dentine lecture

DentinDentin

Dentin is the mineralizedDentin is the mineralized hard tissue forming hard tissue forming the main bulk of the tooth. Covered by enamel the main bulk of the tooth. Covered by enamel

in the crown and cementum in the root.in the crown and cementum in the root.2 major properties distinguishes D from E. 12 major properties distinguishes D from E. 1st st

D is sensitive, 2D is sensitive, 2ndnd D is formed throughout life at D is formed throughout life at the expense of pulp. the expense of pulp.

Formation of dentin begins when the tooth germ reach the bell stage. The dental papilla is the formative organ of dentin, formed of ectomesenchymal spindle shaped cells in loose intercellular substance, separated from the inner dental epithelium by cell free zone. Dentin is formed by cells called odontoblasts that differentiate from ectomesenchymal cells of the dental papilla following an organizing effect (induction) that coming from the inner dental epithelium.

Dentinogenesis

A good blood supply and alkaline phosphatase E are required 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 of dentin formation beginning.

As differentiation progresses, the cells grow in length, the acellular zone gradually disappeared and reaches about 40 µ in height and 7 µ in width. The newly differentiated cells are characterized by their nuclei positioned away from inner dental epithelium.

Unlike amelogenesis which has a well defined end point, dentinogenesis will continue throughout life.

1. Odontoblast Differentiation (Pre-odontoblasts).

2. Formative (secretory) stage:

a. Mantle dentin formation.

b. Odontoblastic process appearance.

3. Quiescent (resting) stage.

Life cycle of odontoblasts

1. Odontoblast differentiation:

Under the inductive effect of the inner dental epithelium, the peripheral ectomesenchymal cells of the dental papilla differentiate into odontoblast.

1. Odontoblast differentiation:Before differentiation, the inner dental epithelium is separated from the dental papilla by a thin basement membrane. The peripheral cells of the papilla are spindle and separated by great amount of ground substance. As induction occur, they come into contact with the basement membrane. They assume a short columnar shape and aligned in a single raw along the basement membrane.

2. Formative stage:

L/M: it is large, plump cell with an open faced nucleus situated basally and a basophilic cytoplasm.E/M: the apical basophilic cytoplasm contains the organelles required for the synthesis of dentin matrix (pronounced Golgi complex- prominent rough endoplasmic reticulum- increased mitochondria)The secretory odontoblasts form extensive junction complexes and gap junction to form distinct row of odontoblasts, the cell also exhibit alkaline phosphatase activity which is necessary for Ca++ transport into the cell.

2. Formative stage (Mantle dentin formation beginning):

secretory odontoblasts are aligned along the periphery of the pulp. Functionally, this cell is considered to consists of 2 distinct parts: cell body in which synthesis and secretion of proteins occurs and cell process whereby secretion occur. The odontoblastic process consists of one main bulk with numerous lateral branches along its length. The first sign of dentin formation is the appearance of distinct, large-diameter collagen fibrils called Von Kroff’s fibersVon Kroff’s fibers.

These fibers consist of collagen type III. They originate deep among the odontoblasts, extend toward the inner dental epithelium, and fan out in the structurless ground substance immediately below the epithelium.

2. Formative stage (Odontoblastic process formation):

As the odontoblasts continue to increase in size, they also produce smaller collagen type I fibrils that orient themselves parallel to the future dentino-enamel junction. As the first layer of dentin is deposited, the odontoblastic layer retract from the basement membrane. The cells when they move into pulpal direction, they leave behind a single process which become enclosed in a tube formed of dentin called dentinal tubule. With the successive deposition of dentin both the process and the tubule grow in length.

3. Quiescent stage:

This stage occurs after completion of the circumpulpal dentin. The odontoblast cell loses most of their protein forming organelles to accommodate the decrease in their function.

The fully differentiated and actively secreting odontoblasts decrease slightly in size and the cell process stop to elongate as dentin formation is reduced. Meanwhile the odontoblasts had reached the quiescent stage, however, they produce dentin in a very slow rate but may be reactivated after injury.

