Dentin is hard mineralized connective tissue. Pulp is a soft tissue of
mesenchymal origin with specialized cells (odontoblasts) arranged peripherally in
direct contact with dentin matrix.
Although dentin and pulp have different structures and compositions once formed
they react to stimuli as a functional unit. Exposure of dentin through attrition,
trauma or caries, procedures profound pulpal reaction that tends to reduce dentin
permeability and stimulate formation of additional dentin. These reactions are
brought about by changes in fibroblasts, nerves, blood vessels, odontoblasts,
leukocytes and the immune system. This close anatomical and functional
relationship between pulp and dentin is referred to as Pulp Dentin complex.
There is great deal of evidence that dentin and pulp are functionally coupled and
hence integrated as a tissue.
Dentin is the hard, elastic, yellowish white, avascular mineralized connective
tissue portion of the pulp dentin complex which surrounds and encloses pulp. It
forms the bulk and general form of the tooth. It supports the enamel and
compensates for its brittleness. Dentin is bone like matrix characterized by the
multiple closely packed dentinal tubules that transverse its entire thickness and
contain the cytoplasmic extensions of odontoblasts that once formed dentin and
then maintain it. The cell bodies of the odontoblasts are aligned along the
peripheral boundary of dental pulp, against the Predentin.
Dentin is light yellowish in colour and darkens with age. It is viscoelastic and is
harder than bone but softer than enamel. It is harder in the central part than near
Composition of Dentin:-
- It is the first dentin deposited.
- It is a layer of unmineralised organic matrix, about 10-15 micro meter
- It lines the inner most (pulp) portion the dentin; situated between the
odontoblast layer and the mineralized dentin.
- It consists of collagen and non collagenous components. It gradually
mineralizes into dentin as various non-collagenous matrix proteins
- Its thickness remains constant by addition of new mineralized matrix
- It is thickest during dentinogenesis and diminishes with age.
Inorganic Material - 70% by weight or 45% by volume
Organic Material - 20% by weight or 33% by volume
Water - 10% by weight or 22% by volume
minerals and interstices
-Inorganic component consists of substituted hydroxyapatite in form of plates.
Each hydroxyapatite crystal is composed of several thousands of unit cells with a
3Ca3 (PO) 4. Ca (OH) 2
The inorganic component also consists of fluorine, magnesium, zinc,
metalphosphates and sulphates.
-Organic substitute consists of 30% collagenous fibrils (mainly Type I with small
amounts of Type II and III) and a ground substance of mucopolysaccharides
(proteoglycans and glycosaminoglycans).
-Small amounts of phosphates, carbonates and sulfates are also present.
-Miscellaneous components- acidic protein, growth related factors, lipids, serum
-Organic and inorganic substances can be separated by either decalcification or
Dentin is formed by cells called odontoblasts, which differentiate from
ectomesenchymal cells of dental papilla. Thus the dental papilla is the formative
organ of dentin and eventually becomes the pulp of the tooth. Dentinogenesis is
a 2 stage or phase sequence in which the collagen matrix is formed first and then
calcified. Von Korff’s fibers have been described that the initial deposition begins
at the cusp tips after odontoblast differentiation.
As odontoblasts differentiate, they change from an ovoid to a columnar shape
and their nuclei become basally oriented at early stage of development. One or
several processes arise from the apical end of cells in contact with basal lamina.
Length of the odontoblast then increases to 40 m and remains constant.
Proline appears in rough surface endoplasmic reticulum and golgi apparatus.
This proline migrates into cell process in dense granules and is emptied into the
extracellular collagenous matrix of predentin.
As cell recedes, it leaves behind a single extension and several initial processes
join into one, which becomes enclosed in a tubule.
As matrix formation continues, the odontoblast process lengthens, as does
dentinal tubules. The odontoblasts secrete both the collagen and other
components of the extracellular matrix.
Initially daily increments of approximately 4 micro meter of dentin are formed.
As each increment of predentin is formed along the pulp border, it remains a day
before it is calcified and the next increment of predentin forms. All the predentin
is formed in the apical end of the cell and along the forming tubule wall. This
continues until crown is formed and teeth erupt and move into occlusion. After
this time dentin production slows to about 1 micro met /day.
After root development is complete, dentin formation may increase further.
The earliest crystal deposition is in the form of very fine plates of hydroxyapatite
on the surface of the collagen fibrils and in the ground substance.
Subsequently, crystals are laid down within the fibrils themselves.
The crystals associated with the collagen fibrils are arranged in an orderly
fashion, with their long axis paralleling the fibrils long axes and in rows
conforming to the 64 mm striation pattern.
Within the globular islands of mineralization, crystal deposition appears to take
place radially from common centers in a so called spherulite form. These are
seen as the first sites of calcification of dentin.
General calcification process is gradual, but the peritubular region becomes
highly mineralized at a very early stage.
Although there is some crystal growth as dentin matures, the ultimate crystal size
remains very small, about 3 nm in thickness and 100 nm in length.
The apatite crystals resemble those found in bone and cementum but they are
300 times smaller than those found in enamel.
