Dentin 2 (dentinogensis) l Oral histology and biology MCQs for dental students

Dentin 2 (dentinogensis) l Oral histology and biology MCQs for dental students

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dentin histology

• now we discussed the histology and component of dentin 

Histological Structure of dentin

 Odontoblastic processes:

•  Each odontoblast gives cytoplasmic  process which crosses the whole  thickness of dentin in a fine tubule called  dentinal tubules.

• The processes are wider in diameter of the pulpal end than in the dentin surface.

• It contains mainly microtubules and  filaments and some mitochondria and vesicle.

• Each process gives off fine branches along its course called Lateral  branches, originate at right angle to the dentinal tubules and attach the neighboring branch.

• At the outer dentin surface, the tubules divide into 2 main branches called Terminal branches forming a plexus beneath the outer dentin surface near ADJ.

Dentinal tubules:

• The dentinal tubules contain cytoplasmic  processes of odontoblasts or Tom's fibers.
• The ratio between the numbers of tubules/unit  area on pulpal to outer surface is 4:1.
• Diameters of tubules are 2.5 μm near the pulpal  surface, 1.2 μm in mid root portion, and 900 nm  near ADJ.

Course of dentinal tubules:

Primary curvature:

• The tubules take S shaped along its course where first curve convex facing root ward.
• In the root portion and under cusp or incisal edge, tubules take straight  course.

Secondary curvature:

• These are the result of the spiral track taken by odontoblasts during its course from the outer dentin to the pulp.

Inter-tubular dentin:

• It is the dentin located between the dentinal  tubules.
• It consists of randomly arranged network of  type I collagen fibrils in which the apatite crystals are deposited.
• It forms the main bulk of dentin.

Intra-tubular dentin (Peri-tubular dentin):

• It’s a rim of more highly calcified matrix that immediately surrounds the dentinal tubules.

• In transverse ground section, peri-tubular dentin appears as translucent rings around odontoblastic processes spaces with inter-tubular dentin in between.

• Periodontoblastic space is the space exists between  the wall of the tubules and odontoblastic processes. It contains tissue fluid, dentinal lymph.

Inter-globular dentin:

• These are areas of hypomineralized dentin where globular zones of mineralization failed to fuse into homogenous mass. Areas of organic matrix between globules will remain uncalcified bounded by curved outlines of adjacent globules.

• It is seen most frequently in circumpulpal dentin just  below mantle dentin in crown.

• Because it is defect in mineralization (not of matrix formation) dentinal tubules pass through it with no peri-tubular dentin exists In ground section, it is called interglobular dentin spaces.

Granular Layer of Tomes:

• It is a constant feature of root dentin immediately adjacent to cementum . It appears granular and best seen in ground section as black granules

Incremental lines of dentin

1. Incremental lines of Von-Ebner:

• They reflect normal rhythmic pattern of dentin deposition of  daily 4-8 microns in crown, much less in root.

• These incremental lines run at right angles to the dentinal  tubules in an inward and root ward direction.

2. Contour line of Owen:

• It is more accentuated incremental line,  resulted from disturbance in matrix mineralization process. It is easily seen in longitudinal ground section.

3. Neonatal line:

• Neonatal line presents in deciduous teeth and in first molars.

Dentin Sensitivity

• Any stimulus to dentin is felt as PAIN. The nerve fibers form a plexus of nerves called Subodontoblastic plexus or plexus of Raschkow. Some nerve fibers lose their schwann cell coating pass between the odontoblasts cell bodies and enter the dentinal tubules not far than 100-150μm from the pulp.  

• Three mechanisms have been proposed to explain dentin sensitivity DENTIN

Direct neural stimulation theory:

• Dentin contains nerve endings, which respond when dentin is stimulated.

Arguments:

1. No evidence that nerve ending exists in the outer parts of dentin, which is the most sensitive region.
2. The application of local anesthetic to the surface of dentin doesn’t abolish the sensitivity.
3. Newly erupted teeth are sensitive, although plexus of Raschkow do not establish until a tooth had erupted.

Odontoblastic transduction theory:

• It proposed that odontoblastic process is the primary structure excited by stimulus then the impulse is transmitted to nerve endings in inner dentin.

Arguments:

• Supporting these theory that odontoblasts are of neural crest origin. But No neurotransmitter vesicles in odontoblastic process.

Fluid or hydrodynamic theory:

• It is the most popular one, which proposed that  various stimuli as heat, cold, air affect fluid movement in dentinal tubules which stimulate nerve endings in plexus of Rashcow.

Supporting:

1. When the cavity is dried with air or cotton wool. A greater loss of fluid is induced, leading to more movements and more pain
2. The increased sensitivity at ADJ due to profuse branching of tubules in this region.
3. This theory explains why local anesthetics, applied to exposed dentin, fail to block sensitivity.

Age changes of dentin

1. Regular or secondary dentin (physiologic secondry dentin):

• Secondary dentin develops in a slow rate after root  formation has been completed.

• Has few number of dentinal tubules and often change  their direction.

• It is resulted from mild stimuli as slow attrition or slowly progressive caries.

• Resulted in ʺpulp recessionʺ due to gradual decrease in pulp size. Making the pulp exposure lesser to occur.

• Between the primary and secondary dentin there is often  darkly staining line where the tubules change their  direction.

 

2. Irregular secondary or tertiary dentin (reparative dentin):

• It occurs due to sever stimuli as abrasion, caries or cavity preparation.

• It is formed on the area of exposed tubules to seal off zone of injury.

• Histologically, Tubules are fewer in number, less regular in arrangement

• Atubular dentin: Some area may not contain tubules.

• Osteodentin: when the dentin forming cells are often included in the rapidly forming matrix, then these cells degenerate and vacate the space that they were occupied.

3. Transparent/ translucent sclerotic dentin:

• It is a calcification of dentinal tubules where the  odontoblastic processes undergo fatty degeneration  then calcification.

• It occurs as a sign of normal aging process and  because of dentin injury by slow caries or abrasion.

• This dentin has uniform refractive index because  dentinal tubules are occluded by calcium salts.

• It appears translucent when viewed by transmitted  light and dark by reflected light.

4. Dead tracts:

• Sever stimulation results in destruction of the odontoblastic processes.

• In ground section, these empty dentinal tubules are filled with air, they appear black in transmitted light and white in reflected light, this optical phenomenon is due to difference in the refractive index.

• Dead tract is bounded by narrow zone of sclerotic dentin.

• It may be seen in normal dentin, due to the death of odontoblasts crowded in the narrow pulpal horn leaving empty tubules which may be filled with air, and these tubules give an appearance like dead tract.