The dentin
By
Dr . Suhair Majeed
Dentinogenesis
is the formation of dentin , a substance
that forms the majority of teeth.It’s
performed by odontoblasts, a special type of
cells on the outer wall of dental pulps,and it
begins at late bell stage of a toot
development, dentinogenesis begins before
amelogenesis,it’s unlike amelogenesis ,that it
occurs throughout life of individual.
Cont.
dentinogenesis occur in two stages :
1- secretion of dentin matrix
2- mineralization of dentin
1- secretion of dentin matrix
odontoblasts begin to secrete
unmineralized dentin matrix, The first sign of
dentin formation is the appearance of distinct,
large-diameter collagen fibrils . In this way, a
layer of mantle predentin appeared.
As more dentin matrix is deposited ,
odontoblasts retreat towards pulp direction
leaving an elongated process known as
odontoblastic process , a narrow layer of
predentin is always present on the surface
of the pulp.
predentin
ameloblast
odontoblast
pulp
2- mineralization of dentin
two patterns of dentin mineralization can be
observed—globular and linear calcification that
seem to depend on the rate of dentin formation.
Globular (or calcospheric) calcification, involves
the deposition of crystals in several discrete areas of
matrix , globular masses are formed that continue
to enlarge and finally fuse to form a single calcified
mass. This pattern of mineralization is seen in the
mantle dentin region.
The predentin (PD)/ pulp (P)/ calcified round
globules (CRG)/ Interglobular Dentin (IGD)
Cont.
the first layer of mineralized dentin is called
mantle dentin and the remaining bulk of the
mineralized dentin is known as circumpulpal
dentin. In circumpulpal dentin the mineralization
can progress in a globular or linear pattern.
When the rate of formation progresses slowly,
the mineralization appears more uniform and the
process progressed in linear pattern.
Linear mineralization
dentin
pd
od
pulp
Globular mineralization
dentin
pd
od
pulp
Formation of predentine:
It’s the first unmineralized layer secreted
of dentin matrix , it consists of collagen fibers
and mucopolysaccharides.The principal
proteins in predentin are :
1- collagen type 1
2- dentin phosphoprotein
3- osteocalcin , osteonectin and osteopontin
4- matrix Gla –protein
(Gamma carboxyglutamic acid )
Cont.
the first indication of the first layer of predentin
which is formed by newly differentiated
odontoblasts is the development of Von Korff’s
fibers ,which are large-diameter collagen fibrils (0.1
to 0.2 mm in diameter). These fibers consist of
collagen type III associated with fibronectin. These
fibers originate deep among the odontoblasts,
extend toward the inner enamel epithelium, and
fan out in the structureless ground substance.
Cont.
As the odontoblasts continue to increase in
size, they also produce smaller collagen type I fibrils
that orient themselves parallel to the future
dentinoenamel junction. In this way, a layer of
mantle predentin appears.
FORMATION OF ROOT DENTIN
The epithelial cells of Hertwig’s root sheath
initiate the differentiation of odontoblasts that form
root dentin . Root dentin forms similarly to coronal
dentin, with some differences , the outer layer of
root dentin, differs in collagen fiber orientation
and organization, in part because the collagen
fibers from cementum blend with those of dentin ,
and its degree of mineralization differs from that of
coronal dentin.
dentin root formation
dentin
od
pd
pulp
Hertwig’s ep.
Root sheath
Life cycle of odontoblasts
1- Differentiation of Odontoblasts
- Odontoblasts differentiate from undifferentiated
ectomesenchymal cells.
- Before the beginning of dentinogenesis, the cells
of inner enamel epithelium are supported by
basement membrane, which separates the
epithelium from the dental papilla .
- in the early bell stage ,under the inductive
influence of the inner dental epithelium ,
Cont.
the periphral ectomesenchymal cells differentiate
first to preodontoblasts , they assume to a
columnar shape and aligned as a single row along
the basement membrane.
- several projections (odontoblastic process) arise
from the upper part of the cells.
- the cells grow in length to become long columnar.
- now the fully diffrentiated odontoblasts begin
their work.
2- Formative stage
- production of first amount of dentin ( dentin
matrix ).
- odontoblasts retreat from the basement
membrane ,leaving a single odontoblastic process
which become enclosed in the dentinal tubule
(Tome’s fiber).
Life cycle of odontoblasts (1-4: differentiation stage,
5-7 formative stage ,after 7: Quiescent stage )
3- Quiescent stage :
- odontoblasts decrease in size and function.
- dentine formation is reduced .