Calcospherite mineralization is seen occasionally along the pulp predentin
HISTOLOGY OF DENTIN:-
When viewed microscopically following structural features can be identified
- Dentinal Tubules
- Peritubular Dentin
- Intertubular Dentin
- Interglobular Dentin
- Incremental growth lines
- Granular layer of Tomes
The dentinal matrix of collagen fibers are arranged in a random network. As
dentin calcifies, the hydroxyapatite crystals mask the individual collagen fibers.
Collagen fibers are visible only under electron microscope and have a diameter
A characteristic of human dentin is the presence of tubules that occupy from 20
to 30 % of the volume of intact dentin. These tubules house the major cell
processes of odontoblasts. Tubules extend through the entire width of the Dentin
from Dentinoenamel junction or Cemento dentinal junction to pulp and form a
network for diffusion of nutrients throughout dentin. Their configuration indicates
the course taken by the Odontoblasts during dentinogenesis. They have a gentle
S shape curve in the coronal dentin, as they extend from Dentinoenamel junction
to Pulp. This S – shaped curvature least pronounced beneath the incisal edges
and cusps, where they may run almost straight course. This curvature is
presumed as result of crowding of odontoblasts as they migrate towards the
center of the pulp. As they approach the pulp, tubules converge because the
surface of the pulp chamber has a much smaller area than surface of dentin
along Dentinoenamel junction. These tubules end perpendicular to
Dentinoenamel junction and
Cementodentinal junction. Along the entire lengths, they exhibit minute
secondary curvatures that are sinusoidal in shape. Tubules are longer than the
entire thickness (3 to 10 mm) of dentin due to their curve through dentin. The
ratio between outer and inner dentin is about 5:1. Accordingly Tubules are further
apart in peripheral layers and are more closely packed near pulp. They are larger
in diameter near the pulpal cavity (3 to 4 micro meters) and smaller at the outer
ends (1 micro meter). The ratio between the number of tubules per unit area on
pulpal and outer surface of dentin is about 4:1. There is more number of tubules
per unit area in the crown than in root.
The dentinal tubules have lateral braches throughout dentin, which are called
Canaliculi / microtubules, which are 1 um or less in diameter. They originate
more or less at right angles to the main tubule every 1-2 um along its length.
Some may enter adjacent or distant tubules while others end in intertubular
dentin. Thus form anastomosing canalicular system. They contain branches of
main odontoblastic process. Researchers have demonstrated they form
pathways for movement of materials between main processes and the more
Major branches occur frequently in root dentin than in coronal dentin. This 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. Few dentinal tubules extend through
dentinoenamel junction into enamel for several mm and are termed enamel
Dentin lining the dentinal tubules is termed peritubular dentin, which is a highly
calcified matrix and forms the walls of tubules in all but dentin near pulp.
It is presumed to be the precursors of dentin matrix that is deposited around each
odontoblast processes are synthesized by the odontoblast transported in
secretory vesicles out into the process, and released by reverse pinocytosis.
With the formation of peritubular dentin, there is a reduction in diameter of the
process near dentinoenamel junction.
It is more mineralized than intertubular dentin. Therefore, harder. Hence this
hardness provides added structural support for intertubular dentin thus
strengthening the tooth. It is twice as thick in outer dentin than inner dentin.
It contains few collagen fibrils and higher proportion of sulphated proteoglycans.
Due to its decreased collage content, dissolves more quickly in acid than
By removal of peritubular dentin, acid etching agents used during dental
restorative procedures enlarge the openings of the dental tubules, thus making
the dentin more permeable.
It is rich in proteins like dentinsialoprotein.
After decalcification the odontoblastic processes appear to be surrounded by
The calcified tubule wall has an inner organic lining termed as Lamina limitans,
which is a thin organic membrane high in glycosaminoglycans (GAG) and similar
to the lining of lacunae in cartilage and bone.
This is located between the rings of peritubular dentin and constitutes bulk of
dentin. Its organic matrix consists mainly of interwoven network of collagen fibrils
having diameter of 50-200 um. Fibrils are arranged randomly at right angles to
Ground substance consists of noncollagenous proteins proper to calcified tissues
and some plasma proteins. Although highly mineralized, it is retained after
decalcification. Hydroxy appetite crystals are formed along the fibers with their
long axis oriented parallel to collagen fibers.
INTERGLOBULAR DENTIN (Or Spaces):-
The term describes areas of unmineralised/ hypomineralised dentin where
globular zones / areas of mineralization (calcospherites) have failed to fuse into a
homogenous mass within mature dentin. (mineralization of dentin begins in small
globular areas which fail to fuse- zones of hypomineralization between globules.
This is prevalent in people who had ( vit D deficiency resistant rickets or
exposure to high levels of fluorides at time of dentin formation,
It is seen in circumpulpal dentin just below mantle dentin, where pattern of
mineralization is largely globular. It follows incremental pattern.
INCREMENTAL GROWTH LINES (Von Ebner / Imbrication Lines):-
These are fine lines? Striations in dentin.
They run at right angles to dentinal tubules and mark normal daily rhythmic linear
pattern of dentin deposition in an inward and rootward direction.
Organic matrix of dentin is deposited incrementally at a daily rate of 4 um and
mineralized in 12 hr cycle.
5 day increment can be seen as Incremental lines of Von Ebner.
