Dentinogenesis lectures for undergraduate students

Introductions

1. Dentinogesis By: Dr. abubakar mohammad kaura (BDS, FMCFD, FIMC, Cert. LMIH, Msc public health)

2. Introduction: The dentin provides the bulk and general form of the tooth and is characterized as a hard tissue with tubules throughout its thickness. It determines the shape of the crown, including the cusps, ridges, the number and size of the roots since it usually begins to form slightly before the enamel. It contains within its tubules the processes of the specialized cells, called the odontoblasts . Although the cell bodies of the odontoblast are arranged along the pulpal surface of the dentin, the cells are morphologically cells of the dentin. The dentin share some physical and chemical properties with the bone. The major morphologic difference between bone and dentin is that the osteoblasts can exist on the surface of bone and sometimes can become embedded within its matrix to form osteocyte. However, the odontoblasts ’ cell bodies remain external to dentine but their processess exist within tubules in dentin.

3. Physical and chemical properties The dentin is usually light yellowish in color in young individuals and becomes darker with age. Dentine is viscoelastic and subject to slight deformation unlike enamel which is very hard and brittle. It is harder than bone but softer than enamel. Dentin hardness varies slightly between tooth types and between crown and root dentin. It is harder in its central part than near the pulp or on its periphery. The dentine of primary teeth is slightly softer than that of the permanent teeth. It is more radiolucent than enamel due to the lower content of minerals salts. Dentin consists of 35% organic matter and water and 65% inorganic material.

4. The organic substance consists of collagenous fibrils and ground substance of mucopolysaccharides (proteoglycans and glyco -aminoglycosides ) The inorganic component consist of hydroxyapetite as in bone, cementum, and enamel. However, the crystals are smaller than that of enamel.

5. The structure of dentine Overview: The dentinal matrix of collagen fibers is arranged in a random network. The hydroxyapetite crystals mask the individual collagen fibers as dentin calcifies. The body 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 odontobalst 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 the enamel or cementum. Tubules are found throughout normal dentin and are therefore its characteristics.

6. Dentinal tubules: It follows a gentle S-shaped curve in the crown and a more straighter contour in the root. Starting at right angles from the pulpal surface and end perpendicular to the dentinoenamel and dentinocementum junctions. They are almost straight near root tip and along the incisal edges and cusps. The tubules are longer than the thickness of the dentine due to their curvatures. They are larger in diameter near the pulpal cavity (3 to 4 micrometers) and smaller at their outer ends (1 micrometer). The dentinal tubules have lateral branches throughout dentin, which are termed canaliculi or microtubules. Few dentinal tubules extend through the dentinoenamel junction into the enamel for several millimeters and are termed eneamel spindles.

7. Peritubular dentin This is the dentin that immediately surrounds the dentinal tubules. It forms the walls of the tubules in all but the dentin near the pulp. It is more highly mineralized (about 9%) than intertubular dentin. It is twice as thick in outer dentin (approx. 0.75 micrometer) than in the inner dentin (0.4 micrometer).

8. Intertubular dentin The main body of dentin is composed of intertubular dentin. It is located between the dental tubules or more specifically, between the zones of peritubular dentin. Although it is highly mineralized, this matrix, like bone and cementum, is retained even after decalcification whereas peritubular dentin is not. About half of its volume is organic matrix, specifically collagen fibers.

9. Predentin : The predentin is located adjacent to the pulp tissue and is 2 to 6 micrometer wide depending on the activity of the odontoblast . It is the first-formed dentin and is not mineralized. As the collagen fibers undergo mineralization at the predentin -dentin junction, the predentin becomes dentin and a new layer of predentin forms circumpulpally .

10. Odontoblast process These are the cytoplasmic extension of the odontoblasts . The odontoblast cells resides in the peripheral pulp at the pulp- predentin border and their processes into the dentinal tubules. The processes are largest in diameter near the pulp (3 to 4 micrometer) and taper to approximately 1 micrometer further into the dentin. Controversy exist as to the extent of the odontoblast process. Some authors demonstrated that it extends 200 to 300 micrometer into the dentin, others demonstrated that it is at the dentinoenamel juction , and recently other researchers have demonstratd it into the DEJ.