DentinogenesisDentinogenesis

1. Formation of predentin (dentin 1. Formation of predentin (dentin matrix formation):matrix formation):

The first indication of predentin formation is the development of bundles of fibrils among the fully differentiated odontoblast. These bundles were known as Von Kroff’s fibers, that form the major component of the first formed thickness of dentin and are attached to the basement membrane of the inner dental epithelium.

These fibers ( Korff’s These fibers ( Korff’s fibers) , were thought to fibers) , were thought to be secreted by the be secreted by the subodontoblastic cells of subodontoblastic cells of the dental papilla. They the dental papilla. They have an argyrophilic have an argyrophilic reaction ( stain black reaction ( stain black with silver). Under E/M, with silver). Under E/M, it was found that this it was found that this black stain is of the black stain is of the ground substances ground substances among the cells and not among the cells and not due to the thick collagen due to the thick collagen fibers. So, the formation fibers. So, the formation and secretion of these and secretion of these fibers is proved to be fibers is proved to be from odontobasts and from odontobasts and not from other cells. not from other cells.

After odontoblasts differentiated, the collagen formation begins in ribosomes sites of RER as procollagen, then pass to Golgi complex where they are glycosylated to be transferred as secretory vesicles towards the secretory poles of the cells.

Once the secretory vesicles secreted outside the cell, the procollagen molecules aggregated as large fibers of type I collagen fibers in ground substance which is the product of odontoblasts incorporated with some of pre-existing substance of the cell free zone to form Mantle dentin.The large collagen fibrils are 0.1-0.2 µm in diameter; these fibrils are aligned at right angles to the basement membrane, while in the mantle dentin of the root, they are parallel to it.

The first formed thickness of dentin is the mantel dentin. As dentin is further deposited, the first formed fibers fade gradually and smaller fibrils constitute a network in the dentin subsequent to the mantle dentin, i.e. circumpulpal dentin. Odontoblasts function in the formation of both the collagen fibers and the acid mucopolysaccharides of the dentine matrix

•Formation of circumpulpal dentin:

Once the layer of mantle dentin is formed, dentinogenesis continue in a slightly different manner to form circumpulpal dentin which is the basic structure of dentin and forms its bulk.The odontoblasts increase in size obliterating the intercellular spaces with extensive junctional complexes develops to form distinct row of odontoblasts.

As the matrix is formed, the odontoblasts begin to move towards the pulp. The plasma membrane of the odontoblasts adjacent to the inner dental epithelium pushes out several short processes called Odontoblastic Process (Tom’s Fiber). Occasionally, one of them may penetrate the basement membrane and interpose itself between the cells of the inner dental epithelium to form Enamel Spindle.

Circumpulpal dentin is formed in a similar way to mantle but differ from mantle dentin in: •The collagen fibers are smaller in diameter 0.05 µm and more closely packed and interwoven with each other.•The fibers are generally present at right or oblique angle to the tubules (parallel to dentin surface).•The ground substance is exclusively a product of odontoblasts.

2. Maturation (mineralization) of predenitn:2. Maturation (mineralization) of predenitn:It occurs at a rate that parallel to matrix formation, and both formation and maturation of predentin begin at the tip of the crown and proceeding in a rhythmic pattern to be gradually completed cervically. It does not occur until a fairly wide band of matrix is formed. Thus until the matrix is completed , the width of predentin remain constant (10-20 um). After the odontoblasts form a wide band of predentin, they bud off matrix vesicles which are small vesicle exit from their plasma membrane into the extra cellular organic matrix.These vesicles are rich in calcium and phosphate ions and contains alkaline phosphatase enzyme, their function is to provide a special micro-environment to form the first hydroxyapatite crystals.

Once the first crystal forms within such vesicle it grows rapidly and rupture through the vesicle wall to spread as a cluster of crystallites and fuse with adjacent clusters to form a fully mineralized matrix. Apetite crystals will obsecure the collagen fibrils of the dentin matrix. However, when these globules do not fuse with each other, areas of uncalcified dentin are left and known interglobular dentin. The predentin is then calcified in a linear pattern or occasionally by globular pattern.

Mineralization sequence of matrix appears primary by crystal deposition in the form of fine plates of hydroxy-apatite on the surface of collagen fibrils and the ground substance.