Contour Lines Of Owen: Incremental lines are accentuated because of
deficiencies in mineralization process. They can be demonstrated by longitudinal
ground sections- Soft x-ray shows hypocalcified bands.
Neonatal Line:- Is the zone of hypocalcification found in enamel and dentin of
deciduous teeth and permanent molar. They separate post and pre natal dentin.
Reflects abrupt change in environment /physiological trauma of birth.
GRANULAR LAYER OF TOMES:-
This is an optical phenomenon seen when root dentin is dry ground and viewed /
visualized in transmitted light, a granular-appearing zoneis seen below the dentin
surface, where root is covered by cementum. This is known as granular layer.
This zone increases slightly in amount from the cementoenamel junction to root
Number of interpretations proposed about these structures were:-
- They were thought to be associated with minute hypomineralised
areas of interglobular dentin.
- Thought to be true spaces.
- Finally spaces representing sections made through looped terminal
portions of dentinal tubules found only in root dentin and seen only
because of light refraction in thick ground sections.
- Recently interpretation relates this layer to a special arrangement of
collagen and non-collagenous matrix at the interface between
dentin and cementum.
- The cause of development was a result of odontoblasts turning on
themselves during early development of teeth.
This is the free fluid which occupies about 22% of total volume of dentin. It is an
ultrafiltration of blood in pulpal capillaries, and its composition resembles plasma
in many respects. This fluid flows outwards between odontoblasts into the
dentinal tubules and eventually escape through small pores in enamel.
Tissue pressure of pulp is 14 cmH2O ( 10.3mmHg)
Pressure gradient between pulp and oral cavity results in the outward flow of
Exposure of tubules by tooth fracture or during cavity preparation often results in
the outward movement of fluid to the exposed dentin surface in the form of tiny
droplets. Dehydrating surface of dentin with compressed air, dry heat or
application of absorbent paper can accelerate this outward movement of fluid.
Rapid flow of fluid through tubules is thought to be the cause of dentin sensitivity.
Dental caries, restorative procedures or growth of bacteria beneath restorations
result in bacterial products or other contaminants to be found in dentinal fluid.
Dentinal fluid serves as a sump from which injurious agents can percolate into
pulp producing an inflammatory response.
TYPES OF DENTIN:-
1) PRIMARY DENTIN (Developmental Dentin)
Development Dentin is one which forms during tooth development
Mantle dentin is the 1st
formed Dentin in the crown underlying DEJ
• Sub adjacent to Enamel / Cementum
• Not present in root dentin
• Outer most peripheral part of dentin about 20 Micro meter
• Bounded by DEJ & zones of interglobular Dentin
• Consists of thick fan shaped collagen fibers, deposited
immediately subjacent to basal lamina during initial stages of
• fibers run perpendicular to DEJ
• It is less mineralized than the rest of the primary dentin with
organic matrix derived from dental papilla.
Circumpulpal Dentin forms remaining primary Dentin or bulk of tooth
• Represents all Dentin formed before root completion after layer of Mantle
Dentin is deposited and
• Organize matrix is composed of collagen fibers which are smaller in
diameter and are oriented right angle to long axis to ditubules and are
closely packed together & form an interwoven network
• May contain more mineral then mantle dentin.
2) SECODARY DENTIN
• It is a narrow band of Dentin bordering the pulp & representing that Dentin
formed after root completion
• Has a tubular structure which is continuous with the primary dentin is most
• Contains fewer incremental and tubular than Primary Dentin
• Not deposited evenly around the periphery of the pulp chamber (molar
teeth) and greater amounts of deposition on roof & floor of coronal pulp
chamber leads to asymmetric reduction in its size & shape. These
changes in pulp space are clinically referred to as pulpal recession.
Important in determining form of cavity preparation for dental restorations.
Young Patients – risk of involving dental pulp by mechanically exposing pulp
Tubules of Secondary Dentin scleroses more rapidly than those of the Primary
Dentin. Thus reduce overall permeability of dentin thereby protecting the pulp.
3) TERTIARY DENTIN : - ( Reactive / Reparative Dentin)
• This localized formation of Dentin on pulp dentin border , formed in
reaction to trauma such as caries or restorative procedures ( chemical ,
thermal , microbial stimuli ) attrition.
• Forms along entire pulp dentin border by cells directly affected by the
• Quantity depends on the intensity, duration of stimuli
• Cells forming it line its surface or become included into Dentin latter case
It can be sub classified into :
• Refractory Dentin- deposited by pre-existing odontoblasts in response to
mild dentinal stimuli
• Reparative Dentin- deposited by newly differentiated ( secondary)
odontoblast like cells in response to more intense
Extensive abrasion, erosion, caries or operative procedures may lead death of
the odontoblast processes or deposition of reparative dentin, if they survive.
When the odontoblast processes die, they are replaced by migration of cells in
cell rich zone, undifferentiated perivascular cells arising in deeper regions of the
pulp to dentin interface.
Both remaining and newly differentiated odontoblasts begin to deposit reparative
dentin, which seals off the zone of injury as healing is initiated by pulp, resulting
in resolution of inflammation.
Permeability of dentin has been well characterized.
Dentin tubules are the major channels for fluid diffusion across dentin.
Fluid permeation is proportional to the diameter and number of tubules.
