Dentin is the mineralized tissue found underneath enamel that surrounds the dental pulp. It is composed mainly of hydroxyapatite crystals (70%) along with collagen (20%) and water (10%). Dentin formation (dentinogenesis) occurs in two stages - first, odontoblast cells secrete an unmineralized dentin matrix called predentin, then mineralization of the matrix occurs from the inside out in either a globular or linear pattern. Odontoblasts undergo differentiation, formation, and quiescence stages as they secrete predentin, retreat into the pulp canal leaving behind dentinal tubules, and reduce activity over time.
2. 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.
3. 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.
4. 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.
5.
6.
7. Formation of predentin
The 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
8. 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.
9. 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).
10. 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.