The long axis of crystals are paralleling the fibril axis in rows. Occasionally, the crystals appear to be deposited in the fibrils themselves.

The dentin mineralization follows two different patterns, linear and globular depending on the rate of dentin formation:

*Globular calcification: deposition of crystals in several areas of the matrix at one time, with continued calcification, globular masses develops, which enlarge and fuse to form a single mass, usually present in mantel dentin where matrix vesicle give rise to mineralization fossi that grow and coalesce. The size of globules depends on the rate of dentin deposition with the largest globules occurs when dentin deposition is fast. When it slow down the mineralization front appears uniform and mineralization is linear.* In circumpulpal dentin , mineralization front can progress in a linear or globular pattern.

Dentin

1.The physical and chemical properties of dentin.

2.The histological structure and ultrastructure of dentin

3. Age changes and clinical consideration.

Dentin is primarily formed from secretory products of the odontoblast and their processes. It is the hard tissue that constitute the body of each tooth serving as both a protective covering of the pulp and as support for the overlying enamel. Unlike enamel, dentin is a vital tissue containing the cell processes of odontoblasts.

Physical properties

• Colour

• Hardness

• Brittleness

• Permeability

• Thickness

• Radiograph

Thickness : 3-Thickness : 3-10mm or 10mm or even moreeven more

Radiograph: Radiograph: more more radiolucent radiolucent than enamel, than enamel, more more radiopaque radiopaque than than cementum cementum and bone due and bone due to lower to lower mineral mineral contentcontent

Chemical properties

Mature dentin composed of approximately: 70% inorganic material, 20% organic material, 10% water by weight.

•Inorganic component: consists mainly of calcium hydroxyapatite crystals. The crystals are plate like-shape, appear needle shape in edge view.Crystals are 0.05-0.06 µm in length and may reach up to 0.1µm.•Organic component: consists of fibrils embedded in an amorphous ground substance. The fibrils are collagen over 90% of the organic content, small inclusion of non-collagenous protein matrix

Classification of dentinAccording to the sequence of

formation, dentine classified as:

•Primary dentin.

•Secondry dentin.

•Tertiary dentin.

Primary dentin

It is the dentin formed before complete root formation. Most of the tooth is formed by primary dentin, which outlines the pulp chamber and is referred to as circumpulpal dentin. The outer layer is called mantel dentin and differs from the rest of the primary dentin in the way it is mineralized and its collagen content.

Secondary Dentin

It develops after root formation has been completed and representing the continuing but much slower, deposition of dentin by odontoblast. The ratio of mineral to organic material is the same as for primary dentin.

The greater deposition of secondary dentin on the roof and floor of the chamber leads to an asymmetric reduction in its size and shape. These changes in the pulp space, clinically referred to as pulp recession.

Tertiary Dentin

Tertiary dentin is reparative, response, or reactive dentin this is localized formation of dentin on the pulp-dentin border, formed in reaction to trauma such as caries or restorative procedures.

Histological Structure

Adjacent to the pulpal end of dentin, the odontoblasts are arranged in a well defined layer, sending their odontoblastic processes through dentin. Each odontoblast sends one odontoblastic process that passes in one dentinal tubule where it traverse the dentin thickness. Adjacent to outer dentin surface, the odontoblastic processes end by formation of several branches

I. Odontoblas

t

1. It is the unit structure of dentin, which form a shallow S shape at the middle part of the crown (primary curvature), and straight at the cuspal and root portions of the tooth. 2. Over the course of dentinal tubule, a regular secondary curvatures are seen.

II. Dentinal Tubules

Histological StructureII. Dentinal Tubules with secondary branches


3. The tubules are packed at their pulp side and further apart at the dentinoenamel junction. This corresponds to the small diameter of the tubule at the dentinoenamel junction and the longer diameter at its pulpal end.



4. The number of tubules is greater in the crown than in root/unit area.5. The tubules have lateral branches through their course known canaliculi, in which the lateral branches of odontoblastic processes traverse.