Dentin permeability increases as the tubules converge on pulp.
Total tubular surface near dentinoenamel junction is 1% of total surface of dentin
while close to pulp chamber tubular surface is 45 %
Therefore, dentin below deep cavity preparation is more permeable than dentin
underlying a shallow cavity when the formation of sclerotic or reparative dentin is
Study has shown that permeability of radicular dentin lower than coronal dentin
because decrease in density of dentin tubules.
Factors modifying dentin permeability is the presence of odontoblast processes
in the tubules and the sheath like lamina limitans that lines the tubules.
In dental caries, inflammatory reaction develops in pulp long before pulp actually
gets infected. This indicates that bacterial products reach pulp in advance of
Dentinal sclerosis below carious lesion reduces permeation by obstructing the
tubules thus decreasing the concentration of irritants into pulp.
Cutting of dentin during cavity preparation produces microcrystalline grinding
debris that coats the dentin and clogs orifices of dentin tubules. This layer of
debris is called smear layer (small particle size, therefore is capable of
preventing bacteria from penetrating dentin).
Removal of grinding debris by acid etching greatly increases permeability of
dentin by using surface resistance and widening orifices of the tubules.
Consequently, incidence of pulpal inflammation may be increased significantly if
cavities are treated with an acid cleanser, unless a cavity liner, base or dentin
bonding agent is used.
Cells of the Pulp:-
3) Undifferentiated ectomesenchymal cells
5) Dentritic cells
Odontoblast is the most distinctive and easily recognized cells of dental pulp.
Because it is responsible for dentinogenesis, both during tooth development and
in mature tooth, they are the characteristic cell of pulp dentin complex.
They form a layer lining the periphery of the pulp and during dentinogenesis the
odontoblasts form the dentinal tubules and their presence in dentin makes dentin
a living tissue.
Odontoblasts, osteoblasts and cementoblats have the general characteristics of
protein secreting cells. The most significant difference between odontoblasts,
osteoblasts and cementoblasts are their morphologic characteristics and the
anatomic relationship between the cells and the structures they produce.
Odontoblasts in the crown of fully developed tooth are columnar and measure
approximately 50 um in height in midpoint of pulp they are cuboid and apical part
Ultrastructural features of odontoblasts:-
The cell body of the active odontoblast has a large nucleus that may contain up
to four nucleoli and the nucleus is situated at basal end of cell and is within a
nuclear envelope. Golgi apparatus is located centrally in the supranuclear
cytoplasm and it consists of an assembly of smooth walled vesicles and
Numerous mitochondria are evenly distributed.
Rough endoplasmic reticulum is prominent, consisting of closely stacked
cisternae (dispersed diffusely within the cytoplasm)
Ribosomes mark the site of protein synthesis.
The odontoblast synthesis mainly Type I, collagen (small amounts of type V
collagen have been seen)
They secrete proteoglycans and collagen. They also secrete dentin sialoprotein
and phosphophoryn, a highly phosphorylated phosphoprotein. (This is unique to
The odontoblast also secretes alkaline phosphotase.
They resting or inactive odontoblast, has decreased number of organelles and
may become progressively shorter.
A dentinal tubule forms around each of major process of odontoblast. The
odontoblast process occupies most of the space within the tubule.
Microtubule and microfilaments are principal ultrastructural components of the
Microtubules extend from the cell body out into the process. These straight
structures are parallel to the long axis of the cell and input the impression of
The plasma membrane of the odontoblast process closely approximates the wall
of dentinal tubules. Localised constrictions in the process occasionally produce
relatively large spaces between tubule wall and process. These spaces may
contain collagen fibrils and ground substance.
The extent to which the process extends outwards in the dentin has been a
matter of considerable controversy. But this knowledge is important in restoring a
tooth, cavity or crown preparation. It has long been thought that the process is
present throughout the full thickness of dentin. However, ultrastructural studies
using electron microscopy, describe the process being limited to inner third of
dentin. This could possibly be the result of shrinkage during fixation and
dehydration during histologic processing.
In an attempt to resolve this issue, monoclonal antibodies were directed against
microtubules to demonstrate tubulin in microtubules of process. Immunoreactivity
was seen throughout the tubule suggesting the process extends throughout the
entire thickness of dentin.
The life span of odontoblasts generally is believed to equal that of viable tooth
because the odontoblasts are end cells, which means, once differentiated, they
cannot undergo further division.
When pulp tissue gets 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
The cells occurring in greatest numbers in the pulp are fibroblasts. They are
particularly in the coronal portion of the pulp, where they form cell rich zone.
The early differentiating fibroblasts are polygonal and well separated and evenly
distributed within ground substance.
Cell to cell contacts are established between the multiple processes that exceeds
out from cells and these contacts take the form of gap junctions, which provide
for electronic coupling of one cell to another.
As the cells mature, the cells become stellate in form and golgi complex
enlarges, RER proliferates, secretory vesicles appear and fibroblasts take
appearance of protein-secreting cells.
With an increase in the number of blood vessels, nerves and fibers, there is a
relative decrease in the number of fibroblasts in pulp.
These cells synthesize type I and III collagen, as well as proteoglycans and
GAGS. Thus they produce and maintain matrix proteins of the extracellular
matrix. Fibroblasts are also responsible for collagen turnover in the pulp.
