INTRODUCTION:It provides the bulk and general form of the tooth and

is characterized as a hard tissue with tubules throughout its thickness.

Since it begins to form slightly before the enamel, it determines the shape of the crown including the cusps and ridges and the size of the roots.

As a living tissue it contains within its tubules the processes of the odontoblasts. Although the cell bodies are arranged along the pulpal surface of the dentin they are morphologically cells of dentin because they produce dentin.

Physically and chemically the dentin closely resembles bone. The main difference is that some of the osteoblasts exist on the surface of bone and when one of these cells becomes enclosed within the matrix it is called an osteocyte.

The odontoblast cell bodies remain external to dentin, but their processes exist within tubules in dentin. Both are considered as vital tissues because they contain living protoplasm.

The topic will be discussed under the following sub titles.1. Physical and chemical properties.2. Structure of Dentin.3. Types of Dentin.

PrimarySecondaryTertiaryPre-dentin

4. Histology of Dentin.Dentinal tubules.Intratubular / peributular dentin.Interglobular dentin.Icremental lines.Granular layer of Tomes.

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Odontoblast process.5.Innervation of Dentin:

Intratubular nerves.Theories of pain transmission.

6. Age and functional changes:Vitality of Dentin.Reparative Dentin.Dead tracts.Sclerotic Dentin.

7. Development:Dentinogenesis.Mineralization.

8. Clinical considerations.9. Dentin Anomalies (developmental)

Dentin Dysplasia.Dentinogenis Imperfecta.

PHYSICAL PROPERTIES AND CHEMICAL PROPERTIES:Dentin is yellowish. Because light can readily pass

through thin highly mineralized enamel and be reflected by the underlying dentin, the crown of a tooth has a yellowish appearance. Physically dentin has an elastic quality, which provides flexibility and prevents fracture of the overlying brittle enamel. It is somewhat harder in its central part than near the pulp or on its periphery.

Mature dentin is chemically by weight, approximately 70% inorganic, 20% organic material and 10% water. Its inorganic component consists mainly of hydroxyapatite which is composed of several thousand unit cells (Unit = 3 Ca (PO4)2, Ca(OH)2). Organic phase is type-I collagen with fractional inclusions of glycoproteins, proteoglycans, phosphoproteins and some plasma proteins. The inorganic phase makes dentin slightly harder than bone and softer than enamel. This difference can be readily distinguished

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on radiographs, where dentin appears more radiolucent than enamel and more radioopaque than pulp.

STRUCTURE OF DENTIN:

The dentinal matrix of collagen fibres is arranged in a random network. As dentin calcifies, the hydroxyapatite crystals mask the individual collagen fibres.

The bodies of the odontoblasts are arranged in a layer on the pulpal surface of the dentin and only their cytoplasmic processes are included in the tubules in the mineralized matrix. Each cell gives rise to one process, which traverses the predentin and calcified dentin within one tubule and terminates in a branching network at the junction with enamel or cementum. Tubules are found throughout normal dentin and are therefore characteristic of it.

TYPES OF DENTIN:

Primary Dentin:

Most of the tooth is formed by primary dentin, which outlines the pulp chamber. The outer layer of primary dentin called mantle dentin which is about 20 m thick differs from the rest of the primary dentin.

The fibrils formed in this zone are perpendicular to the DEJ and the organic matrix is composed of larger collagen (loose) fibrils. Circumpulpal dentin forms the remaining primary dentin or bulk of the tooth. This is the dentin formed prior to the root completion. The collagen fibrils in CPD are much smaller in diameter (0.05 m) and are more closely packed. It may contain slightly more mineral than mantle dentin.

Secondary Dentin:

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Secondary dentin is narrow band of dentin bordering the pulp and representing that dentin formed after root completion. It contains fewer tubules than primary dentin. There is usually a bend in the tubules where primary and secondary dentin interface. It was once thought that secondary dentin was formed only in response to functional stimuli, but it has been found in un-erupted teeth as well.

There is a greater deposition of secondary dentin on the roof and floor of the pulp chamber leads to an asymmetric reduction in the size and shape of the chamber and of the pulp horns. These changes which is clinically referred to as pulp recession can be readily detected on radiographs and are important in determining the form of cavity preparation for certain dental restorative procedures. For e.g. Preparation of the tooth for a full crown in a young patient presents a substantial risk of involvement of the dental pulp by mechanically exposing a pulp horn, but in older patient the horn has recorded, presenting less danger. There is also evidence suggesting that secondary dentin scleroses more readily than primary dentin. This leads to reduce the overall permeability of the dentin, thereby protecting the pulp.

