Topic-6-Dentin-Pulp-Complex_202.pdf.pdff

Dentin is a bonelike matrix characterized by multiple closely packed dentinal tubules that traverse its
entire thickness and contain the cytoplasmic extensions of odontoblasts that once formed the dentin and
then maintain it
The cell bodies of the odontoblasts are aligned along the inner aspect of the dentin, against a layer of
predentin, where they also form the peripheral boundary of the dental pulp.
BASIC STRUCTURE OF DENTIN

The dental pulp is the soft connective tissue that occupies the central portion of the tooth. The space it
occupies is the pulp cavity, which is divided into a coronal portion (or pulp chamber) and a radicular
portion (the root canal).
The pulp chamber conforms to the general shape of the anatomic crown. Under the cusps the chamber
extends into pulp horns, which are especially prominent under the buccal cusp of premolar teeth and the
mesiobuccal cusp of molar teeth.

The root canal (or root canal system, as it is called in multirooted teeth) terminates at the apical foramen,
where the pulp and periodontal ligament meet and the main nerves and vessels enter and leave the tooth.
Connections between the pulp and the periodontal tissues also may occur along the lateral surface of the
root through the lateral canals.

COMPOSITION,
FORMATION, AND
STRUCTURE OF
DENTIN

Dentin is first deposited as a layer of unmineralized matrix called predentin that varies in thickness (10 to
50 mm) and lines its innermost (pulpal) portion.
Predentin consists principally of collagen and is similar to osteoid in bone; it is easy to identify in
histologic sections because it stains less intensely than mineralized dentin
COMPOSITION, FORMATION, AND STRUCTURE OF DENTIN

The thickness of predentin remains constant because the amount that calcifies is
balanced by the addition of new unmineralized matrix. Predentin is thickest at
times when active dentinogenesis is occurring and diminishes in thickness with
age
Mature dentin is made up of approximately 70% inorganic material, 20% organic
material and 10% of water. The inorganic component of dentin consists of
substituted hydroxyapatite in the form of small plates. The organic phase is about
90% collagen (mainly type I with small amounts of types III and V) with
fractional inclusions of various noncollagenous matrix proteins and lipids

Collagen type I acts as a scaffold that accommodates a large proportion
(estimated at 56%) of the mineral in the holes and pores of fibrils. The
noncollagenous matrix proteins regulate mineral deposition and can act as
inhibitors, promoters, and/or stabilizers; their distribution is suggestive of their
role.
Physically, dentin has an elastic quality that is important for the proper
functioning of the tooth because the elasticity provides flexibility and prevents
fracture of the overlying brittle enamel.

Most of the tooth is formed by primary dentin, which outlines the pulp
chamber and is referred to as circumpulpal dentin.
The outer layer, near enamel or cementum, differs from the rest of the
primary dentin in the way it is mineralized and in the structural
interrelation between the collagenous and noncollagenous matrix
components.
This outer layer is called mantle dentin; the term, however, generally is
used to refer to the outer layer in coronal dentin.
PRIMARY DENTIN

Secondary dentin develops after root formation has been completed and
represents the continuing, but much slower, deposition of dentin by odontoblasts.
Secondary dentin has a tubular structure that, though less regular, is for the most
part continuous with that of the primary dentin. The ratio of mineral to organic
material is the same as for primary dentin.
Secondary dentin is not deposited evenly around the periphery of the pulp
chamber, especially in the molar teeth. The greater deposition of secondary dentin
on the roof and floor of the chamber leads to an asymmetrical reduction in its size
and shape.
SECONDARY DENTIN

Tertiary dentin (also referred to as reactive or reparative dentin) is
produced in reaction to various stimuli, such as attrition, caries, or a
restorative dental procedure.
Unlike primary or secondary dentin that forms along the entire
pulp-dentin border, tertiary dentin is produced only by those cells directly
affected by the stimulus.
The quality (or architecture) and the quantity of tertiary dentin produced
are related to the cellular response initiated, which depends on the
intensity and duration of the stimulus.
TERTIARY DENTIN