Macrophages are the monocytes that have left the blood stream, entered the
tissues and differentiated into subpopulations.
Macrophages appear as large oval or spindle shaped cells that under light
microscope exhibit a dark stained nucleus.
A major subportion of macrophages is quite active in endocytosis and
phagocytes. Because of these activities and mobility, they are able to act as
scavengers, remaining dead cells, dead RBCs, foreign bodies from tissues.
Another subset participates in immune reaction by processing antigen and
presenting it to memory T cells. These help in T cell dependent immunity.
UNDIFFERENTIATED ECTOMESENCHYMAL CELLS:-
These cells represent the pool from which connective tissue cells of pulp are
derived. Depending on the stimulus, these cells may give rise to odontoblasts
They are found throughout the cell rich area and pulp core and often are related
to blood vessels.
They appear as large polyhedral cells possessing a large, lightly stained,
centrally placed nucleus.
In older pulps the number of differentiated mesenchymal cells diminishes, along
with number of other cells in pulp core. This reduction reduces the regenerative
potential of the pulp.
Dentritic cells are accessory cells of the immune system. These cells are termed
as “antigen-presenting cells” and are characterized by dentritic cytoplasmic
process and the presence of cell surface class II antigens. Their function is
similar to langerhan’s cells.
They are known to play a central role in the induction of T cell-dependent
Like antigen-presenting macrophages, they engulf protein antigens and present
an assembly of peptide fragments of antigens and class I molecules. The
assembly binds to T-cell receptor and T cell activation occurs.
In normal pulps T-lymphocytes are found but B-lymphocytes are scarce.
The presence of macrophages, dentritic cells and T-lymphocytes indicate that ulp
is well equipped with cells required for the initiation of immune response.
Mast cells are seldom found in the normal pulp tissue, although they are
routinely found in chronically inflamed pulp. The granules of mast cells contain
heparin, an anticoagulant and histamine, an important inflammatory mediator.
MATRIX AND GROUND SUBSTANCE:-
Connective tissue is a system consisting of cells and fibers, both embedded in
the pervading ground substance or extra cellular matrix.
Fibers and cells have recognizable shapes; extra cellular matrix is described as
being amorphous. It is considered as gel rather sol and therefore considered to
differ from tissue fluids.
Because of its content of polyelectric polysaccharides, extra cellular matrix is
responsible for water holding properties.
The matrix consists of collagen fibers and ground substance.
The fibers are principally type I and II collagen. The ratio of these two remains
stable whereas the overall collagen content of pulp increases with age. The
increased amount organizes into fiber bundle. The greatest concentration occurs
in most apical portion. This is of practical significance when a pulpectomy is
preformed. Engaging the pulp with a barbed broach in the region of the apex
affords a better opportunity to remove the tissue intact than does engaging the
broach more coronally, where the pulp is more gelatinous and liable to tear.
The ground substance resembles any other connective tissue. It is principally
composed of GAG, glycoprotein and water.
GAG acts as adhesive molecules that can bond to cell surfaces and other matrix
Fibronectin is a major surface glycoprotein. In pulp the principal proteoglycans
include hyaluronic acid, heparin sulphate and chondroitin sulphate. The
proteoglycan content of pulp tissue decreases approximately 50% with tooth
The long GAG chains of proteoglycan molecules from relatively rigid coils
constituting a network that holds water, this forming gel.
Ground substance also acts as a molecular sieve in that it excludes large
proteins and urea cell metabolites, nutrients and waste pass through the ground
substance between cells and blood vessels.
Degradation of ground substance can occur in certain inflammatory lesions in
which there is a high concentration of lysosomal enzymes.
Connective Tissue Fibers of the pulp:-
Two types of structural proteins seen are:-
Elastins are confined to the walls of arterioles and unlike collagen are not seen in
extra cellular matrix.
Type I and III collagen are seen in the pulp.
Collagen Fibers in the pulp exhibit typical cross striations at 64 nm (640 A ) and
range in length from 10-100nm or more. Bundles of these fibers appear
throughout the pulp. In very young pulp fine fibers ranging in diameter from 10-
12nm (100-120 A ) have been observed. Their significance is unknown. Pulp
collagen fibers do not contribute to dentin matrix production, which is the function
of the odontoblast. After root completion the pulp matures and bundles of
collagen fibers increase in number. They may appear scattered throughout the
coronal or radicular pulp, or they may appear in bundles. These are termed
diffuse or bundle collagen depending on their appearance, and their presence
may relate to environmental trauma. Fiber bundles are most prevalent in the root
canals, especially near the apical region.
MORPHOLOGIC ZONES OF THE PULP:-
1) Odontoblast layer
2) Cell poor zone
3) Cell rich zone
4) Pulp proper
The outer most stratum of cells of the healthy pulp is the odontoblast layer.
This layer is located immediately subjacent to the predentin; the odontoblast
processes, however pass on through the predentin into the dentin.
