumpulpal dentin as it is a defect of mineralization and not of matrix
formation. The dentinal tubules run uninterrupted through these areas.
Interglobular dentin is more prevalent in teeth with vitamin D deficiency and
dentinal fluorosis [1].
In this study, circumpulpal
dentin was found to be absent in primary teeth but prominent in permanent
teeth. However this finding was statistically insignificant and a detailed
explanation for this could nit be found.
Circumpulpal dentin:
Circumpulpal dentin forms a
part of the primary dentin with collagen fibrils (0.05µm in diameter) more
closely packed surrounding the pulpal chamber [1].
There was no significant
difference observed in this structure in primary and permanent teeth.
Incremental growth lines of dentin:
Incremental growth lines represent the
rhythmic pattern of dentin deposition during dentinogenesis and run roughly at
right angles to the dentinal tubules. Lines related to the disturbances during
dentinogenesis are called Von Ebner lines. The dentin matrix is laid down at a
rate of 4µ per day and changes in orientation occur approximately every 5 days,
accounting for the presence of these lines. The incremental lines take up the
shape of the mineralizing front at different stages. The angle of orientation
of odontoblastic processes determines the angle of orientation of Von Ebner
lines [4].
In this study, incremental
lines were not found in primary teeth but prominent in permanent teeth. However
this finding was statistically insignificant and a detailed explanation for
this could nit be found.
Dead tracts:
Severe irritation of dentinal tubules
causes the loss of its contents and failure to mineralize. Atrophic
degeneration of odontoblastic processes will lead to the dentinal tubules
getting filled with air in ground sections. These dead tracts appear black in
transmitted and white in reflected light [1].
This study showed that there
was no difference in the occurrence and structure in primary and permanent
teeth. Since dead tracts occur as a result of death of odontblasts due to
crowding, attrition, erosion and abrasion, there is no preferential occurrence
of dead tracts in either of the two dentitions [9].
Tomes granular layer:
When dry ground sections of
root dentin are visualized, a granular unmineralised zone adjacent to the
cementum called Tomes granular layer is seen. It is caused by coalescing and
looping of the terminal portions of the dentinal tubules [1].
No significant differences were
found in the appearance and occurrence in primary and permanent teeth.
CONCLUSION:
The following observations were
noted:
1. Dentinal tubules followed an
S-shaped course in permanent molars but are found to be more divergent and
prominent in primary molars.
2. There was no significant
difference in the occurrence of peritubular dentin, intertubular dentin, circumpulpal
dentin, dead tracts and tomes granular layer.
3. Interglobular dentin was not
seen in the primary molars.
4. Incremental growth lines of
dentin could not be identified in primary molars but were prominently seen in
permanent molars.
Hence this study was able to
establish that significant structural differences exist in the dentin of the
primary and permanent dentition.
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