Teeth are made of four distinct layers: enamel, dentin, cementum, and pulp. Unlike bone, which is living tissue that constantly regenerates, most of a tooth’s structure is mineralized and cannot repair itself once damaged. Each layer has a different composition and job, working together to let you bite, chew, and speak without thinking twice about it.
Enamel: The Outer Shield
The visible white part of your tooth is enamel, and it’s the hardest substance in the human body. About 95% of enamel is made of calcium and phosphate minerals, packed into a crystalline structure called hydroxyapatite. The remaining 5% is water and organic material. On the Mohs Hardness Scale, enamel scores a 5 out of 10, the same rating as bone and roughly half as hard as a diamond.
That extreme hardness is what lets you crack a nut or tear through tough food without damaging your teeth. But enamel has a major limitation: your body can’t grow more of it. During tooth development, specialized cells called ameloblasts build enamel layer by layer. Once a tooth fully emerges, those cells are gone for good. So when enamel wears down from acid exposure, grinding, or decay, the damage is permanent. This is why cavities don’t heal on their own the way a broken bone does.
Dentin: The Bulk of the Tooth
Beneath the enamel sits dentin, which makes up most of a tooth’s mass. Dentin is softer and slightly yellowish, composed largely of type I collagen and protein compounds along with hydroxyapatite minerals. Think of it as the structural backbone: collagen gives it flexibility so the tooth can absorb the force of chewing without shattering, while the mineral content keeps it firm.
Dentin is produced by cells called odontoblasts that line the inner surface of the dentin, right next to the pulp. Unlike ameloblasts, odontoblasts stick around throughout your life and can lay down new dentin in response to irritation or slow-progressing decay. This is a limited form of self-repair, not enough to fill a cavity, but enough to add a protective barrier if the tooth senses a threat. Dentin also contains microscopic tubes that run from the pulp outward. When enamel wears thin and exposes these tubes, hot, cold, or sweet foods can trigger sharp sensitivity because the stimulus travels through the tubes straight to the nerve.
Cementum: The Root Coating
You never see cementum because it covers only the root of the tooth, the portion buried below the gumline. It’s a thin, mineralized connective tissue made primarily of type I collagen and hydroxyapatite, similar in composition to both dentin and bone.
Cementum’s main job is anchoring the tooth to the jawbone. It does this through the periodontal ligament, a network of collagen fibers that stretches between the cementum and the bone socket. These fibers embed directly into the cementum (and into the bone on the other side), acting like tiny suspension cables that hold the tooth firmly in place while still allowing a slight amount of natural movement. That microscopic give is what keeps a tooth from cracking under the full force of your bite. When gum disease progresses and destroys cementum or the periodontal ligament, the tooth loosens, which is why advanced gum disease is a leading cause of tooth loss in adults.
Pulp: The Living Core
At the center of every tooth is the pulp, and it’s the only part that’s truly alive in the way most people think of living tissue. Pulp is a soft, jelly-like mass of connective tissue packed with blood vessels, nerves, and several types of immune cells. The blood vessels supply nutrients and oxygen to the tooth. The nerves are responsible for sensation, which is why a deep cavity or crack that reaches the pulp causes intense, throbbing pain rather than the mild zing of surface sensitivity.
Pulp also houses the odontoblasts that produce dentin, and immune cells that fight bacteria if infection reaches the interior. When bacteria do breach the pulp, the resulting infection can become an abscess. A root canal procedure removes the infected pulp entirely, and the tooth can survive without it because the surrounding bone and ligament still supply the outer structures. A tooth that has had its pulp removed becomes more brittle over time, though, since it no longer receives internal nutrients.
How Teeth Differ From Bone
People often assume teeth are bones, but the two are fundamentally different in one critical way: bones heal and teeth don’t. Bone is living tissue in a constant cycle of breakdown and rebuilding. When you fracture a bone, your body immediately forms a soft collagen callus at the break site, then gradually replaces it with new hard bone tissue. Teeth have no such mechanism. Once enamel chips or decay creates a hole, no biological process will fill it back in.
The composition is different too. Bone relies heavily on collagen for its framework, with calcium phosphate minerals providing rigidity. Enamel is almost entirely mineral with very little organic material, which is what makes it so hard but also so brittle compared to bone. Dentin and cementum fall somewhere in between, sharing characteristics with both enamel and bone. Another key distinction: bone contains its own blood supply woven throughout its structure, while a tooth concentrates all its living tissue in the pulp chamber at the center. Remove the pulp, and the tooth can still function. Remove the blood supply from a bone, and the bone dies.
What Damages These Layers
Each layer faces different threats. Enamel erodes when acids from bacteria, citrus foods, or stomach acid (from conditions like acid reflux) dissolve its mineral surface. This process is gradual and painless until enough enamel is lost to expose the dentin underneath. Grinding your teeth at night also wears enamel down mechanically over years.
Dentin decay progresses faster than enamel decay because dentin is softer and more porous. Once a cavity breaks through the enamel, it can spread quickly through the dentin toward the pulp. Cementum becomes vulnerable when gums recede, exposing the root surface to bacteria and acid it was never designed to handle on its own. Root cavities are common in older adults for exactly this reason.
Pulp damage is usually the final stage. Bacteria from deep cavities, cracks, or repeated dental procedures can inflame the pulp, a condition called pulpitis. Early inflammation is sometimes reversible if the source of irritation is removed. Once the pulp tissue dies, though, the only options are removing it or extracting the tooth entirely.