The tight packing together of these tall, slender cells produces the appearance of
a palssade. The odontoblasts vary in sheight and often produce the appearance
of a layer 3-5 cells in thickness. Between odontoblasts there are small
intercellular spaces ( app. 300-400 A in width)
Between adjacent odontoblasts there are seriesof specialized cells to cell
junctions (i.e. junctional complexes) including desmosomes (i.e. zonula
adherens), gap junctions (i.e. nexuses) and tight junctions(i.e. zonula occludens)
Gap junctions provide low resistance pathways through which electrical
excitation can pass between cells.
Cell Poor Zone (Weil’s zone):-
Immediately subjacent to the odontoblast layer, in the coronal pulp, there
is often a narrow zone approximately 40 mm in width. This is relatively free of
It is traversed by blood capillaries, unmyelinated nerve fibers, and the
cytoplasmis process of fibroblasts.
The zones presence is dependent on functional status of pulp. It may be
apparent in young pulp, where dentin forms rapidly, or in older pulps, where
reparative dentin is being produced.
Cell rich Zone: -
This is a stratum containing a relatively high proportion of fibroblasts,
compared with the more central region of pulp.
It is more prominent in coronal pulp than in radicular pulp.
Besides fibroblasts, the cell rich zone includes number of macrophages, dentritic
cells and lymphocytes.
On the basis of few evidence, it has been suggested that cell rich zone is formed
as a result of peripheral migration of cells in the central region of pulp at the time
of tooth eruption.
Although cell division is rare within this zone, death of odontoblasts causes a
great increase in rate of mitosis.
The pulp proper is the central mass of pulp. It contains the larger blood vessels
and nerves. The connective tissue cells in this zone are fibroblasts or pulpal
The metabolic activity of pulp has been studied by measuring rate oxygen
consumption and the production of carbon dioxide or lactic acid by pulp tissue in
Because of relatively sparse cellular composition of pulp, the rate of oxygen
consumption is low compared to other tissue.
During active dentinogenesis, metabolic activity is much higher than after crown
completion. The greatest metabolic activity is found in the region of odontoblast
The pulp has the ability to produce energy through a phosphogluconase shunt
type of carbohydrate metabolism (in addition to usual glycolytic pathway),
suggesting that the pulp may be able to function under varying degrees of
ischemia. (This explains pulp functioning during vasoconstriction as in local
Several commonly used dental materials ( e.g. eugenol, calcium hydroxide, zinc
oxide and eugenol, silver amalgam have shown to inhibit oxygen consumption by
pulp tissue, indicating the capability of depressing metabolic activity of pulp cells.
Even orthodontic forces interfere the metabolic activity.
The blood vessels enter and exit the dental pulp by the way of the apical and
One or sometimes two vessels of arteriolar size (about 150 um) enter the apical
foramen along with nerve bundles.
Smaller vessels without any nerve bundles, enter the pulp through minor
Vessels leaving the dental pulp are associated closely with arterioles and nerve
bundles entering the apical foramen.
The arterioles occupy a central portion within the pulp and as they pass through
radicular portion of pulp, give off smaller lateral branching that extend and branch
into subodontoblastic area.
The number of branches given off in this manner increases coronally, so as to
form an extensive vascular capillary network.
Occasionally U-looping of pulpal arterioles is seen, and tought to be related to
the regulation of blood flow.
The capillaries in the subodontoblastic area range from 4-8 um in diameter, and
the main portion of capillary bed is located just below the odontoblasts.
During dentinogenesis they extend to about predentin. On the periphery of
capillaries at random intervals, pericytes are present. These cells are thought to
be contractile capable of reducing the size of vessel lumen.
Arteriovenous anastomoses (AVAS) may be present in both the coronal and
radicular portions of pulp, such vessels provide direct communication between
arterioles and venules, thus bypassing the capillary bed.
REGULATION OF BLOOD SUPPLY:-
Several systems are involved in regulation of pulpal blood flow:-
- Sympathetic adrenergic vasoconstriction
- B-adrenergic vasodilation
- Lymphatic cholinergic vasoactive system
- Antidromic vasodilation system involving sensory nerves,
including axon reflex vasodilatation.
The walls of arterioles and venules are associated with smooth muscle that is
innervated by unmyelinated sympathetic fibers. When stimulated, these fibers
transmit impulse causing muscle fibers to contract, thereby decreasing the
diameter of vessel.
Activation of adrenergic receptor by administration of epinephrine containing
local anesthetic solution results in a marked decrease in pulpal blood flow.
A unique feature of pulp is that it is rigidly encased within the dentin. This
process it in a low compliance environment, much like brain, bone marrow, nail
bed. Thus pulp has limited ability to expand. So, vasodilation and increased
vascular permeability (as inflammation) result in increase pulpal hydrostatic
Theoretically, if tissue pressure increases to the point equal to into intravascular
pressure, the venules would be compressed, thereby increasing vascular
resistance and reducing pulpal blood flow. This explains why injection of
vasodilators into an artery leading to pulp results in a reduction rather than
increased blood flow.
The presence of lymph vessels in the dental pulp is questioned. Support for this
system stems from the investigators who use injection of fine particulate
substances into dentin or peripheral pulp, which are subsequently reported
present in some of the thin walled vessels that exit apical foramen.
Lymph capillaries are described as endothelium lined tubes that join thin walled
lymph venules or veins in central pulp. The larger vessels have an irregular
shaped lumen composed of endothelial cells surrounded by an incomplete layer
of pericytes, also there is absence of RBC and presence of lymphocytes.
