Tooth enamel is one of the four major tissues that make up the tooth in humans and many other animals, including some species of fish. It makes up the visible part of the tooth, covering the crown. The other major tissues are dentin, cementum, and dental pulp and it is a very hard, white to off-white, highly mineralised substance that acts as a barrier to protect the tooth but can become susceptible to degradation, especially by acids from food and drink. Enamel is the hardest substance in the body and contains the highest percentage of minerals, 96%, with water. The primary mineral is hydroxyapatite, which is a calcium phosphate. Enamel is formed on the tooth while the tooth is developing within the gum, once fully formed, it does not contain blood vessels or nerves. Remineralisation of teeth can repair damage to the tooth to a certain degree, the maintenance and repair of human tooth enamel is one of the primary concerns of dentistry. In humans, enamel varies in thickness over the surface of the tooth, often thickest at the cusp, up to 2.5 mm, the normal color of enamel varies from light yellow to grayish white. At the edges of teeth where there is no dentin underlying the enamel, since enamel is semitranslucent, the color of dentin and any material underneath the enamel strongly affects the appearance of a tooth. The enamel on primary teeth has a more crystalline form. The large amount of mineral in enamel accounts not only for its strength, Tooth enamel ranks 5 on Mohs hardness scale and has a Youngs modulus of 83 GPa. Dentin, less mineralized and less brittle, 3–4 in hardness, Enamel does not contain collagen, as found in other hard tissues such as dentin and bone, but it does contain two unique classes of proteins, amelogenins and enamelins. While the role of proteins is not fully understood, it is believed that they aid in the development of enamel by serving as a framework for minerals to form on. Once it is mature, enamel is almost totally without the softer organic matter, Enamel is avascular and has no nerve supply within it and is not renewed, however, it is not a static tissue as it can undergo mineralization changes. The basic unit of enamel is called an enamel rod, measuring 4–8 μm in diameter, an enamel rod, formally called an enamel prism, is a tightly packed mass of hydroxyapatite crystals in an organized pattern. In cross section, it is best compared to a keyhole, with the top, or head, oriented toward the crown of the tooth, the arrangement of the crystals within each enamel rod is highly complex. Both ameloblasts and Tomes processes affect the crystals pattern, Enamel crystals in the head of the enamel rod are oriented parallel to the long axis of the rod. When found in the tail of the rod, the crystals orientation diverges slightly from the long axis
Histologic slide showing a developing tooth. The mouth would be in the area of space at the top of the picture.
Histologic slide showing enamel formation
The effects of bruxism on an anterior tooth, revealing the dentin and pulp which are normally hidden by enamel