Dentin is made of roughly 70% mineral (by weight), 20% organic material, and 10% water. The mineral component is hydroxyapatite, a crystalline form of calcium phosphate. The organic portion is predominantly type I collagen fibers, with a smaller fraction of specialized proteins that help regulate how the tissue mineralizes. This combination makes dentin harder than bone but softer and more flexible than the enamel that covers it.
The Mineral Component
The mineral crystals in dentin are hydroxyapatite, the same calcium phosphate mineral found in enamel and bone. In dentin, these crystals take the form of tiny flattened plates, roughly 60 to 70 nanometers long, 20 to 30 nanometers wide, and only 3 to 4 nanometers thick. For comparison, enamel crystals are larger and arranged in tightly packed rods, which is why enamel is significantly harder. The smaller, more dispersed crystals in dentin give it a degree of flexibility that actually helps absorb chewing forces and protects the more brittle enamel above from cracking.
The Organic Matrix
About 90% of dentin’s organic content is type I collagen, the same protein that forms the structural scaffold of bone, skin, and tendons. These collagen fibers create a mesh that mineral crystals deposit onto and grow within, much like rebar inside concrete. Small amounts of other collagen types may also be present.
The remaining 10% of the organic matrix consists of non-collagenous proteins that play outsized roles despite their small quantity. These include phosphoproteins, proteoglycans, and various acidic glycoproteins. Together, they control where and how fast mineralization happens, essentially directing hydroxyapatite crystals to form in the right places and at the right density.
Thousands of Tiny Tubes
What makes dentin truly distinctive isn’t just what it’s made of, but how it’s organized. The tissue is riddled with microscopic channels called dentinal tubules that radiate outward from the pulp (the nerve-containing center of the tooth) toward the enamel. Each tubule contains fluid and a thin extension of the cell that originally built the dentin.
These tubules are not uniform. Near the pulp, they’re about 4.3 micrometers in diameter and packed closely together. Toward the outer surface, they narrow to around 2.4 micrometers. In permanent teeth, the average density is roughly 46,000 tubules per square millimeter. Primary (baby) teeth are even more densely packed, averaging about 124,000 per square millimeter.
This tubular structure is directly responsible for tooth sensitivity. The most widely accepted explanation, known as the hydrodynamic theory, holds that temperature changes, sweet foods, or a blast of air cause the fluid inside these tubules to shift. That tiny movement stimulates nerve endings at the pulp end, producing a sharp pain. The more open the tubules are (from worn enamel, receding gums, or a cavity), the more easily fluid moves and the more sensitive the tooth becomes.
How Dentin Forms
Dentin is produced by specialized cells called odontoblasts that line the inner surface of the tooth, right at the border of the pulp. These cells secrete the collagen-rich organic matrix first, creating a soft layer called predentin. Mineral crystals then gradually infiltrate this matrix, hardening it into mature dentin. The odontoblasts leave their long, thin extensions behind inside the tubules, which is why dentin remains a living tissue connected to the nerve supply even after it fully mineralizes.
Three Types of Dentin
Not all dentin in your tooth formed at the same time or under the same conditions.
- Primary dentin forms during tooth development, before the tooth erupts and begins functioning. It makes up the bulk of the tooth’s structure.
- Secondary dentin starts forming as soon as the tooth comes into contact with its opposing tooth and continues slowly throughout life. This gradual addition is why the pulp chamber inside your teeth gets smaller as you age.
- Tertiary dentin is produced on demand, as a defense response. When a tooth is damaged by decay, wear, or even irritation from dental materials, odontoblasts ramp up production to lay down an extra protective layer between the injury and the pulp. If the odontoblasts themselves are destroyed, backup cells from deeper in the pulp can step in, though the dentin they produce is less organized and sometimes resembles bone more than true dentin.
How Dentin Changes With Age
Dentin doesn’t stay the same over a lifetime. After about age 20, a process called physiological sclerosis begins: hydroxyapatite crystals slowly fill in the open spaces inside the dentinal tubules. This starts at the tip of the root, near the outer surface, and gradually progresses inward toward the pulp and upward toward the crown.
As the tubules become plugged with mineral, dentin grows denser and less permeable. This actually reduces sensitivity in older teeth, since fluid can no longer move as freely through the clogged tubules. The trade-off is that sclerotic dentin is more brittle. Research has documented roughly a 20% reduction in fracture toughness in sclerotic root dentin compared to normal dentin, which partly explains why older teeth are more prone to cracking.
You can sometimes see this aging process directly. When a cross-section of an older tooth root is held up to light, the sclerotic areas appear translucent because the filled tubules transmit light more evenly. This creates a distinctive butterfly-shaped pattern first described in the 1930s and still used by forensic scientists to estimate age from teeth.
Dentin vs. Enamel vs. Bone
Dentin sits between enamel and bone on the hardness spectrum. Enamel is about 96% mineral by weight, making it the hardest tissue in the body but also the most brittle. Bone is roughly 65% mineral, with more organic material and a blood supply that allows it to remodel and heal. Dentin, at 70% mineral, splits the difference: harder and more wear-resistant than bone, but with enough collagen to give it some flex and enough living connections to mount a biological defense when damaged.
Unlike bone, dentin cannot remodel itself. Once a section of dentin mineralizes, it stays in place permanently. The tooth can only add new dentin on the inner surface, never replace or reshape what already exists. This is a key reason why cavities don’t heal on their own: once decay destroys dentin, the body can wall off the damage with tertiary dentin but cannot regenerate the lost tissue.