Dentin
A- Definition :
Dentin is a calcified tissue that surrounds the pulp parenchyma except at the level of the apex orifice, its external face is covered in its coronal part by enamel, and in its radicular part by cementum.
This is the result of the mineralization of the matrix substrate produced by the odontoblasts.
B-Dentinogenesis:
Dentinogenesis goes through successive stages from the differentiation of odontoblasts to the mineralization of the matrix substrate.
Differentiation of odontoblasts:
Peripheral cell of the ectomesenchymal papilla located at a short distance from the basement membrane:
-non-polarized
– oval and central core
-cellular organelles distributed evenly in the cytoplasm.
-these are cells which will differentiate in the cytoplasm:
1)-formation of pre-odontoblasts:
-stopping cell proliferation
-increase in cell size
-the cells attach to the anchoring fibrils of the basement membrane, they are called pre-odontoblasts.
2)-differentiation of odontoblasts:
Cell polarization: we distinguish an apical or secretory pole close to the basal membrane and a nuclear pole close to the future dental pulp.
-REG parallel to the long axis of the cell
-more central golgi apparatus
-the cell body elongates to reach a height of approximately 50µm.
-formation of a cellular extension at the apical pole and its elongation causes the odontoblastic cell bodies to move back towards the center of the ectomesenchymal papilla, the latter will take the name of dental pulp as soon as the first odontoblasts appear.
-the extension branches out to give several branches which will extend laterally in relation to the main trunk.
The cell has a cilium near the centriole and around the Golgi apparatus; cilia are thought to be involved in dentin sensitivity.
-formation of a terminal web which separates the extension from the cell body (actin and vimentin filament) allowing passage only to exocytosis vesicles.
3-release of the dentin matrix:
Odontoblasts synthesize and secrete pre-dentin (the organic matrix) between the anchoring fibrils of the basement membrane, later around the odontoblastic extensions, then maturation of this matrix and mineralization.
The first layer of dentin is called the dentinal mantle.
The formation of the dentine mantle marks the end of the dental bell stage and the beginning of the crown formation stage, with the dental papilla becoming the dental pulp.
-Mineralization begins when the pre-dentine reaches a thickness of 20 to 30 µm at the coronal level and a few microns at the root level.
Mineralization is the result of impregnation of the matrix substrate by calcium phosphate salts which crystallize in the form of hydroxyapatite Ca10(PO4)6OH2.
Dentin formation occurs in successive layers with periods of rest, leaving traces in the form of VON EBNER striations or lines, the most pronounced of which bear the name OWEN contour.
-The interface between mineralized and non-mineralized dentin is called mineralization front or Metadentine.
The continuous deposition of pre-dentine pushes the cell body of the odontoblast towards the center of the dental pulp, this phenomenon progressively increases the size of the extension which is included in a small tube of dentine, called dentinal tubuli, which lengthens at the same time as it.
This very thin tube is approximately 2.5 µm in diameter.
Throughout the life of the tooth, odontoblasts are in close relationship with cells of the odontoblastic region: pulp fibroblasts, capillary endothelial cells (supply of oxygen and nutrients), pulp immune cells (defense), pulp nerve fibers, some of which penetrate the odontoblasts and even the dentinal tubules for a short distance.
C-Composition:
Schematically, dentin is composed of a cellular compartment and an extracellular matrix.
CA/ The cellular compartment :
It is made up of odontoblasts whose cell bodies are located on the periphery of the pulp and whose extensions cross a large part of the dentin in fine canaliculi (the dentinal tubules)
The cell body is the seat of all biosynthesis, while the cell extension is involved in the phenomena of exocytosis and endocytosis.
CB/ The extracellular matrix:
It consists of a mineral phase (70%), the organic matrix (20%) and an aqueous phase (10%).
B1: the organic matrix : it is rich in matrix proteins, it is composed of proteins and lipids.
B-1-1 Proteins:
1-1-1-collagen proteins: collagen fibrils are synthesized and secreted by odontoblasts, they constitute the framework of the dentin matrix (90%), and they have a role of support for the dentin mineral consisting essentially of carbonated hydroxyapatite crystals.