Tertiary dentin may have tubules continuous with those of secondary
dentin, tubules sparse in number and irregularly arranged, or no tubules at
all.
The cells forming tertiary dentin line its surface or become included in
the dentin; the latter case is referred to as osteodentin.
Tertiary dentin is subclassified as reactionary or reparative dentin, the
former deposited by preexisting odontoblasts and the latter by newly
differentiated odontoblast-like cells.
TERTIARY DENTIN

Dentin formation begins at the bell stage of tooth development in the papillary tissue adjacent to the
concave tip of folded inner enamel epithelium , the site where cuspal development begins.
From that point, dentin formation spreads down the cusp slope as far as the cervical loop of the enamel
organ, and the dentin thickens until all the coronal dentin is formed.
PATTERN OF DENTIN FORMATION

Root dentin forms at a slightly later stage of development and requires the proliferation of epithelial cells
(Hertwig’s epithelial root sheath) from the cervical loop of the enamel organ around the growing pulp to
initiate the differentiation of root odontoblasts.
Completion of root dentin formation does not occur in the deciduous tooth until about 18 months after it
erupts and in the permanent tooth until 2 to 3 years after it erupts.
PATTERN OF DENTIN FORMATION

Dentin is formed by cells called odontoblasts that differentiate from ectomesenchymal cells
of the dental papilla following an organizing influence that emanates from the inner enamel
epithelium.
Thus the dental papilla is the formative organ of dentin and eventually becomes the pulp of
the tooth, a change in terminology generally associated with the moment dentin formation
begins.
DENTINOGENESIS

The differentiation of odontoblasts from the dental papilla in
normal development is brought about by the expression of
signaling molecules and growth factors in the cells of the
inner enamel epithelium.
The dental papilla cells are small and undifferentiated, and
they exhibit a central nucleus and few organelles. At this
time they are separated from the inner enamel epithelium by
an acellular zone that contains some fine collagen fibrils.
ODONTOBLAST DIFFERENTIATION

The ectomesenchymal cells adjoining the acellular zone rapidly enlarge
and elongate to become preodontoblasts first and then odontoblasts as
their cytoplasm increases in volume to contain increasing amounts of
protein-synthesizing organelles.
The acellular zone between the dental papilla and the inner enamel
epithelium gradually is eliminated as the odontoblasts differentiate and
increase in size and occupy this zone.
ODONTOBLAST DIFFERENTIATION

After the differentiation of odontoblasts, the next step in the production of
dentin is formation of its organic matrix. The first sign of dentin
formation is the appearance of distinct, large-diameter collagen fibrils
(0.1 to 0.2 mm in diameter) called von Korff ’s fibers.
These fibers consist of collagen type III associated, at least initially, with
fibronectin.
These fibers originate deep among the odontoblasts, extend toward the
inner enamel epithelium, and fan out in the structureless ground
substance immediately below the epithelium.
FORMATION OF MANTLE DENTIN

Topic-6-Dentin-Pulp-Complex_202.pdf.pdff

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.
Coincident with this deposition of collagen, the plasma
membrane of odontoblasts adjacent to the differentiating
ameloblasts extends stubby processes into the forming
extracellular matrix.

As the odontoblast forms these processes, it also buds off a number of small, membrane-bound vesicles
known as matrix vesicles, which come to lie superficially near the basal lamina.
The odontoblast then develops a cell process, the odontoblast process or Tomes’ fiber, which is left
behind in the forming dentin matrix as the odontoblast moves away toward the pulp.

The mineral phase first appears within the matrix vesicles as single
crystals believed to be seeded by phospholipids present in the vesicle
membrane
These crystals grow rapidly and rupture from the confines of the vesicle
to spread as a cluster of crystallites that fuse with adjacent clusters to
form a continuous layer of mineralized matrix. The deposition of mineral
lags behind the formation of the organic matrix so that a layer of organic
matrix, called predentin, always is found between the odontoblasts and
the mineralization front

Throughout dentinogenesis, mineralization is achieved by
continuous deposition of mineral, initially in the matrix
vesicle and then at the mineralization front.
In the case of dentinogenesis, some dispute exists because
the junctions holding the odontoblasts together in a palisade
arrangement are incomplete and thus leaky. Conceptually,
simple percolation of tissue fluid supersaturated with calcium
and phosphate ions could take place.
CONTROL OF MINERALIZATION

However, calcium channels of the L type have been demonstrated in the
basal plasma membrane of the odontoblast; significantly, when these are
blocked, mineralization of the dentin is affected.
The presence of alkaline phosphatase activity and calcium
adenosinetriphosphatase activity at the distal end of the cell also is
consistent with a cellular implication in the transport and release of
mineral ions into the forming dentin layer.

