Enamel is the hard outer coat of a tooth, and it is the hardest part of the human body, although it is made by cells called ameloblasts only before the tooth comes in. Once a tooth is in the mouth, mature enamel is not alive. It has no living cells and cannot feel or repair itself. It cannot grow back after it is formed. That is why protecting it matters, since the adult teeth that come after the baby teeth have to last for the rest of a person's life.

Dentin is the layer under the enamel, and unlike enamel it is made of living cells that make up the biggest part of a tooth, while dentin keeps forming throughout life, so it can thicken and repair, which enamel cannot do. Secondary dentin keeps forming as the years pass, and reparative dentin forms when the tooth is irritated or injured, which means that a tooth can repair its dentin in these ways even though it has no way to rebuild the enamel above it.

The soft pulp in the middle of a tooth holds its nerves and blood vessels, and the dentin around it is full of microscopic tubes called dentinal tubules, which hold fluid and the thread-like extension of a dentin-making cell. These tubes connect with the nerve in the pulp, and this is how heat, cold and decay cause pain. Because dentin is alive and can sense hot and cold, it is the part of a tooth that hurts when something is wrong.

Tooth decay can happen at any age, from childhood to old age, and as of November 2026 cavities are still one of the most common childhood health problems, with about half of children aged 6 to 9 in the US having had at least one. As of November 2026, nearly all adults over 65 have had a cavity too. Tooth care matters for life, and since early decay usually does not hurt, a dentist check is how it gets found.

Plaque is a sticky film of bacteria that keeps forming on your teeth, and when you eat or drink sugary foods, the bacteria in plaque make acid that attacks the enamel. One of these bacteria, Streptococcus mutans, turns sucrose into dextran, a sticky polysaccharide that helps it stick to the tooth surface, and it is the leading cause of tooth decay. Because sugar feeds the mouth bacteria that cause cavities, a review of the research finds that decay is strongly linked to diet.

Enamel, dentin and cementum are always swapping minerals with the mouth in a back and forth process called demineralization and remineralization. Before a cavity forms, this process can be reversed. The first sign of a cavity starting can be a white spot on a tooth, where acid has pulled minerals out of the enamel. At this early stage the damage can be reversed, because minerals from saliva and fluoride can go back into the enamel, so it may be possible to stop early decay with fluoride.

Fluoride works by strengthening the hard outer surface of a tooth, the enamel, so that it resists decay better. It reaches teeth in both children and adults from toothpaste and from tap water in many towns. Almost all water has some natural fluoride, but usually too little to prevent cavities, and the amount depends on the water source. The CDC does not require any town to add it, because state and local governments decide.

Water fluoridation began with a puzzle. Dentist Frederick McKay spent thirty years investigating the Colorado brown stain, a mottling of enamel, and found that stained teeth were also cavity-free, and a UK dentist, Norman Ainsworth, had reported the same link in 1925. Trendley Dean set out in 1931 to study fluoride's harm, but by 1950 he had shown that small amounts prevent cavities. Today more than 100 health organizations recognize the cavity-fighting benefits of fluoridated water.

Sealants work like raincoats for the chewing surfaces of back teeth. Molars are rough and uneven, so food and bacteria hide in them, and a sealant keeps them out, which is why sealants have been shown to cut the risk of decay in molars by nearly 80 percent. Your first adult molars come in around age 6 and your second molars around age 12. Dentists like to seal these new back teeth as soon as they come in, before decay has a chance to start.

Most vertebrates replace their teeth over and over during life, but people are an exception because we get only two sets. Baby teeth have roots, and the adult front teeth form behind them, so cells called odontoclasts break down the root of a baby tooth to make room for the adult tooth coming in. They dissolve the root with acids and enzymes and leave small pits along the root surface, and the process speeds up and slows down in phases.

Some animals replace their teeth again and again. A shark keeps growing new teeth all its life, and when a front tooth falls out, a fresh one from the rows behind moves forward like a conveyor belt. Some sharks shed about 35,000 teeth during their lives. A few mammals also keep getting new molars. The silvery mole rat of East Africa grows new ones at the back of its jaw that slowly move forward while the worn front ones are absorbed.

In a Tufts University study, damaged human tooth roots were filled with a gel carrying dental stem cells and implanted in rodents, and within weeks new cells and blood vessels grew. As of November 2026, researchers say this supports a possible future route to restoring natural tooth function.

As of November 2026, scientists are also testing ways to regrow the living pulp inside damaged teeth using stem cells. In a small trial of 40 children with injured adult teeth, 30 got stem cells taken from their own baby teeth, and blood flow rose in the treated teeth. The authors call it a first step, and growing whole teeth is not yet possible.

Words to know

• ameloblasts: enamel-making cells
• dentin: tooth middle layer
• fluoride: cavity-fighting mineral
• pulp: soft tooth center
• odontoclasts: root-dissolving cells