- odontoblasts produce now secondary and
reparative dentin.
Physical and chemical properties
- The color of dentin varies from light yellow in
deciduous teeth to pale yellow in permanent
dentition, becoming darker with age.
-The lower content of mineral salts in dentin
renders it more radiolucent than enamel.
-It is less hard than enamel but more hard than
bone or cementum . Dentin hardness is more in the
central part than near the pulp or at its periphery.
Dentin is harder in permanent teeth than in
deciduous teeth.
Cont.
-Dentin is mildly elastic and subject to slight
deformation, so it supports the brittle enamel
under masticatory forces .
-It is semitransparent and highly permeable due
to the presence of dentinal tubules. Permeability
decreases with advancing age.
-dentin is a bone – like matrix that is porous and
yellow –hued (colored) material.it’s made up of
70% inorganic materials ( mainly hydroxyapatite and
some non-crystalline amorphous calcium phosphate
).
Cont.
20% organic materials (90% of which is collagen
type 1 and the remaining 10% ground substance
,which includes dentin-specific proteins ),and 10%
water (which is absorbed on the surface of the
minerals or between the crystals ).
Dentin structural units :
-dentinal tubules (D.T.)
* D.T.are fine canals that contain tissue fluid
and odontoblastic process (cytoplasmic process of
odontoblasts).
*the course of D.T.is sigmoid (S) shaped curved
structure knowm as primary curvature.starting at
right angles from pulpal surface, the first convexity
of this doubly curved course directed toward the
apex of the root ending perpendicular to D.E.J.,this
configuration indicate the course taken by
odontoblasts during dentinogenesis.
Cont.
secondary curvature can be seen over the entire
length of D.T.,they probably reflect the minor changes
in the direction of movement of odontoblasts.
*in the root ,and in the area of incisal edge or
cusps, the tubules almost straight.
*The diameter of the dentinal tubules is larger at
the pulpal side (1.5 to 3 microns) than at the dentinoenamel junction (one-micron).
*the tubules are widely separated at the outer
surface of the dentin while, they are crowded
together near the pulpal cavity. The root dentin has
lesser tubules compared to the crown.
Dentinal tubules course
p/pulp
SDT/sigmoid dentinal tubule
Odontoblast and it’s process in the dentinal tubule
Cont.
D.T. make dentin permeable and providing a pathway
for invasion of bacteria caused dental caries and also
play an important role in dentin sensitivity .
*The dentinal tubules show more branching at
their terminal parts. The lateral branches are called
canaliculi or microtubules. These microtubules
originate at right angles to the main tubules every
one to two microns along it’s length.
*Afew dentinal tubules extend through DEJ into
the enamel for several millimeters, these are called
enamel spindles.
Terminal branching (TB) of dentinal tubules (DT) near
cementodentinal junction (CDJ)
2- peritubular dentin :
*it’s the dentin that surrounds D.T. and form one
micron thick sheath around each tubule, also called
intratubular dentin .
*it has more mineral content and less collagen
fibers than intertubular dentin.
*intratubular dentin is formed after the
intertubular dentin formation occurs.
Increased mineral density in the peritubular
dentin
3- intertubular dentin :
*The main mass of dentin is composed of
intertubular dentin that is present between the
dentinal tubules or peritubular dentin.
*less mineralized than peritubular dentin ,and
it’s consist of network course of collagen fibers
in which apatite crystals deposited on it .
intertubular dentin is in
between dentinal tubules
Interglobular dentin :
* are areas of unmineralized or hypomineralized
dentin which persist within the normally mineralized
dentin.(appear as darker ark like areas in stained
section of dentin ).
*in areas where both primary and secondary
mineralization have occurred with complete
crystalline fusion ,these appear as lighter rounded
areas on a stained section of dentin and are
considered globular dentin.
*in IGD ,only primary mineralization has occurred
within predentin ,and globules of dentin do not fuse
completely.
Cont.
*IGD is slightly less mineralized than globular
dentin.
*IGD is seen in both crown and root dentin,near
DEJ and CEJ.,also in certain dental anomalies such
as dentinogenesis imperfecta.
Hypomineralized or unmineralized interglobular dentin
within normally mineralized dentin
Tome‘s granular layer :
In ground sections , a thin layer of D. adjacent to
cementum almost appears granular and only
found in the root, it’s called Tome’s granular
layer.
It’s thought to represent an interference with
mineralization of the entire surface layer of root
D. prior to the beginning of cementum
formation.
TGL- tome’s granular layer
Incremental lines in D.