1-1-2: non-collagenous proteins:
we find SIBLING ( Small Integrin – bending Ligand N – linked G lycoproteins):
They are present mainly in bone and dentin, there are 5 of them:
1-Dentin sialophosphoprotein-2-Dentin matrix phosphoprotein-3-Bone sialoprotein-4-Osteopontin-5-Extracellular matrix phosphoglycoprotein.
B-2-lipids (1.75%):
Rich in cholesterol, mono-di- and triglycerides, free fatty acids and phospholipids.
Some phospholipids could be involved in dentin biomineralization.
B2/the mineral matrix
It is rich in hydroxyapatite crystals, and some trace mineral ions.
D-HISTOLOGY
Depending on the location, two types of dentin are described:
-the peripheral dentin, the outermost.
-the Circumpulpal dentin, the most internal.
D-1-Peripheral Dentin
It includes 3 different layers, from the most superficial to the deepest:
-at the coronal level: it is the dentine mantle
-at the root level: the hyaline layer of Hopewell-Smith on the periphery and the granular layer of Tomes.
1-1-The dentin mantle : this is the first to be deposited at the coronal level in contact with the enamel.
Its thickness is between 7 and 30 µm (GOLDBERG), it is devoid of canaliculi and does not contain phosphoproteins (PIETT and GOLDBERG 2001).
1-2-The HOPEWELL-SMITH hyaline layer:
Located between the granular layer of Tomes and the acellular cementum; it would contain enamel proteins, it is highly calcified, non-collagenous, highlighting the root dentin.
It is a narrow layer 7–15 µm thick and is also devoid of dentinal tubules.
1-3-The granular layer of TOMES:
Located between the Hopewell-Smith layer and the Circumpulpal dentin
It is hypomineralized and 8 to 15 µm thick, it contains fine canaliculi (GOLDBERG 1989), characterized by the presence of interglobular spaces persisting between the calcospherites at the time of dentin mineralization which do not fuse uniformly, giving a granular appearance to this layer.
1-4Email dentine reports :
The junction between the enamel and the dentin has a scalloped path with concavities facing the enamel.
The enamel and dentin are joined together, the two tissues exhibit remarkable cohesion between them.
1-5-The dentin-cementum junction
At the root level, the external limit of the peripheral dentin and its junction with the cementum are poorly defined.
-The mineralization, due to the non-fusion of calcospherites at this level, appears heterogeneous, this zone is called the TOMES granular layer.
D-2) Circumpulpular Dentin :
D-2-a- Histological structure:
It is characterized by the presence of dentinal tubules which run through it from the enamel-dentinal junction to the pulp.
The tubules surround the cytoplasmic extensions of the odontoblasts; these extensions lengthen as the cell bodies recede during dentinogenesis and successive layers of dentin are applied.
– the tubule, the cytoplasmic content and the peri-cytoplasmic material, the peritubular zone form a dentin metabolic unit.
-between 2 neighboring units is the intertubular dentin.
a-1-Dentinal tubules:
These are molds that envelop the cytoplasmic extensions of the odontoblasts and protect them.
-the dentinal tubules are parallel to each other, at the coronal level, they have an elongated S-shaped path; at the root level they have a straight path, they are connected to each other by secondary tubules; their number varies between 20,000 and 65,000/mm2. Their diameter gradually decreases from the pulp to the enamel-dentin junction (2.5 to 0.5 µm), at the level of the JAD, the tubules end with -bi- or tri- furcations.
a-2-Cytoplasmic (odontoblastic) extensions
They arise from the cell body of the odontoblasts which are located on the periphery of the pulp.
They are housed in the dentinal tubules, they bathe in a liquid called intra-tubular liquid or transdentinal fluid and contribute to the formation of peri-tubular dentin which is affixed along the dentinal tubules between the extension and the wall of the dentinal tubule.
The cytoplasmic content decreases as one moves away from the cell body and the ergastoplasm and mitochondria become rarer.
The cytoplasmic extensions may present lateral expansions (included in the secondary tubules), notably in the internal dentine.
