Humans have two sets of teeth, the baby teeth (also called the primary teeth) and the permanent teeth. Children have 20 primary teeth; they are replaced by the permanent teeth by about age 13. Adults have 32 permanent teeth. A tooth is a hard calcified object. The surface of a tooth is formed from types of tissues that are very high in mineral content. These tissues are called enamel and dentin. Enamel The vast majority of the surface portion of a tooth that is visible is covered by enamel. You've probably heard that tooth enamel is the hardest tissue found in the human body. This is true. Enamel is over 95% mineral in composition. Most of this mineral is a compound called hydroxyapatite which, as you probably already know, has a high calcium content.   Dentin You may be surprised to learn that teeth are not solid enamel. Only the portion of a tooth nature intended to lie at and above the gum line is covered by enamel. The bulk of a tooth, both its root and inner aspects, is composed of another calcified material called dentin. Dentin also contains the mineral hydroxyapatite, but to a lesser degree than enamel. Only about two thirds of the content of dentin is mineral so, relatively speaking, dentin is "softer" than enamel.   Tooth Cross Section   Pulp This is the soft centre of the tooth. The pulp contains blood vessels and nerves; it nourishes the dentin. Root Canal Root canals are very small, thin divisions that branch off from the top pulp chamber down to the tip of the root. A tooth has at least one but no more than four root canals.