A tooth is built from four distinct tissues layered around each other, each with a different job. The outermost layer is enamel, a mineral shell that ranks 5 on the Mohs hardness scale, making it harder than iron or steel. Beneath that sits dentin, then a soft core called the pulp, and a thin coating on the root called cementum. A separate set of structures anchors the whole thing into your jawbone.
Enamel: The Outer Shield
Enamel is the white, visible part of a tooth and the hardest substance your body produces. It’s roughly 96% mineral by weight, with just 3% water and 1% organic matter. That mineral is a crystalline form of calcium phosphate called hydroxyapatite, arranged in tightly packed rods that run from the surface down toward the dentin underneath.
Because enamel is almost entirely mineral, it’s extremely resistant to the forces of chewing and biting. But it has a vulnerability: acid. Bacteria in your mouth feed on sugars and produce acids that dissolve hydroxyapatite crystals over time, which is how cavities start. Fluoride helps counter this by swapping into the crystal structure, replacing some of the hydroxyl groups in hydroxyapatite. The result is a modified mineral that’s harder, less soluble in acid, and more resistant to erosion. This is the basic chemistry behind fluoride toothpaste and fluoridated water.
One important detail: enamel contains no living cells. Once it forms, your body cannot regrow or repair it. The cells responsible for building enamel, called ameloblasts, do their work during tooth development and then disappear before the tooth ever breaks through the gum. Any damage to enamel is permanent.
Dentin: The Structural Core
Dentin makes up the bulk of a tooth’s structure. It sits directly beneath the enamel on the crown and beneath the cementum on the root, surrounding the pulp chamber like a thick wall. It’s about 70% mineral, 20% organic material, and 10% water by weight, making it significantly softer and more flexible than enamel. That flexibility is actually useful. It absorbs mechanical stress and prevents the brittle enamel above it from cracking under pressure.
The organic portion is almost entirely type I collagen, the same protein found in bone and skin. This collagen forms a scaffold that the mineral crystals deposit onto, giving dentin a combination of strength and slight give that pure mineral alone couldn’t achieve.
What sets dentin apart from enamel is that it’s alive. Thousands of microscopic channels called dentinal tubules run through it, each one housing a thin extension of a living cell. These tubules are about 2 to 4 micrometers wide, and there are roughly 18,000 to 21,000 of them packed into every square millimeter. The cells that maintain dentin, called odontoblasts, sit at the inner border where dentin meets the pulp. They can produce new dentin throughout your life, especially in response to damage or decay, which is why dentin can partially repair itself in ways enamel cannot.
Those tubules are also why exposed dentin is sensitive. When enamel wears away or gums recede, the tubules create a direct path for temperature changes and pressure to reach the nerve-rich pulp. That sharp sting from cold water hitting a sensitive tooth is fluid moving through dentinal tubules.
Pulp: The Living Center
At the core of every tooth is a soft tissue called the pulp, filling a hollow chamber that extends down through the root in narrow canals. This is the tooth’s lifeline. It contains blood vessels, nerves, and connective tissue populated by several types of cells, including the odontoblasts that maintain dentin, plus fibroblasts, immune cells like macrophages and mast cells, and undifferentiated stem cells.
Blood reaches the pulp through tiny openings at or near the tip of each root called apical foramina. Typically, one or two small arteries and a single vein enter through each root tip and branch into a dense capillary network inside the pulp chamber. This blood supply delivers oxygen and nutrients that keep the tooth’s living tissues healthy.
Two types of nerves run alongside those blood vessels. Sensory nerve fibers from branches of the trigeminal nerve (the main nerve of the face) detect pressure, temperature, and pain. Autonomic nerve fibers control blood flow by tightening or relaxing the walls of small arteries within the pulp. This is why tooth pain can be so intense: the pulp is packed with sensory nerve endings inside a rigid, enclosed space, so any inflammation creates pressure with nowhere to go.
Cementum: The Root Coating
Cementum is a thin, bone-like layer covering the outside of each root. It’s less mineralized than either enamel or dentin and often only a fraction of a millimeter thick. Its primary job is to serve as the attachment point for the fibers that hold your tooth in its socket.
There are two main types. Primary cementum forms during tooth development and covers most of the root surface. Secondary cementum forms later, particularly near the root tip, and continues to build up over your lifetime. Secondary cementum can adapt to changes in how you bite and chew, thickening in areas that experience more force. This adaptability helps your teeth stay stable even as wear patterns shift over the years.
The Periodontium: What Holds It All In
A tooth doesn’t sit rigidly fused to the jawbone. It’s suspended in its socket by four supporting structures collectively called the periodontium: cementum (described above), the periodontal ligament, alveolar bone, and gingival tissue.
The periodontal ligament is a thin band of connective tissue fibers that span the gap between cementum and the bone of the socket. These fibers act like tiny shock absorbers, cushioning each tooth against the forces of chewing. The ligament also contains cells that can remodel both the bone and cementum around it, which is how orthodontic braces work: sustained pressure triggers the ligament’s cells to break down bone on one side and build it up on the other, gradually shifting the tooth’s position.
Alveolar bone is the specific ridge of jawbone that forms the sockets teeth sit in. It exists only to support teeth. If a tooth is lost, the alveolar bone in that area slowly resorbs over time, which is why long-term tooth loss changes the shape of the jaw. The gingiva, or gum tissue, seals everything off from the outside, creating a barrier that protects the root, ligament, and bone from bacteria in the mouth.
How Tooth Types Differ
Adults have 32 permanent teeth (28 if the wisdom teeth are removed or never developed), and children grow a set of 20 primary teeth that start appearing around 4 to 6 months of age. All teeth share the same four-tissue structure, but they vary in shape to handle different tasks.
- Incisors (8): Flat, chisel-shaped front teeth designed for cutting and biting into food.
- Canines (4): Pointed teeth flanking the incisors, built for gripping and tearing.
- Premolars (8): Broader teeth with two cusps, used for crushing food. These exist only in the permanent set; children don’t have premolars.
- Molars (12): The largest teeth, positioned in the back of the mouth with wide, flat surfaces for grinding. This count includes the four wisdom teeth.
Front teeth typically have a single root with one pulp canal, while molars can have two or three roots, each with its own canal and apical foramen. The basic composition is identical across all types, but molars have proportionally thicker enamel and dentin to withstand the heavy grinding forces they absorb.
How a Tooth Builds Itself
Teeth form through a tightly coordinated exchange between two cell types. Odontoblasts, which develop from the dental mesenchyme (a type of embryonic connective tissue), begin laying down dentin by secreting a collagen scaffold that then mineralizes. As they deposit dentin, chemical signals from these cells trigger nearby epithelial cells to differentiate into ameloblasts, which then begin secreting the proteins that form enamel.
This process is sequential. Dentin always forms first, and enamel follows. The two cell types essentially signal back and forth: the collagen that odontoblasts produce physically anchors the developing ameloblasts in position, while proteins from the pulp side promote ameloblast maturation. Once enamel formation is complete, the ameloblasts degenerate. Odontoblasts, by contrast, survive for the life of the tooth, lining the inner wall of the pulp chamber and maintaining the ability to produce new dentin when needed.