Dentin 1 (dentin structure) l Oral histology and biology MCQs for dental students

 Dentin 1 (dentin structure) l Oral histology and biology MCQs for dental students 

 

Dentin

 

Definition :

• Dentin is the mineralized tissue that forms the bulk of the tooth.

 

Physical properties:

• Color: light yellowish.
• Hardness:

1. It is elastic to provide flexibility which prevent  fracture of the overlying brittle enamel.
2. It is less hard than enamel, harder than cementum and bone.

• Thickness: 3 -10mm or even more.
• Radiograph: more radiolucent than enamel, more  radiopaque than cementum and bone.

 

Chemical properties:

1. Inorganic component (70%): consists mainly of calcium hydroxyapatite crystals.
2. Organic component (30%): consists mainly of collagen fibrils embedded in an amorphous ground substance (20%) and water (10%)

 

DENTINOGENESIS

• The formation of dentin begins when the tooth germ reaches the bell stage.  

• The dental papilla is forming the dentin from the ectomesenchymal cells separated from the inner dental epithelium by cell free zone.

• With induction, the peripheral cells of dental papilla differentiate into odontoblasts.

 

Life cycle of odontoblasts

1. Undifferentiated mesenchymal cell (Pre-odontoblasts):

• Small ovoid cells with few rough endoplasmic reticulum (RER) and poorly developed Golgi apparatus.

2. Fully differentiated Odontoblast (Secretory odontoblast):

• The cells increase in length.

Light microscope :

• it is large cell with an open-faced nucleus with a basophilic cytoplasm.

Electron microscope :

• it shows the 5 criteria of active cell.
• As the dentin matrix is formed, the odontoblasts move towards the pulp leaving a cytoplasmic process behind.

3. Resting odontoblasts:

• After dentin formation and mineralization is completed, the cells enter a rest state.
• They produce dentin in a very slow rate but may be reactivated after injury.
• The cells undergo reduction in length and cytoplasmic organelles with increase in lysosome and phagosome.

Formation of dentin matrix

Formation of Mantle dentin:

1. After odontoblasts differentiated, it became a protein forming cell (active cell).
2. Nucleus (proximal end) → Rough Endoplasmic Reticulum → Golgi complex → secretory vesicles (distal end) → type I collagen (thick vonkorff’s fibers) → arranged perpendicular on dentin surface → mantle dentin.

Formation of circumpulpal dentin:

• After the matrix is formed, the odontoblasts begin to move towards the pulp leaving a cytoplasmic process behind it called Odontoblastic process (Tom’s fiber). Occasionally, one of them may penetrate the basement membrane and appears between the cells of the inner dental epithelium as Enamel Spindle.
• Once the layer of mantle dentin is formed, Dentinogenesis continue to form the circumpulpal dentin which forms the bulk of the dentin.

 

Mantle Dentin

Thickness

•  10-20 M

Diameter of collagen fibers

• Large (0.1-0.2 um)

Direction of collagen fibers

• Perpendicular to DEJ and parallel to the dentinal tubules

Ground substance :

• Pre- existing from dental papilla cell free zone and the odontoblasts.

Mineralization:

• linear form (contains matrix vesicles).

Circumpulpal Dentin

Thickness

• Bulk of the tooth

Diameter of collagen fibers

• Small (0.05um)

Direction of collagen fibers

• Parallel to DEJ and perpendicular to the dentinal tubules

Ground substance :

• Formed by the odontoblasts only

Mineralzation:

• Globular below the mantle dentin then become mixed in the remaining circumpulpal dentin (no matrix vesicles ).

 

 

Mineralization of dentin

• A constant layer (10-20 μm) of dentin matrix (predentin) remains adjacent to the odontoblast.
• Formation and mineralization begins at the tip of cusps or incisal ridge and proceed cervically.
• After the odontoblasts form a wide band of predentin, they secrete the matrix vesicles (rich in hydroxy apatite crystals) into the extra cellular organic matrix.

 

The dentin mineralization follows three different patterns

• Linear calcification: deposition of crystals directly with no interruption front occurred in mantle dentin.
• Globular calcification: deposition of crystals in several areas of the matrix in globules which enlarge and fuse to form a single mass, usually present in circumpulpal dentin just below mantle dentin.
• Combined pattern: of both types as in the rest of circumpulpal dentin.

Age changes of dentin:

1-Vitality:

• decrease with old age.

2-Attrition:

• May there is attrition of dentin.

3-Permeability:

• If the calcification in dentin is happened, permeability will decrease.

4-Secondary dentin:

• Definition: Any dentin that formed after complete root formation.
• Primary dentin is larger than secondary dentin.
• It occurs on the entire pulpal surface.
• Dentinal tubules in primary dentin are larger than in secondary dentin.
• Course of dentinal tubules is regular in primary dentin while it is more irregular in secondary dentin where dentinal tubules change their direction to a more wavy course.

5-Transparent/sclerotic dentin:

• Sclerotic means high calcification.
• Happens due to mild stimulus.
• Calcification occurs intratubular.
• Appears translucent/white by transmitted light.
• Appears black by reflected light.

6-Dead tracts:

• Happens due to severe stimulus.
• Severe stimulation leads to destruction of the odontoblastic processes and odontoblasts. This leads to empty and wide (dead) dentinal tubules called dead tracts.
• We use transmitted light to see dead tracts.
• Appears black by transmitted light.
• Appears white by reflected light.
• Under the dead tracts from the pulpal surface, reparative dentin will be formed to protect pulp.
• The dead tract is surrounded by sclerotic dentin to protect the surrounding dentinal tubules and the odontoblastic processes.

7-Reparative or tertiary dentin:

• Happens due to severe stimulus.

Types of reparative dentin:

• Osteodentin (entrapped cells).
• Vasodentin: (entrapped blood vessels).
• Atubular dentin: (area without dentinal tubules).

Clinical consideration of dentin :

1. The rapid penetration & spread of caries in the dentin is the result of the tubule system in the dentin.
2. The dentinal tubules form a passage for invading bacteria that may thus reach the pulp through a thick dentinal layer.
3. Air driven cutting instruments cause dislodgement of the odontoblasts from the periphery of the pulp & their aspiration within the dentinal tubule.

 

Incremental lines of dentin :

1- Imbrication or von Ebner lines:

• It appear as fine lines, which in cross section run at right angles to the Dentinal tubules .The course of the lines indicates the growth pattern of the Dentine. The distance between the lines corresponds to the daily rate of opposition, which in crown varies from 4-8 µm and becomes decreasingly less as root formation Progress

2- Counter lines of owens;

• Its hypocalcified line, it distinguish in longitudinal ground section as accentuated few lines. These lines arises due to disturbances in Dentin matrix and mineralizing process.

3- Neonatal lines:

• This line separating between prenatal and postnatal Dentin and mostly found in deciduous and first permanent molar. This line is the result of incomplete calcification, due to metabolic disturbances at the time of birth to the abrupt changes in environment and nutrition.