Histologically, two patterns of dentin mineralization can be observed—globular and linear calcification.
Globular (or calcospheric) calcification involves the deposition of crystals in several discrete areas of
matrix by heterogeneous capture in collagen. With continued crystal growth, globular masses are formed
that continue to enlarge and eventually fuse to form a single calcified mass. This pattern of mineralization
is best seen in the mantle dentin region, where matrix vesicles give rise to mineralization foci that grow
and coalesce.
CONTROL OF MINERALIZATION

● In circumpulpal dentin the mineralization front can
progress in a globular or linear pattern. The size of the
globules seems to depend on the rate of dentin
deposition, with the largest globules occurring where
dentin deposition is fastest. When the rate of formation
progresses slowly, the mineralization front appears more
uniform and the process is said to be linear.

The epithelial cells of Hertwig’s root sheath initiate the differentiation of
odontoblasts that form root dentin .
The outermost layer of root dentin, the equivalent of mantle dentin in the
crown, shows differences in collagen fiber orientation and organization,
in part because the collagen fibers from cementum blend with those of
dentin
Some reports also indicate that the phosphoprotein content of root dentin
differs, that it forms at a slower speed, and that its degree of
mineralization differs from that of coronal dentin.
FORMATION OF ROOT DENTIN

SECONDARY AND
TERTIARY
DENTINOGENESIS

Secondary dentin is deposited after root formation is
completed, is formed by the same odontoblasts that formed
primary dentin, and is laid down as a continuation of the
primary dentin.
Secondary dentin can be distinguished histologically from
primary dentin by a subtle demarcation line, a slight
differential in staining, and a less regular organization of
dentinal tubules.
SECONDARY AND TERTIARY
DENTINOGENESIS

Tertiary dentin is deposited at specific sites in response to
injury by damaged odontoblasts or replacement cells from
pulp. The rate of deposition depends on the degree of injury;
the more severe the injury, the more rapid the rate of dentin
deposition.
As a result of this rapid deposition, cells often become
trapped in the newly formed matrix, and the tubular pattern
becomes grossly distorted

When the dentin is viewed microscopically, several structural features can be identified: dentinal tubules,
peritubular and intertubular dentin, areas of deficient calcification (called interglobular dentin),
incremental growth lines, and an area seen solely in the root portion of the tooth known as the granular
layer of Tomes.
HISTOLOGY OF DENTIN

Odontoblast processes, similar to osteocyte processes, run in canaliculi
that traverse the dentin layer and are referred to as dentinal tubules.
Dentinal tubules extend through the entire thickness of the dentin from
the dentinoenamel junction to the mineralization front and form a
network for the diffusion of nutrients throughout dentin.
The tubules follow an S-shaped path from the outer surface of the dentin
to the perimeter of the pulp in coronal dentin. This S-shaped curvature is
least pronounced beneath the incisal edges and cusps.
DENTINAL TUBULES

These curvatures result from the crowding of and path
followed by odontoblasts as they move toward the center of
the pulp.
In root dentin, little or no crowding results from decrease in
surface area, and tubules run a straight course.
The dentinal tubules are tapered structures being larger near
the pulp and thinnest at the dentinoenamel junction.

A significant reduction in the average density of tubules also occurs in
radicular dentin compared with cervical dentin
Dentinal tubules branch to the extent that dentin is permeated by a
profuse anastomosing canalicular system
Major branches occur more frequently in root dentin than in coronal
dentin.
The tubular nature of dentin bestows an unusual degree of permeability
on this hard tissue that can enhance a carious process and accentuate the
response of the pulp to dental restorative procedures.
DENTINAL TUBULES

● Tubules in carious lesions may fill with bacteria
and appear darkly stained in histologic sections.
The processes in these tubules may disintegrate
or retract leaving behind an empty tubule,
referred to as a dead tract. Reparative dentin
seals off such dead tracts at their pulpal
extremity, thereby protecting the pulp from
infection.