Lymph vessels draining pulp and periodontal ligament have a common outlet.
Those draining anterior teeth pass to submental lymph nodes, those draining
posterior teeth pass to submandibular and deep cervical lymph nodes.
The dental pulp is innervated richly. Nerves enter the pulp through the apical
foramin, along with afferent blood vessels and together form neurovascular
Regardless of the native of sensory stimulus, whether it is thermal change,
mechanical deformation, injury to tissues, are afferent impulses from the pulp
result in sensation of pain.
The innervation of the pulp includes both afferent neurons, which conduct
sensory impulses, and autonomic fibers, which provide neurogenic modulation of
microcirculation and perhaps regulate dentinogenesis.
Nerve fibers are usually classified according to their diameter, conduction
velocity and function.
Sl.No. Type of
1. A o Motor, proprioception 12-20 70-120
2. AB Pressure, Touch 5-12 30-70
3. A r Motor, to muscle
spindles 3-6 15-30
4. A d Pain, temperature,
Touch 1-5 6-30
5. B Preganglionic
autonomic <3 3-15
6. C dorsal root Pain 0.4-1 0.5-2
7. sympathetic Postganglionic
sympathetic 0.3-1.3 0.7-2.3
In pulp there are two types of sensory nerve fibers:-
1) Myelinated ( A fibers)
2) Unmyelinated ( C fibers)
The might be overlapping between pulpal A and C fibers.
The A fibers include both – A beta and A delta.
The A beta fibers may be slightly more sensitive to stimulation than A delta
The sensory nerves of the pulp arise from trigeminal nerve and pass into
radicular pulp in bundles via the foramen. Each of the nerves entering the pulp is
invested within Schwann cells.
Most of the unmyelinated C fibers entering the pulp are located within these
fibers bundles, the remainder are situated towards the periphery of the pulp.
It is noticed that single pulpal nerve fibers have been reported to innervated
multiple tooth pulp.
The A fibers gradually increase after the eruption of teeth. This relatively late
appearance of A fibers in the pulp may help to explain why electric pulp test
tends to be unreliable in young teeth.
The nerve bundles pass upward through radicular pulp together with blood
vessels. Once they reach coronal pulp, they act beneath cell rich zone, branch
into smaller bundles and ramify into a plexus of single nerve axons- Plexus of
Raschkow. Full development of this plexus doesn’t occur until the final stages of
It is in the plexus that A fibers emerge from their myelin sheath, and while still in
schwann cells, branch repeatedly to form subodontoblastic plexus.
Finally terminal axons exit from Schwann cells and pass between odontoblasts
as free nerve endings.
With the exception of intracellular fibers, dentin is devoid of sensory nerve fibers.
So pain producing agents don’t elicit pain when applied to exposed dentin.
On basis of their location and pattern of branching several types of nerve endings
have been described and it has been found that some simple fibers run from
subodontoblastic nerve plexus toward the odontoblast layer. But these fibers
donot reach predentin, they terminate in extracellular spaces in cell rich zone,
cell poor zone or odontoblast layer.
Some fibers enter the dentinal tubule. Most of the intracellular fibers extend into
the dentinal tubules only for a few mm, but few may penetrate as far as 100
The nerve fibers lie in a groove or gutter along the surface of odontoblast
process, and toward their terminal ends they twist around the process like
corkscrew. The cell membranes of odontoblast process and nerve fiber are
closely approximately and run parallel but are not synaptically linked.
If odontoblasts were acting as a receptor cell, it would synapse with adjacent
nerve fiber. But researches have been unable to find synaptic junctions.
Another study showed that a reduction in pulpal blood flow induced by
stimulation of sympathetic fibers leading to pulp, results in depressed excitability
of pulpal a fibers.
Of considerable clinical interest is the evidence that nerve fibers of the pulp are
relatively resistant to necrosis. This is because nerve bundles, in general, are
more resistant to autolysis than other tissue elements
The electric pulp tester delivers a current sufficient to overcome the resistance of
enamel and dentin and stimulate sensory A fibers at pulp dentin border zone.
Bendel et al found that in anterior teeth the optional placement site of electrode is
incisal edge of anterior teeth, as the response threshold is lowest here and
increases as electrode is moved towards the cervical region or tooth.
Cold tests using carbon dioxide snow or liquid refrigerants and heat tests
employing heated gutta percha or hot water activated hydrodynamic forces within
the dentinal tubules, which in turn excite the intradental A fibers.
It has been shown that cold tests do not injure the pulp. Heat tests have a greater
potential to produce injury.
One of the most unusual features of the pulp dentin complex is its sensitivity. The
extreme sensitivity of this complex is difficult to explain.
Converging evidence indicates that movement of fluid in the dentinal tubules is
the basic event in arousal of pain.
It now appears that pain producing stimuli, such as heat, cold, airblasts and
probing with the tip of an explorer, have in common the ability to displace fluid in
the tubules. This is referred to as hydrodynamic mechanism of dentin sensitivity.
Thus fluid movement in the dentinal tubules is translated into electrical signals by
sensory receptors located within the tubules or subjacent odontoblast layer.