Tertiary Dentin:

Also referred as reactive, reparative, or irregular secondary dentin.

It is produced in reaction to noxious stimuli, such as caries or a restorative dental procedure.

Unlike primary or secondary dentin, which forms along the entire pulp dentin border, only cells directly affected by the stimulus produce tertiary dentin.

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The quality and quantity of tertiary dentin produced are related to the cellular response initiated, and this depends on the intensity and duration of the stimulus.

For e.g.: The stimulus of an active carious lesion causes extensive destruction of dentin and considerable rapidly by odontoblast like cells that have differentiated from pulpal prevascular cells. Such as dentin displays a sparse and irregular tubular pattern with frequent cellular inclusions and is sometimes called Osteodentin.

If the stimulus is less active tertiary dentin is deposited less rapidly, its tubular pattern is more regular and there are fewer, cellular inclusions.

The main function of reparative dentin is to protect the pulp by “sealing off” the dentinal tubules or minimizing the permeability of potentially harmful contents.Pre-Dentin (10-47 m):

It lines the innermost portion of the dentin, which is first formed but unmineralized.

It consists of collagen, glycoproteins and proteoglycans.

As the collagen fibres undergo mineralisation at the predentin-dentin junction the pre-dentin becomes dentin and a new layer of pre-dentin forms circumpulpally.

Pre-dentin is the thickest where active dentinogenesis is occurring and its presence is important in maintaining the integrity of dentin, since its absence appears to leave the mineralized dentin vulnerable to restoration of odontoclasts.HISTOLOGY:

When dentin is viewed microscopically, structural features identified area:1. Dentinal tubules.

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2. Intertubular (Peritubular) Dentin and Sclerotic Dentin.3. Intertubular dentin.4. Interglobular dentin.5. Incremental growth lines.6. Granular layer of Tomes.7. Odontoblast process.

Dentinal Tubules:

The course of the dentinal tubules follows a gentle curve in the crown, less so in the root where it resembles an “S” in shape.

They start at right angles from the pulpal surface and ends perpendicular to the dentinoenamel and dentinocementum junctions.

Near the root tip and along the incisal edges and cusps the tubules are almost straight.

Over their entire lengths the tubules exhibit minute, relatively regular secondary curvatures that are sinusoidal in shape.

The ratio between the outer and inner surfaces of dentin is about 5:1. Accordingly the tubules are further

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apart in the peripheral layers and are most closely packed near the pulp.

They are larger in diameter near the pulpal cavity and smaller at their outer ends (1 m).

The ratio between the number of tubules/unit area on the pulpal and outer surfaces of dentin is about 4:1.

The dentinal tubules have lateral branches throughout dentin called canaliculi originate more or less at right angles to main tubule entering the intertubular dentin or to distant tubules.

A few dentinal tubules extend through the DEJ into the enamel for several mms. There are termed as enamel spindles.

Dentinal tubules makes the dentin permeable, providing a pathway for invasion of caries, drugs and chemical present in restorative materials causing pulpal injury.

Intra-tubular Dentin / Peri-tubular Dentin:

The dentin that immediately surrounds the dentinal tubules is termed peritubular dentin.

The peritubular dentin is the best calcified of all the mineralized components of dentin, so it is clearly demonstrated in cross sections of ground section of un-decalcified dentin with the light microscope (because it is so highly mineralized, it is lost in de-calcified sections).

It is twice as thick in outer dentin (0.75 m) than in inner dentin (0.4 m). By its growth it constricts the dentinal tubules to a diameter of 1 m near the DEJ.

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The deposition of intra-tubular dentin causes a progressive reduction in the tubule lumen and eventually obliterates the tubule.

When this occurs in several tubules in the same area the dentin obtain a glassy appearance. This dentin is termed Sclerotic dentin.

Inter-tubular Dentin:

The main body of the dentin is composed of intertubular dentin. It is located between the dent tubules.

Although it is highly mineralized, this matrix like bone and cementum is retained after decalcification where as peritubular dentin is not.

About one half of its volume is organic matrix, especially collagen fibres, which are randomly oriented around the dentinal tubules.

Hydroxyapatite crystals, which average 0.1 m in length are formed along their fibres with their long axis oriented parallel to the collagen fibres.

