Dentin. Dentin – Composition , Formation , and Structures. Dentinogenesis. O rigin of the dentin.
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Dentin is formed by cells called odontoblasts that defferentiated from ectomesenchymal cells of the dental papilla following an organizing influence that emanents from the inner dental epithelium.
Thus the dental papilla is the formative organ of dentin and eventually becomes the pulp of the tooth.
For dentinogenesis and amelogenesis to take place normally, the differentiating odontoblasts and ameloblasts will receive
signals form each other – “reciprocal induction”.
and beginning of morphodifferentiation(tooth crown assumes its final shape).
4 and 5 - subodontoblastic cells.
It is brought about by the expression of signaling molecules and growth factors in the cells of the inner epithelium;
a.The dental papilla cells are small and undifferentiated and exhibit a central nucleus and few organelles;
At this time they are separated from inner epithelium by an acellular zone that contains some fine collagen fibrils;
b. Committed dental ectomessenchymal cells that are in a state of mitosis or cell division;
c.Daughter cells that are competent to become odontoblasts remain in the peripheral zone;
d.Differentiatedodontoblasts with a polarized nucleus and citoplasmicextentions;
Almost immediately after cells of the inner dental epithelium reverse polarity, changes also occur in the adjacent dental papilla;
Ectomesenchymal cells rapidly enlarge and elongate to become preodontoblasts first and then odontoblasts as their cytoplasm increases in volume to contain increasing amount of protein-synthesizing organelles.
Short columnar cells bordering the dental papilla – inner dental epithelium will eventually become ameloblasts ;
5. The odontoblasts as they differentiate will start elaborating organic matrix of dentin, which will mineralize.
6. As the organic matrix of dentin is deposited, the odontoblasts move towards the center of the dental papilla, leaving behind cytoplasmic extensions which will soon be surrounded by dentin;
7. Therefore, a tubular structure of dentin is formed.
The odontoblasts appear like protein-producing cells;
Dentin formation proceeds toward the inside of the tooth;
This area of mineralization is known as mantle dentin and is a layer usually about 150 μm thick.
Some types (of 15) of known collagen
Korff\'sfibers (corkscrew fibers) passing between odontoblasts and reach predentin.
The question of the origin of these fibers is controversial.
The next step in the production of dentin is formation of its organic matrix;
Odontoblasts differentiate in the preexisting ground substance of the dental papilla;
The first dentin collagen syntesized by them is deposited inthis groun substance.
Layer of polarized odontoblasts with Tomes\' fibers.
Dentin is formed throughout the life of the tooth if the tooth pulp is alive
Line of demarcation:
The increase of the dentin thickness and the closure of the pulp
horns make it much less possible to expose the pulp chamber during preparation.
Line of demarcation:
Functions as a barrier against caries.
The size of the pulp cavity decreases and obliteration of the pulp horns.
The course of the dentin canals is more irregular.
in the formed dentin
Is defined as a tertiary dentin matrix secreted by a new generation of odontoblast-like cells in response to an appropriate stimuls after the death of the original postmitotic odontoblsts responsible for primary and fisyologic secondary dentin secretion;
Such a response will be made to stronger stimuli and represents a much more complex sequence of biologic processes.
The conduction theory (intratubularinnervation theory) contend that dentin is richly innervated and those nerves mediate the impulse to the brain.
Some new studies show that predentin and the first layer of circumpulpal dentin (0.2mm) is innervated with nerve fiber from the raschkows plexus.
The fibers run parallel to the tomes fiber in the dentin tubules.
The density of those fiber is much higher in the coronal dentin than cervical dentin. Root dentin doesn’t include such fibers.
Some authors contend that those fibers end at the DEJ, but can not be seen in histological slides.
The “hydrodynamic theory”, developed in the 1960’s is the widely accepted physiopathological theory of Dentin Sensitivity.
Temperature, physical osmotic changes or electrical and chemical stimuli and dehydration are the most pain-inducing stimuli.
According to this theory, those stimuli increase centrifugal fluid flow within the dentinal tubules, giving rise to a pressure change throughout the entire dentine.
The movement stimulates intradentinal nerve
receptors sensitive to pressure (BARORECEPTORS),
which leads to the transmission of the stimuli .
This simulation generates pain.
Berman describes this reaction as: “The coefficient of thermal expansion of the tubule fluid is about ten times that of the tubule wall.
Therefore, heat applied to dentin will result in expansion of the fluid and cold will result in contraction of the fluid, both creating an excitation of the \'mechano-receptor\'.”
The fluid moves through the tubules and excites nerves
The incentives on dentine causes movement of fluid in the tubules