Their main role is the intracellular transport of secretory vesicles.
a-3-The peri-cytoplasmic space:
The pericytoplasmic space, located between the cellular extension and the tubule wall, is filled by polysaccharide complexes and non-collagenous proteins.
This intra-tubular peri-cytoplasmic material constitutes a potential mineralizable matrix reserve which serves as support for the formation of peritubular dentin.
It is in the pericytoplasmic space that the nerve endings from the pulp innervation travel.
a-4-Intertubular dentin :
Intertubular dentin occupies the space between two neighboring dentin units.
It consists of a mineral framework organized into small flakes 60 µm long and 3 to 4 nm wide.
Its organic matrix is composed of 85 to 90% type I collagen, and numerous non-collagenous proteins.
It has a certain resilience and lends itself easily to abrasion (Piette and Goldberg)
a-5-Peritubular dentin :
It forms a dense and homogeneous sheath, highly mineralized, it describes a ring of dentine around the cellular extensions, conferring resistance to the tissue.
It has a thickness ranging from 0.5 to 1.5 nm, it is more resistant to pressures exerted on the tooth such as abrasion, its organic matrix does not contain fibrillar collagen and it is more mineralized than intertubular dentin.
D-2-b- The different types of Circumpulpal dentin:
We can distinguish 3 types according to their chronology of formation: primary dentin, secondary dentin and tertiary dentin.
b-1-Primary dentin:
It is secreted by first generation odontoblasts, its synthesis begins during intrauterine life and continues until the tooth becomes functional on the arch, it has a thickness of 150 µm.
b-2-Secondary dentin:
It is secreted throughout life, in the absence of pathological processes, it appears when the root has finished its edification. It is in continuity with the primary dentin.
It is located on the periphery of the pulp chamber and accumulates mainly over the entire pulp roof and floor.
b-3-Tertiary dentin:
It is a dentin formed during a pathological process, it is only found on teeth that have been subjected to trauma or irritation.
Its purpose is to remove and isolate the pulp from the septic oral environment and therefore protect it, it allows for good healing and maintenance of the vitality of the pulp tissue.
It is less permeable than the others, and depending on the intensity of the stimulus exerted on the tooth, it is classically divided into two (2) types: reactional dentin and reparative dentin.
b-3-1-L a reactional dentin:
It is developed in the presence of a slowly evolving carious process or a wear process.
It is secreted by first generation post-mitotic odontoblasts surviving inflammation (Cooper et al 2010), from a topographical point of view, two types are distinguished:
b-3-1-1- sclerotic reaction dentin:
It is formed in the dentinal tubules by the odontoblasts. It is highly mineralized.
b-3-1-2-peripulpal reaction dentin (DRP):
It results from the reactivation of post-secretory odontoblasts and the acceleration of the synthesis, secretion and mineralization of Predentin.
Reactive dentin is an effective protection for the pulp in the event of chronic carious lesions.
b-3-2-Reparative dentin:
It is secreted by second generation odontoblasts or replacement odontoblasts (RO) from the subodontoblastic layer of Hohl, after migration and in contact with the necrosis zone they will differentiate into second generation odontoblasts or replacement odontoblasts (RO).
Reparative dentin is classically divided into fibrodentin and orthodentin.
fibrodentin:
It is characterized by an extracellular matrix rich in collagen (88% collagen and 12% type III) and fibronectin. It is less mineralized and structured than normal dentin and does not contain tubules.
Cellular inclusions are sometimes detected near the junction between fibrodentine and normal dentine, especially when carious progression is rapid. Fibrodentine then shows a structure similar to that of bone and is called OSTEODENTINE.
orthodentine:
It is formed by pulp cells that attach to fibrodentine, then take on an odontoblastic morphology with an extension and a highly polarized cell body. Its matrix mineralizes to form orthodentine, which, unlike fibrodentine, contains tubules surrounding the odontoblastic extensions.
The tubules are more spaced than in normal dentin , have a more convoluted course, and are usually oriented in a different direction.
This difference in orientation would increase the barrier effect against carious progression.
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