1- Imbrication or Von Ebner lines:
appear as fine lines, which in cross section run at
right angles to D.T.,the course of the lines
indicates the growth pattern of the D.,the
distance between the lines corresponds to the
daily rate of opposition ,which in crown varies
from 4-8 microns, and become decreasingly less
as root formation progress.
2- counter lines of Owens :
It’s hypocalcified line ,it distinguished in
longitudinal ground sections as accentuated few
lines. These lines arises due to disturbances in D.
matrix and mineralizing process.
3- neonatal lines :
this line separating between prenatal and
postnatal D., and mostly found in deciduous
and first permanent molar . This line is the result
of incomplete calcification , due to metabolic
disturbances at the time of birth to the abrupt
changes in environment and nutrition.
Types of dentin :
3 types of dentin, primary ,secondary , and
tertiary.
1- primary dentin :
The most prominent dentin in the tooth, lies
between enamel and pulp chamber. The outer
layer closest to enamel is known as mantel dentin
,this layer is unique to the rest of primary D.,
mantel D. is formed by newly differentiated
odontoblasts and forms a layer about 150
micrometers width.
Cont.
Mantel dentin has loosely packed collagen fibrils
and is less mineralized.
Below it lies the circumpulpal dentin, a more
mineralized D., that makes up most of the D.
layer ,and is secreted after the mantel D. by the
odontoblasts.
Circumpulpal D. is formed before the root
formation is completed.
2- secondary dentin :
It’s formed after root formation is complete, normally
after the tooth has erupted and is functional. It grows
much slowly than primary D., but maintain it’s
incremental aspect of growth.
It has similar structure to primary D., although it’s
deposition is not always even around pulp chamber.
The growth of this dentin causes the decrease in the
size of pulp chamber with age,this is clinically known
as pulp recession ,cavity preparation in young
patients may carries a greater risk of pulp exposure,
Cont.
If this happened , pulp can be treated by different
therapies like direct pulp capping.
3- Tertiary dentin (pathologic type )
It’s formed as a reaction to external stimulation like
cavities.it’s of two types :
a- reactionary (where D. is formed from a preexisting
ododtoblasts)
b- reparative ( where newly differentiated
odontoblast -like cells are formed due to death of
the original odontoblasts, from a pulpal progenitor
cell.
Reparative dentin formation in relation to the cavity
preparation and subsequent restoration, is limited to
zone of stimulation
Cont.
it’s formed only by odontoblast directly
affected by a stimulus, thus the architecture and
structure depend on the intensity and duration of
the stimulus, e.g. if the stimulus is a carious lesion,
there is extensive destruction of D. ,and damage to
the pulp, due to the differentiation of bacterial
metabolites and toxins.
thus ,tertiary D.,is deposited rapidly , with a
sparse and irregular tubular pattern and some
cellular inclusions,in this case it is referred to as
“osteodentin “.
The dentinal tubules are filled with organisms (indicated by
arrows) and thus form transport channels for infection to pass
to the pulp.
Cont.
If the stimulus is less active , it’s laid down less
rapidly with more regular tubular pattern and
hardly any cellular inclusions.
Cont.
Stimuli of different nature not only induce
additional formation of reparative D.,but also lead
to changes in the D. itself , calcium salts may be
deposited in or around degenerated odontoblastic
processes and may obliterate the tubules ,
this type of D. called transparent or sclerotic
dentin .
Sclerotic dentin :
it describes dentinal tubules that have become
occluded with calcified material. When this occurs
in several tubules in the same area, the dentin
assumes a glassy appearance and becomes
translucent . The amount of sclerotic dentin
increases with age and is most common in the
apical third of the root and in the crown midway
between the dentinoenamel junction and the
surface of the pulp .
sclerosed dentin (SCD). Dentinal tubules are absent
Age and Functional Changes
With advancing age and for functional
requirements
number of changes are seen in dentin. Important
among them are the following:
1. Formation of secondary dentin
2. Sclerotic dentin
3. Formation of reparative dentin (Tertiary dentin).
4. Dead tracts:
a group of dark lines that follow the course of
dentinal tubules .
Cont.
They are groups of dead degenerated
odontoblastic processes in the dentinal tubules .
These tubules are empty and are filled with air
and therefore appear dark in transmitted light.
Dead tracts are probably the initial step in
the formation of sclerotic dentin. These areas
demonstrate decreased sensitivity and appear to
a greater extent in older teeth due to the aging
process . Reparative D. seals these dead tracts at
their pulpal end.
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