Dentin is a calcified tissue that forms the bulk of a tooth, lying directly beneath the outer layers of enamel on the crown and cementum on the root. This hard, yellowish material surrounds the central dental pulp chamber, which contains the tooth’s nerves and blood vessels. Dentin is a living tissue, unlike the non-cellular enamel, and its health is fundamental to the overall function and survival of the tooth.
Physical Structure and Chemical Makeup
Dentin is structurally complex, consisting of a mineralized matrix and an intricate network of microscopic channels. By weight, it is composed of approximately 70% inorganic material, primarily hydroxyapatite, the same mineral found in bone and enamel. The remaining material consists of 20% organic components, mostly Type I collagen, and 10% water. This composition gives it a slightly yellow hue and makes it less mineralized than enamel but harder than bone.
The most distinctive feature of dentin is the presence of millions of microscopic channels called dentinal tubules, which radiate outward from the pulp chamber. These tubules are widest near the pulp and narrow toward the enamel. Each tubule contains dentinal fluid and a cytoplasmic extension of an odontoblast cell, the specialized cell responsible for dentin formation. This tubular architecture defines dentin as a porous material, facilitating communication between the living pulp and the external environment.
Mechanical Function Supporting the Enamel
Dentin serves as the tooth’s primary structural buffer, providing the foundation for the extremely hard but brittle enamel layer. Enamel is the hardest substance in the human body, but it lacks the flexibility to withstand the pressures of chewing. Dentin possesses viscoelastic properties due to its lower mineral content and high collagen matrix, allowing it to exhibit resilience.
This resilience allows dentin to function as a shock absorber, distributing and dissipating the forces generated during mastication. When biting down, dentin acts like a cushion beneath the enamel, preventing the thin, rigid outer layer from cracking or fracturing under stress. This relationship is maintained by a hypermineralized, scalloped interface called the dentinoenamel junction, which locks the two tissues together.
Biological Function Protection and Sensitivity
The dentin-pulp complex functions as a singular biological unit, with dentin forming a protective barrier for the soft, innervated pulp tissue. This layer shields the pulp from external irritants such as bacteria, temperature changes, and chemical stimuli. The odontoblast processes and dentinal fluid within the tubules are integral to the tooth’s sensory ability.
When dentin loses its outer covering due to gum recession or enamel wear, the open tubules expose the fluid to the oral environment, leading to sensitivity. The hydrodynamic theory explains this sensation: external stimuli like cold air, sweet foods, or pressure cause a rapid shift in the dentinal fluid within the tubules. This fluid movement stimulates mechanoreceptors on the nerve fibers near the pulp, which the brain interprets as the characteristic sharp, transient pain of tooth sensitivity.
The Dynamic Capacity for Repair
Dentin is a dynamic, living tissue capable of continuous formation and change throughout life, distinguishing it from inert enamel. The odontoblasts continuously deposit a small amount of new dentin, known as secondary dentin, even after the tooth has fully formed. This slow deposition gradually narrows the pulp chamber over time, which is a natural aging process.
When the tooth is exposed to injury, such as deep decay or excessive wear, the dentin’s primary response is to form tertiary dentin, a localized defense mechanism. This rapidly deposited, less organized dentin is created by the odontoblasts to wall off the pulp chamber from the invading threat. The formation of this reparative dentin reduces the permeability of the dentin and protects the vulnerable pulp from inflammation or infection.