The evoked pain was of short duration (1-2 sec), on brief application of heat or
cold. The thermal diffusivity of dentin is relatively low, yet the response of the
tooth to tooth stimulation is rapid, often less than a second.
Evidence suggests that –
Thermal stimulation of the tooth results in rapid movement of fluid into dentinal
tubules resulting in activation of sensory nerve terminal in underlying pulp.
Heat expands the fluid within the tubules, causing it to flow towards pulp,
whereas cold cause the fluid to contract, producing outward flow. The rapid
movement of fluid deforms the membrane and activates the receptor.
Some channels are activated by voltage, some by chemicals, and some by
The dentinal tubule is a capillary tube having an exceedingly small diameter.
Therefore the effects of capillary are significant, because the narrower the bore
of capillary tube, the greater the effect of capillarity. Thus if fluid is removed from
the outer end of exposed dentinal tubules by dehydrating the dentinal surface
with an air blast or absorbent paper, or dehydrating solutions, can produce pain if
applied to exposed dentin.
Investigators have shown that it is the A fibers rather than C fibers that are
activated by stimuli applied to exposed dentin. If it is for a longer time, then C
fibers get activated.
It has also been shown that pain producing stimuli are more readily transmitted
from dentin surface when the exposed tubule apertures are wide and the fluid
within the tubules is free to flow outward.
The most difficult phenomenon to explain is pain associated with light probing of
dentin. May be these forces mechanically compress the openings of tubules and
cause displacement of fluid to excite sensory receptors in underlying pulp.
Another example of effect of strong hydraulic forces that are created within the
dentinal tubules is the phenomenon of odontoblast displacement.
The hydrodynamic theory can be applied to an understanding of mechanism
responsible for hypersensitive dentin. Although the dentin may at first be very
sensitive, within a few weeks the sensitivity usually subsides as a result of
gradual occlusion of tubules by mineral deposits.
Currently the treatment of hypersensitive teeth is direct towards reducing the
functional diameter of dentinal tubules to limit fluid movement.
Methods employed are-
1) Formation of a smear layer on sensitive dentin by burnishing the exposed
2) Application of agents such as axolane compounds to form insoluble
precipitates within tubules.
3) Impregnation of tubules with plastic resins.
4) Application of dentin bonding agents to seal tubules.
Dentin sensitivity can be modified by laser irradiation.
The presence of neuropeptides in sensory nerves is of current interest.
Pulpal nerve fibers contain neuropeptides such as calcitonin gene related peptide
( CGRP), substance P (SP), neuropeptide Y, neurokinin A, and vasoactive
intestinal polypeptide (VIP).
Release of these peptides can be triggered by such things as tissue injury,
complement activation, antigen antibody reactions, or antidromic stimulation of
inferior alveolar nerve. Once released, they produce vascular changes that are
similar to those evoked by histamine and bradykinin (vasodilation). It is reported
that mechanical stimulation of dentin produces vasodilation within pulp.
PLASTICITY OF INNERVATION NERVE FIBERS:-
It is has become apparent that the innervation of tooth is a dynamic complex in
which number, size and cytochemistry of nerve fibers can change because of
dying, tooth injury and dentinal caries. For example, nerve fibers sprout into
inflamed tissue surrounding sites of pulpal injury and the content of CGRP and
SP increases in these sprouting fibers. When inflammation subsides there is a
decrease in the number of sprouts. Regulation of such change appears to be a
function of nerve growth factors (NGF)
NGF receptors are found on intradental sensory fibers and schwann cells.
Maximal sprouting of CGRP and SP containing nerves fibers corresponding to
areas of pulp where there is increases production of NGF.
Three characteristics of hyperalgesia are:-
1) spontaneous pain
2) decreased pain threshold
3) increased response to a painful stimulus.
It is recognized that hyperalgesia can be produced by sustained inflammation as
in case of sunburned skin.
It has been seen that sensitivity of dentin is often increased when pulp becomes
acutely inflamed. We also know that when a pulp chamber of a painful tooth with
an abscessed pulp is opened, drainage of pus soon reduces level of pain. This
suggests that pressure may contribute to hyperalgesia.
In addition certain mediators of inflammation ( eg. Bradykinin, 5-AT,
proteoglandin E2) are capable of producing hyperalgesia.
Leucotriene B4 (LTB4) was shown to have a long lasting sensitizing effect on
intradental nerves, suggesting it may potentiate no receptor activity during pulpal
It is apparent that pain associated with the stimulation of A fibers doesnot
necessarily signify pulp is inflamed or tissue injury has occurred. The clinician
should carefully examine symptomatic teeth to rule out-
- Cracked or leaking fillings
- Tooth fracture
Pain associated with inflamed or degenerated pulp may be either provoked or
The hyperalgesic pulp may respond to stimuli that usually do not evoke pain, or
pain may be exaggerated and persist longer. On the other hand, the tooth may
ache spontaneously in the absence of external stimuli. No satisfactory
explanations are there for this pulpal pain.
Narhi has done much to elucidate the role of hydrostatic pressure changes in
activation of pulpal nerve fibers. He theorized that pressure changes produced
local deformities in pulp tissue, resulting in a stretching of sensory nerve fibers.