The primary curvature result from The primary curvature result from crowding and the path followed by crowding and the path followed by the odontoblasts as they move the odontoblasts as they move toward the center of the pulp. toward the center of the pulp. 2ndary curvature due to changes in 2ndary curvature due to changes in direction of much smaller amplitude direction of much smaller amplitude which result in a spiral track taken which result in a spiral track taken by the odontoblast during its course by the odontoblast during its course from the outer dentin surface to the from the outer dentin surface to the pulppulp

Tubules taper from 2.5 um in diameter near Tubules taper from 2.5 um in diameter near the pulp to 1.2 um in the midportion of dentin the pulp to 1.2 um in the midportion of dentin and 900 nm near the ADJ. and 900 nm near the ADJ. No of tubules differ according to tooth age No of tubules differ according to tooth age and thickness of dentin 30000/mm2 in outer and thickness of dentin 30000/mm2 in outer dentin, 40000 in the middle, and 760000 in dentin, 40000 in the middle, and 760000 in inner dentin. ( the ratio between no of inner dentin. ( the ratio between no of tubules/unit area on the pulpal and outer tubules/unit area on the pulpal and outer surface is 4:1surface is 4:1

Contents of dentinal tubulesContents of dentinal tubules

Contain od process, afferent nerve Contain od process, afferent nerve terminals, extracellular fluid called terminals, extracellular fluid called dentinal fluid or dental lymph. If dentinal fluid or dental lymph. If dentin is fractured , fluid exudates dentin is fractured , fluid exudates emit from tubules and form emit from tubules and form droplets on the surface of dentin. droplets on the surface of dentin. This suggest a pressure force from This suggest a pressure force from pulpal tissue outwards that help to pulpal tissue outwards that help to limit the progress of chemicals and limit the progress of chemicals and toxins toward the pulp. The tissue toxins toward the pulp. The tissue changes in dentin occurs through changes in dentin occurs through this fluid.this fluid.


1. It is the cytoplasmic process of the odontoblast that run inside the dentinal tubule.

III. Odontoblastic process

Histological Structure2. It undergoes several branches at its terminal end while along its course it sends out several lateral branches enclosed in the canaliculi. These lateral branches fuse with the lateral branches of the adjacent odontoblastic processes.



3. While the odontoblastic processes usually end at the dentinoenamel junction, some processes traverse this junction to a short distance in the space of enamel and are known as enamel spindle.


http://www.usc.edu/hsc/dental/ohisto/Cards/tth/11_bb.html

Od process Od process structure differ structure differ according to the according to the site in dentin . site in dentin . Near pulp, it Near pulp, it

contains more contains more organelles, away organelles, away

is little is little organellesorganelles

Histological StructureIV. Peritubular dentin

Histological StructureV. Intertubular dentin


I. Incremental lines:A. Incremental lines of Von Ebner.

VI. Hypocalcified structures:

B. Contour lines of Owen.C. Neonatal line.

II. Interglobular dentin.III. Granular layer of Tomes.

Histological StructureI. Incremental lines:A. Incremental lines of Von Ebner:

Histological StructureI. Incremental lines:B. Contour lines of Owen:

Histological StructureI. Incremental lines:C. Neonatal line:

Histological StructureII. Interglobular dentin:

What about What about dentinal tubules dentinal tubules through these through these

areasareas??

Histological StructureII. Interglobular dentin:

Because this irregularity of dentin is a defect of mineralization and not of matrix formation, the normal architectural pattern of the tubules remains unchanged, and they run uninterrupted through the interglobular areas. However, no peritubular dentin exists where the tubules pass through the mineralized areas.

Histological StructureIII. Granular layer of

Tomes.


Theories of granular layer of Tomes occurrence:

III. Granular layer of Tomes.

1. It may be due to interference with the mineralization of the firstly formed layer of dentin.

2. They may represent smaller areas of interglobular dentin than that found in the crown.

3. Looping of the terminal ends of the tubules due to different orientation of odontoblast processes during initial dentin formation.

4- B.V

Age changes (Repair & Defence Mechanisms)I. Physiologic secondry dentin:

Repair & Defence MechanismsI. Physiologic secondry dentin:

II. Irregular secondary dentin (reparative):

This results from attrition, caries and operative cutting procedures.