Inter globular Dentin:

Sometimes mineralisation of dentin begins in small globules areas that fail to coalesce into a homogeneous mass. This results in zones of hypomineralisation between the globules, which are known as globular dentin or inter-globular spaces. Seen commonly in Vitamin D deficiency and exposure to high levels of fluoride during dentin formation.

It is seen most frequently in the circumpulpal dentin where the pattern of mineralisation is largely globular.

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Because this irregularity of dentin is a defect of mineralisation and not of matrix formation, the architectural pattern of the tubules remains unchanged and they run uninterruptedly through the interglobular areas.

Incremental Lines: (Von Ebner)

These are imbrication lines, appear as fine lines or striations in dentin.

They run at right angles to dentinal tubules and correspond to the incremental lines in enamel or bone.

These lines reflect the daily rhythmic recurrent deposition of dentin matrix as well as hesitation in the daily formative process.

The distance between lines varies from 4 – 8 m in the crown to much less in the root.

The daily increment decreases after a tooth reaches functional occlusion. The course of lines indicates the growth pattern of dentin.

Some of the incremental lines are accentuated because of disturbances in the matrix and mineralizaation process. Such lines demonstrated in ground sections are called contour lines (Owen) (hypocalcified bands).

In the deciduous teeth and in the first permanent molars where dentin is formed partly before and partly after birth, there is an accentuated contour line due to the abrupt change in the environment during dentin matrix formation, which may be a zone of hypocalcification termed as Neonatal line (as in enamel).

Granular layer of Tome’s:

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When dry ground sections of the root dentin are visualized in transmitted light, there is a zone adjacent to the cementum that appears granular known as Tomes granular layer.

This zone increases slightly in amount from the CEJ to the root apex and is believed that it is caused by coalescing and looping of the terminal portions of the dentinal tubules.

The cause of development of this zone is probably similar to the branching and beveling of the tubules at the DEJ.Odontoblast process:

Odontoblast processes are the cytoplasmic extension of the odontoblasts.

Odontoblast cells reside in the peripheral pulp at the pulp - predentin border and their processes extend into the dentinal tubules.

The processes are largest in diameter near the pulp (3 - 4 mm) and taper approximately 1 m further into the dentin.

Odontoblast process transverse the thickness of dentin. In other areas a shortened process may be characteristic in tubules that are narrow or obliterated by mineral deposit.

Odontoblast process is composed of microtubules of 20 m in diameter and small filaments 5 to 7.5 m in diameter. Occasionally mitochondria, dense bodies, resembling lysosomes, micro vesicles and coated vesicle that may open to extracellular space are also seen.

The ododontoblast processes divide near the DEJ and may indeed extend into enamel in the enamel spindles.Innervation of dentin:Intratubular nerves:

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Dentinal tubules contain numerous nerve endings in the predentin and inner dentin no farther than 200 to 150 m from the pulp.

Most of these small vesciculated endings are located in tubules in the coronal zone, specifically in the pulp horn (because the tubule diameter is largest here and the peritubular dentin is very little or absent).

The nerves and their terminals are found in close association with the odontoblast process within the tubule.

The nerve endings interdigitate with the odontoblast process indicating an intimate relationship to this cell. It is believed that most of these are terminal processes of the mylenated nerve fibres of the dental pulp.Theories of pain transmission through dentin:

There are three basic theories of pain conduction through dentin.i. Direct neural stimulation:

Stimuli in some manner, reach the nerve endings in the dentin and the dentin is stimulated. There is little scientific support of this theory.ii. Fluid or Hydrodynamic theory:

Various stimuli such as heat, cold, air blast, desiccation, or mechanical or osmotic pressure affect fluid movement in the dentinal tubules.

The fluid movement either inward/outward stimulates the pain mechanism in the tubules by mechanical disturbance of the nerves closely associated with the odontoblast and its process.

Thus these endings may act as mechanoreceptors as they are affected by mechanical displacement of the tubular fluid.iii. Transduction theory:

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This theory presumes that the odontoblast process is the primary structure excited by the stimulus and that the impulse is transmitted to the nerve endings in the inner dentin, through its membrane.

This is not a popular theory since there are no neurotransmitter vesicles in the odontoblast process to facilitate the synapse.

