Dentin is the foundational tissue that makes up the largest proportion of the tooth structure, lying immediately beneath the hard outer shell of enamel in the crown and cementum in the root. Unlike the highly mineralized and inert enamel, dentin is a living tissue that continues to form throughout life, maintaining a close connection with the innermost pulp chamber. This characteristic allows it to respond to damage and changes in the oral environment.
Dentin’s Place in the Tooth Structure
This tissue is strategically positioned between the heavily protected outer surface of the tooth and the delicate, soft pulp at its core, creating a protective layer for the nerves and blood vessels within. The most distinguishing feature of dentin is its microscopic structure, which is permeated by millions of tiny, fluid-filled channels known as dentinal tubules. These tubules radiate outward from the pulp chamber to the junction with the enamel or cementum, following a slight S-shaped curve in the crown.
The dentinal tubules house the long, slender cytoplasmic extensions of specialized cells called odontoblasts, whose main bodies line the inner wall of the pulp. The number and diameter of these tubules are not uniform, being significantly wider and more densely packed closer to the pulp. This internal network allows for a degree of permeability.
The Chemical Makeup of Dentin
The composition of dentin provides a balance between strength and resilience, achieved through a blend of mineral, organic material, and water. By weight, dentin is composed of approximately 70% inorganic material, 20% organic matrix, and 10% water. This makes it substantially less mineralized and therefore softer than enamel, which is about 95% inorganic content.
The inorganic component consists primarily of hydroxyapatite crystals, which are calcium phosphate compounds that give the dentin its hardness. These crystals are smaller and less tightly packed than those found in enamel, contributing to dentin’s relative flexibility. The organic framework is largely made up of Type I Collagen, accounting for about 90% of the matrix. This protein provides a strong, flexible scaffold that prevents the brittle mineral phase from shattering under the forces of chewing.
Essential Functions of Dentin
The unique blend of hard mineral and flexible collagen enables dentin to perform several functions. One primary role is to act as a shock absorber, cushioning the overlying enamel from fracture. The elasticity of the collagen-rich dentin helps to dissipate the powerful forces generated during biting, preventing the more brittle enamel from cracking.
Dentin also functions as a physical barrier, protecting the inner pulp chamber from external threats like bacteria and thermal changes. When the tooth is subjected to injury or decay, the odontoblast cells can respond by producing secondary or tertiary dentin, effectively walling off the pulp to ensure its continued viability. This regenerative capability is a hallmark of dentin as a living tissue.
Finally, dentin is responsible for the sensation and sensitivity experienced by the tooth, a process explained by the hydrodynamic theory. External stimuli, such as cold air or sugary foods, cause a rapid shift in the fluid contained within the dentinal tubules. This fluid movement excites mechanoreceptors on the nerves located in the pulp, which the brain interprets as sensitivity.