It takes many names such as osteodentin, atubular dentin

Repair & Defence MechanismsII. Pathologic secondry dentin:

B. Reparative dentin:

Under the previously mentioned conditions that lead to the formation of the pathological type of dentin, the corresponding odontoblast to the injured area of dentin will be more or less damaged. If the odontoblasts are less damaged they will be stimulated to continue dentin formation.



In case of severly damaged odontoblasts, they are replaced by the underlying undifferentiated mesenchymal cells found in the subodontoblastic layer. Damaged or newly differentiated odontoblast are stimulated to deposit reparative dentin to seal off the injured area of dentin.


C. Sclerotic dentin ( transparent dentin ):

In addition to the formation of reparative dentin as a result of injurious stimuli, changes also occur in the surrounding and damaged dentin. This is seen through the deposition of calcium salts in or around the degenerated odontoblastic process.

Repair & Defence MechanismsIII: Dead tract :

Odontoblastic processes may disintegrate by any injrious stimuli, most oftenly in areas of narrow pulp horn due to odontoblasts crowding. In ground section these dentinal tubules appear black where they are empty. Dentinal tubules with degenerated odontoblastic processes are called dead tracts.

Repair & Defence Mechanisms

IV. Sclerotic dentin ( transparent dentin ):Sclerotic dentin can be seen in:

1. Eldery tooth root.

2. Around dentinal part of type B & C enamel lamellae.

3. Under slowly progressive caries.

And is characterized by:

1. Harder and denser than normal dentin.

2. Appears light in transmitted light, and dark in reflected light.

Dentine sensitivity

The only type of sensation obtained in dentine pulp complex is pain. There are three basic theories of pain conduction through dentin.

Dentine sensitivity

This theory denotes that dentin is innervated, however, the nerve fibers of dentin are proved to be only limited to the predentin and in very thin layer of dentin lying beside the pulp tissue. as a matter of fact, the nerve fibers are not demonstrated beyond these regions or at the dentinoenamel area.

I. The direct neural stimulation:

Dentine sensitivity

The nerve fibers arise from the plexus of Raschkow, enter the predentin, and return to region the plexus again.


Dentine sensitivity

The controversy:


1. sensitivity of dentin is more at the dentinoenamel junction, while the nerve endings didn’t reach that area.

2. The plexus of Raschkow and the intratubular nerves don’t establish themselves until after eruption by time although the newly erupted teeth are sensitive

3. The application of local anesthetics to exposed dentin doesn’t eliminate dentin sensitivity.

Dentine sensitivity

This theory considers the odontoblast to be a receptor cell. This was argued that because the odontoblast is of neural crest origin, it retains an ability to transduce and propagate an impulse.

2. Transduction theory

Dentine sensitivity

The controversy:

1. The demonstration of a synaptic relationship between the odontoblast and pulpal nerves has not found.

2. The membrane potential of odontoblast measured in vitro is too low to permit transduction.

3. The application of local anesthetics to exposed dentin doesn’t eliminate dentin sensitivity.

2. Transduction theory

Dentine sensitivity

This theory proposed to explain dentin sensitivity involves movement of fluid through the dentinal tubules. This hydrodynamic theory, which fits much of the experimental and morphologic data, proposes that fluid movement through the tubule distorts the local pulpal environment and is sensed by the free nerve endings in the plexus of Raschkow. Thus

2. Hydrodynamic theory:

Dentine sensitivity

The agreements:

1. When dentin is first exposed, small blebs of fluid can be seen on the cavity floor. When the cavity is dried with air or cotton wool, a greater loss of fluid is induced, leading to more movement and more pain.

2. The increased sensitivity at the dentinoenamel junction is explained by the profuse branching of the tubules in this region.

3. The hydrodynamic theory also explain why local anesthetics, applied to exposed dentin, fail to block sensitivity and why pain is produced by thermal change, mechanical proping, hypertonic solutions, and dehydration.

2. Hydrodynamic theory

Clinical considerations

1. Metallic restorations are excellent thermal conductors. Therefore it is appreciate to replace a cement base under these restorations to protect the pulp by minimizing pain conduction.

2. Because of the permeability of dentin, in cavity preparation, sealing of dentinal tubule is a requisite of effective restorative dentistry.

dentine lecture

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