Tubules are delimited by a collar of more highly calcified matrix called peritubular dentin which starts at
the mineralization front.
The mechanism by which peritubular dentin forms and its precise composition are still not known;
peritubular dentin has been shown to be hypermineralized compared to intertubular dentin.
PERITUBULAR DENTIN

Sclerotic dentin 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.
The occlusion of dentinal tubules with mineral begins in root dentin of
18-year-old premolars without any identifiable external influence; hence
the assumptions that sclerotic dentin is a physiologic response and that
occlusion is achieved by continued deposition of peritubular dentin
SCLEROTIC DENTIN

Dentin located between the dentinal tubules is called intertubular dentin.
Intertubular dentin represents the primary formative product of the odontoblasts and consists of a tightly
interwoven network of type I collagen fibrils (50 to 200 nm in diameter) in which apatite crystals are
deposited. The fibrils are arranged randomly in a plane at roughly right angles to the dentinal tubules.
The ground substance consists of noncollagenous proteins proper to calcified tissues and some plasma
proteins.
INTERTUBULAR DENTIN

Interglobular dentin is the term used to describe areas of unmineralized or hypomineralized dentin where
globular zones of mineralization (calcospherites) have failed to fuse into a homogeneous mass within
mature dentin
Interglobular dentin is seen most frequently in the circumpulpal dentin just below the mantle dentin,
where the pattern of mineralization is largely globular.
INTERGLOBULAR DENTIN

The organic matrix of primary dentin is deposited incrementally at a daily
rate of approximately 4 mm; at the boundary between each daily
increment, minute changes in collagen fiber orientation can be
demonstrated by means of special staining techniques.
Superimposed on this daily increment is a 5-day cycle in which the
changes in collagen fiber orientation are more exaggerated.
These incremental lines run at right angles to the dentinal tubules and
generally mark the normal rhythmic, linear pattern of dentin deposition in
an inward and rootward direction.
INCREMENTAL GROWTH LINES

The 5-day increment can be seen readily in conventional and ground
sections as the incremental lines of von Ebner (situated about 20 mm
apart).
Close examination of globular mineralization shows that the rate in
organic matrix is approximately 2 mm every 12 hours. Thus the organic
matrix of dentin is deposited rhythmically at a daily rate of about 4 mm a
day and is mineralized in a 12-hour cycle.

When root dentin is viewed under transmitted light in ground sections (and only in ground sections), a
granularappearing area, the granular layer of Tomes, can be seen just below the surface of the dentin
where the root is covered by cementum .
More recent interpretation relates this layer to a special arrangement of collagen and noncollagenous
matrix proteins at the interface between dentin and cementum,
GRANULAR LAYER OF TOMES

The dental pulp is the soft connective tissue that supports the dentin. When its histologic appearance is
examined, four distinct zones can be distinguished:
(1) the odontoblastic zone at the pulp periphery;
(2) a cell-free zone of Weil beneath the odontoblasts, which is prominent in the coronal pulp;
(3) a cell-rich zone, where cell density is high, which again is seen easily in coronal pulp adjacent to the
cell-free zone; and
(4) the pulp core, which is characterized by the major vessels and nerves of the pulp
The principal cells of the pulp are the odontoblasts, fibroblasts, undifferentiated ectomesenchymal cells,
macrophages, and other immunocompetent cells.
PULP

Odontoblasts form a layer lining the periphery of the pulp and have a process extending into the dentin.
In the crown of the mature tooth, odontoblasts often appear to be arranged in a palisade pattern some
three to five cells deep. This appearance is an artifact caused by crowding of the odontoblasts as they
migrate centripetally and also by a tangential plane of section.
The number of odontoblasts corresponds to the number of dentinal tubules and, as mentioned previously,
varies with tooth type and location within the pulp space.
In the crown of the fully developed tooth, the cell bodies of odontoblasts are columnar and measure
approximately 50 mm in height, whereas in the midportion of the pulp they are more cuboid and in the
apical part more flattened.
ODONTOBLASTS