11. Primary dentin Mantle dentin: This is the first formed dentin in the crown underlying the dentinoenamel junction. It is the outer or most peripheral part of the primary dentin and is about 20 micrometer thick. It is bounded by the dentinoenamel junction and the zone of interglobular dentin. The fibrils formed in this zone are papendicular to the dentinoenamel junction. Mantle dentin has fewer defects than circumpulpal dentin. Circumpulpal dentin: Forms the remaining primary dentin or bulk of the tooth. It is the the circumpulpal dentin that represents all of the dentin formed before root completion. The collagen fibrils in the circumpulpal dentin are much smaller in diameter and are more closely packed together compared to the mantle. It is more mineralized than the mantle dentin.

12. Secondary dentin This is the narrow band of dentin bordering the pulp and representing that dentin formed after root completion. It contains fewer tubules and is formed more slowly than primary dentin. Secondary dentin is not formed uniformly and appears in greater amounts on the roof and floor of the coronal pulp chamber where it provides protection to the pulp from exposure

13. 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.

14. Interglobular dentin: Sometimes mineralization of dentin begins in small globular areas that fail to coalesce into a homogenous mass. This results in zones of hypomineralization between the globules. These zones are known as globular dentin or interglobular dentin. This dentin forms in the crowns of teeth in the circumpulpal dentin just below the mantle dentin, and it follows the incremental pattern. The dentinal tubules pass uninterruptedly through interglobular dentin, thus demonstrating more defect of mineralization and not of matrix formation.

15. Incremental lines: The incremental lines otherwise known as von Ebner or imbrication lines, appear as fine lines or striations in dentin. They run at right angles to the dentinal tubules and correspond to the incremental lines in enamel. They represent the daily rhythmic, recurrent deposition of dentin matrix. The distance between lines varies from 4 to 8 micrometers in the crown to much less in the root. The daily increment decreases after a tooth reaches functional occlusion. When the incremental lines are accentuated because of disturbances in the matrix and mineralization, they are known as contour lines of Owen. In the deciduous teeth and in the first permanent molars, the dentin is formed partly before and partly after birth. The prenatal and postnatal dentin are separated by an accentuated contour line and it is termed neonatal line.

16. Tomes’ granular layer When a dry ground sections of the root dentin are visualized in transmitted light, a zone adjacent to the cementum appears granular. This zone slightly increases in amount from the CEJ to the root apex and is believed to be caused by a coalescing and looping of the terminal portions of the the dentinal tubules.

17. Development of dentin: Dentinogenesis : Dentinogenesis begins at the cusp tips after the odntoblasts have differentiated and begin collagen production. The odontoblasts changes from an ovoid to columnnar shape and their nuclei become basally oriented at the early stage of dentinogenesis . One or more processes arise from the apical end of the cell in contact with the basal lamina. The length of the odontoblast then increases to approximately 40 micrometers. Proline appears in the rough surface of endoplasmic reticulum and G olgi apparatus. The proline then migrates into the cell process in dense granules and is emptied into the extracellular collagenous matrix of the predentin .

18. As the matrix formation continues, the odontoblast process and dentinal tubule lengthens. At the onset, daily increment of 4 micrometers of dentin is are formed. This continues until the crown is formed and the teeth erupt and move into occlusion. Once the tooth reached the occlusion, the dentin production slows down to about 1 micrometer/day. After root development is complete, dentin formation may decrease further. Dentinogenesis is a two-phase sequence (collagen matrix formation and calcification). As each increment of predentin is formed along the pulp border, it remains a day before it is calcified and next increment of predentin forms.

19. Mineralization: The earliest crystal deposition is in the form of very fine plates of hydroxyapetite on the surfaces of collagen fibrils and in the ground substance. Subsequently, crystals are deposited within the fibrils themselves. The general calcification process is gradual, but the peritubular region becomes highly mineralized at a very early stage. The apatite crystals of dentin resembles those found in bone and cementum, but they are 300 times smaller than those found in enamel Calcospherite mineralization is seen occasionally along the pulp- predentin -forming front.

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