A tooth is not a solid block of bone. It’s a layered structure with a hard mineral shell on the outside, a thick living tissue underneath, and a soft core of blood vessels and nerves at the center. Each layer has a distinct job, and together they make teeth one of the most durable yet sensitive structures in your body.
Enamel: The Outer Shield
The outermost layer of a tooth is enamel, and it’s the hardest substance your body produces. It ranks 5 on the Mohs hardness scale, the same rating as the mineral apatite, and well above your fingernails at 2.5. That hardness comes from its composition: enamel is roughly 96% mineral by weight, with just 3% water and 1% organic matter. The mineral is a crystalline form of calcium phosphate called hydroxyapatite, packed into tightly organized rods that can withstand decades of biting, chewing, and exposure to acids in food.
What enamel gains in strength, it loses in flexibility. It contains no living cells and cannot repair itself once damaged. Cracks, chips, and erosion from acid are permanent. This is why cavities that break through enamel don’t heal on their own.
Dentin: The Bulk of the Tooth
Beneath enamel sits dentin, a yellowish tissue that makes up most of the tooth’s volume. Dentin is softer than enamel but harder than bone, and unlike enamel, it’s alive. It’s produced by specialized cells called odontoblasts that line the inner surface of the dentin, right at the border with the pulp.
What makes dentin unusual is its structure. It’s threaded with millions of microscopic tubes called dentinal tubules, each filled with fluid and thin cellular extensions from the odontoblasts. These tubes run from the outer edge of the dentin all the way inward toward the nerve-rich pulp. When something hot, cold, or sweet reaches exposed dentin, it causes the fluid inside those tiny tubes to shift. That fluid movement triggers nerve endings near the pulp, which your brain registers as a sharp, sudden pain. This is why a chipped tooth or receding gum line can make you wince when you drink ice water. About 75% of people with dentin sensitivity report cold as the main trigger.
Teeth with sensitive dentin have tubules that are roughly eight times more numerous and wider in diameter than those in non-sensitive dentin. The wider the tube, the faster fluid moves through it, and the rate of fluid flow depends on the fourth power of the tube’s radius. Even a small increase in width dramatically increases sensitivity.
Dentin also has a defensive trick. When bacteria from a cavity begin invading the tubules, odontoblasts respond by producing a modified form of dentin called reactionary dentin. This emergency layer has fewer tubules, which slows bacterial movement toward the pulp. It’s an efficient stalling tactic, and it’s one reason cavities can take months or years to reach the nerve.
The Pulp: Nerves, Blood, and Living Tissue
At the very center of every tooth is the pulp, a soft tissue packed into a small chamber in the crown and extending down through narrow canals in each root. The pulp is what keeps the tooth alive. It contains blood vessels, nerves, and connective tissue, all working together to nourish the surrounding dentin and detect threats.
Each pulp chamber typically holds one to two small arteries and a single larger vein, which branch into a dense network of capillaries in the upper portion of the chamber. These vessels deliver oxygen and nutrients to the odontoblasts and other cells that maintain the tooth from within.
The nerve supply is equally intricate. Two types of nerve fibers run through the pulp. Sensory fibers branch off from the trigeminal nerve, the major nerve of the face, and are responsible for detecting temperature changes and pain. These fibers fan out beneath the odontoblast layer into a dense mesh, with some nerve endings extending partway into the dentinal tubules themselves. A second set of nerve fibers, part of the autonomic nervous system, wraps around the small arteries and controls blood flow within the pulp by tightening or relaxing the vessel walls.
The pulp also contains stem cells with remarkable versatility. These dental pulp stem cells can develop into multiple tissue types and are the subject of growing interest in regenerative medicine, with researchers exploring their potential for conditions ranging from spinal cord injuries to diabetes and liver disease. Even baby teeth contain a version of these stem cells, which is why some parents now choose to bank their children’s lost teeth.
The Root and Its Outer Coating
Below the gum line, the tooth narrows into one or more roots that anchor it in the jawbone. The root is made of the same dentin found in the crown, but instead of enamel on the outside, it’s covered by a thin layer of tissue called cementum.
Cementum is thinner near the gum line (50 to 200 micrometers, roughly the thickness of a sheet or two of paper) and thicker at the root tip, where it can reach 700 to 1,500 micrometers in molars. Its primary job is anchoring. Tiny collagen fibers, some as narrow as 1 to 2 micrometers wide, embed directly into the cementum on one end and into the jawbone on the other. These fibers form the periodontal ligament, a shock-absorbing hammock that holds the tooth in its socket while allowing slight movement during chewing. The ligament contains several types of collagen and runs in both radial and circumferential directions, creating a web-like suspension system around each root.
At the very tip of each root is a small opening called the apical foramen. This is the entry point where blood vessels and nerves pass from the jawbone into the pulp chamber. It’s the tooth’s lifeline to the rest of the body, and it’s also the reason a tooth infection can spread into surrounding bone and tissue.
How the Layers Work Together
Each layer depends on the others. Enamel protects dentin from the harsh environment of the mouth. Dentin cushions the pulp from impact and provides a second line of defense against bacteria. The pulp feeds the dentin and sounds the alarm when something goes wrong. Cementum and the periodontal ligament hold everything in place while absorbing the constant forces of biting and grinding.
When one layer fails, the others are compromised. A cavity that penetrates enamel exposes the softer, tubule-rich dentin, which lets bacteria advance more quickly toward the pulp. If the pulp becomes infected, it can no longer nourish the dentin, and the tooth becomes brittle and prone to fracture. A tooth that has had a root canal, where the pulp is removed entirely, survives on the structural integrity of its remaining dentin and whatever protection a crown or filling provides. It’s no longer alive, but it can still function for years.