Dentin: Structure, Formation, and Classification

Overview of Dentin

• Dentin: Mineralized tissue forming the bulk of the tooth.
• Anatomical Junctions:
  • Dentino-enamel Junction: Junction with enamel.
  • Cemento-dentinal Junction: Junction with cementum.
• Forms walls of the pulp chamber and pulp canals.

Formation of Dentin

• Stage: Advanced Bell Stage during tooth development.
• Process:
  • Cells of the Dental Papilla differentiate into Odontoblasts.
  • Odontoblasts deposit dentin.
• Microscopic Observation:
  • Odontoblasts located adjacent to dentin in the pulp chamber.
  • Dentinal Tubules: Extend from the pulp chamber to the outer surface, containing odontoblastic processes.

Classification of Dentin

Dentin is classified into three types:

1. Primary Dentin

• Definition: Dentin formed before root completion.
• Formation Timeline: From Advanced Bell Stage to root completion.
• Types of Primary Dentin:
  • Mantle Dentin:
    • First formed dentin, located just beneath the dentino-enamel junction.
    • Around 20 microns wide.
    • Collagen Fibers: Type 3 collagen, 0.1 to 0.3 microns in diameter, known as Kroff's fibers.
    • Mineralization: Matrix vesicles, Globular Pattern, slower rate, fewer defects.
  • Circumpulpal Dentin:
    • Forms the bulk of dentin, includes coronal and radicular dentin.
    • Collagen Fibers: Type 1 collagen, 0.05 microns in diameter, randomly arranged and closely packed.
    • More mineralized than mantle dentin; defects like inter-globular dentin are common.
    • Mineralization: Both Globular and Linear patterns.

2. Secondary Dentin

• Definition: Dentin formed after root completion.
• Characteristics:
  • Formed until the death of pulp tissue.
  • Rate of deposition is slower than primary dentin.
  • Within the tooth, rate varies; higher at the roof and floor of the pulp chamber.

3. Tertiary Dentin

• Definition: Deposited in response to mechanical injury or pathology.
• Characteristics:
  • Rapid deposition; less organized structure.
  • Fewer or wider dentinal tubules; irregular course.
  • May entrap odontoblasts, forming osteodentin.
• Types:
  • Reactionary Dentin:
    • Formed when odontoblastic processes are injured but odontoblasts survive.
    • Surviving odontoblasts deposit tertiary dentin.
  • Reparative Dentin:
    • Formed when injury leads to degeneration of odontoblasts and their processes.
    • Undifferentiated mesenchymal cells from the cell-rich zone migrate and differentiate into odontoblasts to deposit this dentin.