hist 4 pract

Dentin formation

(Dentinogenesis)

By:dr.Nafha Shihab. (periodontist)

Dentin is a mineralized, elastic, yellowish- white, avascular tissue enclosing the central pulp chamber. Dentin is characterized by multiple closely packed dentinal tubules that traverse its entire thickness and contain the cytoplasmic extensions of odontoblasts that once formed.

Dentine is composed of 70% inorganic material (hydroxyapatite in form of small plate) 20% Organic material ( about 90% collagen mainly type I with small amount

of type III and type V fibers, noncollagenous matrix proteins and lipids)

10% water.

Life cycle of odontoblasts

1- Differentiating stage: •The undifferentiated peripheral cells of dental follicle are spindle and separated by great amount of ground substance.

•In the early bell stage, under the inductive influence of the inner enamel epithelium, the peripheral ectomesenchymal cells differentiate into preodontoblasts.

•They assume to a columnar shape and aligned as a single row along the basement membrane. Several projections (odontobtastic process) arise from the upper part of the cells.

•The nuclei become basally oriented toward the pulp. The cells grow in length to become columnar. Now the fully differentiated odonroblasts begin their work.

2- Formative stage:

Concentration of the cell organelles, granular components and globular elements. Production of the first amount of dentin (dentin matrix). The odontoblasts retreat from the basement membrane leaving a single odontoblastic process which become enclosed in the dentinal tubule (Tomes' fiber). With successive deposition of dentin, tubule and odontoblastic process (Tomes' fiber) grow in length.

3- Quiescent stage: Odontoblasts decrease in size and function. The dentin formation is reduced. They produce now secondary and reparative dentin.

Formation of predentinThe odontoblasts begin secreting an organic matrix around the area directly adjacent to the internal enamel epithelia, closest to the area of the future cusp of a tooth which contains collagen fibers (Korff's fibers).

The odontoblasts begin to move toward the center of the tooth, forming an extension called the odontoplastic processes. Thus, dentin formation proceeds toward the inside of the tooth.

The odontoblast process causes the secretion of hydroxyapatite crystals and mineralization of the matrix. This area of mineralization is known as mantle dentin

Dentinogenesis occurs in two stages:• Secretion of dentin matrix: odontoblast cells begin to secrete an unmineralized dentine matrix which is termed predentine. As more dentine matrix is deposited, the odontoblast cells retreat in the direction of the pulp leaving an elongated process known as the odontoblastic process. A narrow layer of predentine is always present on the surface of the pulp.

• Mineralization of dentin: it begins when the predentine is approximately 5 µm thick. Spherical zones of hydroxyapatite called calcospherites are formed within the dentine matrix. Mineralization of the dentine matrix starts at random points and eventually these calcospherites fuse together to form mineralized dentine.

Maturation of Predentin:

•It begins at the tip of the crown and then it proceeds in a rhythmic pattern to gradually complete cervically.

• Mineral phase first appears within the matrix vesicles as single crystals. Crystal (calcospherites) grow and rupture and fuses with adjacent crystals to form a continuous layer of mineralized matrix.

• The maturation goes then in linear or occasionally globular pattern in the remnant thickness of dentin (circumpulpal dentin).

Pattern of mineralization in dentin:

1- Globular mineralization Occur usually in mantle dentin

region and in some area of circumpulpal dentin. Deposition of crystals in several discrete areas of matrix.

2- Linear calcification:Occur in circumpulpal

dentin.When rate of formation progress slowly, mineralization front appears more uniform.

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