The morphology of odontoblasts reflects their functional activity and ranges from an active synthetic
phase to a quiescent phase.
The organelles of an active odontoblast are prominent, consisting of numerous vesicles, much
endoplasmic reticulum, a well developed Golgi complex located on the dentinal side of the nucleus, and
numerous mitochondria scattered throughout the cell body.
Decreasing amounts of intracellular organelles reflect decreased functional activity of the odontoblast.
Thus the transitional odontoblast is a narrower cell, with its nucleus displaced from the basal extremity
and exhibiting condensed chromatin.
Resting, or aged, odontoblasts are smaller cells crowded together. The nucleus of such a cell is situated
more apically, creating a prominent infranuclear region in which fewer cytoplasmic organelles are
clustered.
ODONTOBLASTS

The life span of the odontoblasts generally is believed to equal that of the viable tooth because the
odontoblasts are end cells, which means that, when differentiated, they cannot undergo further cell
division.
This fact poses an interesting problem. On occasion, when the pulp tissue is exposed, repair can take
place by the formation of new dentin. This means that new odontoblasts must have differentiated and
migrated to the exposure site from pulp tissue, most likely from the cell-rich subodontoblast zone.
The dentinal tubule and its contents bestow on dentin its vitality and ability to respond to various stimuli.
The tubular compartment therefore assumes significance in any analysis of dentinal response to clinical
procedures, such as cavity preparation or the bonding of materials to dentin.
Dentin is tubular, that each tubule is (or was once) occupied by an odontoblast process, that the tubule is
delimited by a layer of peritubular dentin, and that fluid circulates between dentin and the process.
ODONTOBLASTS

The cells occurring in greatest numbers in the pulp are fibroblasts.
Fibroblasts are particularly numerous in the coronal portion of the pulp, where they form the cell-rich
zone. The function of fibroblasts is to form and maintain the pulp matrix, which consists of collagen and
ground substance.
The histologic appearance of these fibroblasts reflects their functional state. In young pulps the
fibroblasts are actively synthesizing matrix and therefore have a plump cytoplasm and extensive amounts
of all the usual organelles associated with synthesis and secretion. With age the need for synthesis
diminishes and the fibroblasts appear as flattened spindle-shaped cells with dense nuclei.
FIBROBLASTS

UNDIFFERENTIATED
ECTOMESENCHYMAL
CELLS

Undifferentiated mesenchymal cells represent the pool from which connective tissue cells of the pulp are
derived.
Depending on the stimulus, these cells may give rise to odontoblasts and fibroblasts.
These cells are found throughout the cellrich area and the pulp core and often are related to blood vessels.
Under the light microscope, undifferentiated mesenchymal cells appear as large polyhedral cells
possessing a large, lightly stained, centrally placed nucleus.
In older pulps the number of undifferentiated mesenchymal cells diminishes, along with the number of
other cells in the pulp core. This reduction, along with other aging factors, reduces the regenerative
potential of the pulp.
UNDIFFERENTIATED
ECTOMESENCHYMAL CELLS

In normal pulps, T lymphocytes are found, but B lymphocytes are scarce. There are also some leukocytes
(neutrophils and eosinophils) which increase substantially during infection.
Bone marrow–derived, antigen-presenting dendritic cells are found in and around the odontoblast layer in
nonerupted teeth and in erupted teeth beneath the odontoblast layer.
They have a close relationship to vascular and neural elements, and their function is similar to that of the
Langerhans’ cells found in epithelium in that they capture and present foreign antigen to the T cells.
These cells participate in immunosurveillance and increase in number in carious teeth, where they
infiltrate the odontoblast layer and can project their processes into the tubules.
INFLAMMATORY CELLS

The extracellular compartment of the pulp, or matrix, consists of collagen fibers and ground substance.
The fibers are principally type I and type III collagen.
In young pulps, single fibrils of collagen are found scattered between the pulp cells. Whereas the overall
collagen content of the pulp increases with age, the ratio between types I and III remains stable, and the
increased amount of extracellular collagen organizes into fiber bundles.
The greatest concentration of collagen generally occurs in the most apical portion of the pulp. This fact is
of practical significance when a pulpectomy is performed during the course of endodontic treatment.
The ground substance of these tissues resembles that of any other loose connective tissue. Composed
principally of glycosaminoglycans, glycoproteins, and water, the ground substance supports the cells and
acts as the medium for transport of nutrients from the vasculature to the cells and of metabolites from the
cells to the vasculature.
MATRIX AND GROUND SUBSTANCE

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