In short, no single proposed mechanism fully explains all the facts related to dentin sensitivity may be more than one mechanism operates at any one time.Age and functional changes:i. Vitality of dentin.ii. Reparative dentin.iii. Dead tracts.iv. Scelerotic or transparent dentin.Vitality of dentin:

Since the odontoblast and its process are an integral part of the dentin, there is no doubt that dentin is a vital tissue.

If vitality is understood to be the capacity of the tissue to react to physiologic and pathologic stimuli, dentin must be considered a vital tissue.

Dentin is laid down throughout life, although after the teeth have erupted and have been functioning for a short time, dentinogeneis slows and further dentin formation is at a much slower rate.Reparative dentin:

If by extensive abrasion, erosion, caries or operative procedures, the odontoblast process are exposed or cut, the odontoblasts die or if they live, deposit reparative dentin.

The odontoblasts that are killed are replaced by the migration of undifferentiated cells arising in deeper regions

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of the pulp is believed to be originated from the cell rich zone or perivascular cells deeper in the pulp.

Reparative dentin is characterized as having fewer and more twisted tubules than dentin.Dead tracts:

In dried ground sections of normal dentin the odontoblast processes disintegrate and the empty tubules are filled with air.

They appear black in transmitted light and white in reflected light.

They extend from the DEJ to the corresponding area of dentin pulp interface.

In most instances, the dead tracts are sealed at their pulpal aspect by the formation of reparative dentin.

These areas demonstrate decreased sensitivity and appear to a greater extent in older teeth.

Sclerotic / transparent dentin:

In case of caries, abrasion, erosion, cavity preparation, sufficient stimuli are generated to cause collagen fibres and appetite crystals to fill the tubules with a fine meshwork, thus obliterating it.

Such calcified tubular space assumes a difference in refractive index becoming transparent which is observed in the teeth of elderly people especially in the roots.

It is also found under slowly progressing caries reducing the permeability of dentin and may prolong pulp vitality.

It is also found beneath warm enamel, which occurs in the incisal area of anterior teeth in elderly and in cervical

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area of older teeth where cervical cementum has been exposed to the oral cavity due to gingival recession.

DEVELOPMENT:

Dentinogenesis:

Dentinogenesis begins at the cusp tips after the odontoblasts have differentiated and begin collagen production. As the odontoblasts differentiate they change from an ovoid to a columnar shape, and their nuclei become basally oriented.

One or several processes excise from the apical end of the cell in contact with the basal lamina. The length of odontoblasts then increases to approximately 40 m although its width remains constant.

Proline appears in the rough surface endoplasmic reticulum and golgi apparatus. The proline then migrates into the cell process in dense granules and is emptied into the extracellular collagenous matrix of the predentin. As the cell recede it leaves behind a single extension, and the several initial processes join into one, which becomes enclosed in a tubule.

As the matrix formation continues, the odontoblast process lengthens, as does the dentinal tubule. Daily 4 m of dentin are formed initially, which continues until the crown is formed and the teeth erupt and move into occlusion.

After this dentin production slows to about 1 m/day. After root development is complete, dentin formation may decrease further, although reparative dentin may form at a rate of 4 m/day for several months after a tooth is restored.

Dentinogenesis is a two-phase sequence in that collagen matrix is first formed and then calcified. As each

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layer of predentin is formed along the pulp border, it remains a day before it is calcified and the next increment of predentin forms.

Korff’s fibres have been described as the initial dentin deposition along the cusp tips. Because of the argyrophilic reaction (stain black with silver) it was long believed that bundles of collagen formed among the odontoblasts.

Recently, ultrastructural studies revealed that the staining is of the ground substance among the cells and not collagen consequently; all predentin is formed in the apical end of the cell and along the forming tubule wall.

The finding of formation of collagen fibres in the immediate vicinity of the apical ends of the cells is in agreement with the general concept of collagen synthesis in C.T. and bone. The odontoblasts secrete both collagen and other components of the extracellular matrix.

Mineralisation:

The earliest crystal deposition is in the form of very fine plates of hydroxyapatite on the surfaces of the collagen fibrils and in the ground substance.

Subsequently, crystals are laid down within the fibril themselves. The crystals associated with the collagen fibrils are arranged in an orderly fashion, with their long axis paralleling the fibril long axes and in rows conforming to the 64 nm. Striation pattern.

Within the globular islands of mineralisation, crystal deposition appears to take place radically from common centers, in a so-called spherulite form. These are seen as the first sites of calcification of dentin.

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The general calcification process is gradual, but the peritubular region becomes highly mineralized at a very early stage.

Although there is obviously some crystal growth as dentin matures, the ultimate crystal size remains very small, about 3 nm in thickness and 100 nm in length. The appetite crystals of dentin resemble those found in bone and cementum.

They are 300 times smaller than those formed in the enamel. It is interesting that two cells so closely allied at the dentinoenamel junction procedure crystals of such a difference but at the same time produce chemically the same hydroxyapatite crystals.

Calcospherite mineralisation is seen occasionally along the pulp pre-dentin forming front.Clinical considerations / importance:

Bacterial toxins, strong drugs, undue operative trauma, unnecessary thermal changes or irritating restorative materials should not insert the cells of the exposed dentin.

It should be remembered that when 1 mm2 of dentin is exposed, about 30,000 living cells are damaged. It is advisable to seal the exposed dentin surface with a non-irritating insulating substance.

The rapid penetration and spread of caries in the dentin is the result of the tubule system in the dentin. The enamel may be undermined at the DEJ, even when caries in the enamel is confined to a small surface area. This is due to the spaces created at the DEJ by enamel tuffs, spindles and open and branched dentinal tubules.

The dentinal tubules provide a passage for invading bacteria and their products through either a thin or thick dentinal layer.

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Electron micrographs of carious dentin show regions of massive bacterial invasion of dentinal tubules. The tubules are enlarged by the destructive action of the microorganisms.

Dentin sensitivity of pain, unfortunately, may not be a symptom of caries until the pulp is infected and responds by the process of inflammation leading to tooth ache. Thus patients are surprised at the extend of damage to their teeth with little or no warning from pain.

Under trauma from operative instruments also may damage the pulp. Air driven cutting instruments cause dislodgement of odontoblasts from the periphery of the pulp and their “aspiration” within the dentinal tubule. This could be an important factor in survival of the pulp if it is already inflamed.

Repair requires the mobilization of the macrophage system as healing takes place, as this progresses there is the contribution of deeper pulpal cells, through cytodifferentiation into odontoblasts, which produces reparative dentin.

The sensitivity of the dentin has been explained by the concept that alteration of fluid and cellular contents of the dentinal tubules causes stimulation of nerve endings in contact with these cells. This theory explains pain throughout dentin since fluid movement will occur at the DEJ as well as near the pulp.

Teeth with deep carious lesions can be saved by indirect pulp capping. By partial removal of the carious dentin and insertion of a Ca(OH)2 dressing for a few weeks. During this period the odontoblasts form new dentin along the pulpal surface underlying the carious lesion, and then dentist can reopen the cavity and remove the remaining bacteria laden decay without endangering the pulp.

The smear layer formed on the cavity floor during cavity preparation although reduces permeability temporarily, it is a bacteria laden mass and its important to

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remove it. Or else toxic product may migrate to the pulp. A cavity liner is then recommended to line the cavity.DENTIN ANOMALIES:i. Dentin Dysplasia (Rootless teeth)

It is a rare disturbance of dentin formation characterized by normal enamel by atypical dentin formation with abnormal pulpal morphology.

Clinically the tooth appears normal both dentitions.ii. Dentinogenesis imperfecta:Type-I: Occurs in families with osteogenesis imperfecta.Type-II: Not associated with osteogenesis Imperfecta. Both dentitions are equally affected (hereditary opalscent dentin).Type-III: (Brandywine type): Is a racial isolate and is characterized by some clinical appearance as I and II but with multiple pulp exposures of dec teeth both dentition are affected.

Clinically they exhibit a characteristic unusual translucent hue.

Because of abnormal DEJ, the scalloping is lost, leading to early loss of enamel by fracturing away after which the dentin undergoes rapid attrition and teeth are severely flattened.

Radiographically obliteration of the pulp chambers and root canals by continuing formation of dentin is seen.

In type-III, there is a great variability in deciduous teeth, they appear as “shell teeth”. The enamel of the tooth appears but dentin is extremely thin and pulp chambers are enormous. This large size of pulp chambers is not due to resorption, but rather due to insufficient and defective dentin formation.

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Histologocally- irregular tubules often with areas of uncalcified matrix can be seen. In some areas there may be absence of tubules.

Treatment: Cast metal crowns for posteriors and JC with anteriors.

REFERENCES:

1. Textbook of Oral Histology and Embryology by

Orbans.

2. Textbook of Oral Histology by Tencate.

3. Pathways of the Pulp, 7 th Edition by Cohen.

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