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 normally visible part of the tooth, covering the crown. The other major tissues are dentin, cementum, and dental pulp. 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. Calcium hardens the tooth enamel. In rare circumstances enamel fails to form, leaving the underlying dentin exposed on the surface.
Enamel is the hardest substance in the human body and contains the highest percentage of minerals (at 96%), with water and organic material composing the rest. The primary mineral is hydroxyapatite, which is a crystalline calcium phosphate. Enamel is formed on the tooth while the tooth develops within the jaw bone before it erupts into the mouth. Once fully formed, enamel does not contain blood vessels or nerves, and is not made of cells. Remineralisation of teeth can repair damage to the tooth to a certain degree but damage beyond that cannot be repaired by the body. 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, and thinnest at its border with the cementum at the cementoenamel junction (CEJ).
The normal color of enamel varies from light yellow to grayish (bluish) white. At the edges of teeth where there is no dentin underlying the enamel, the color sometimes has a slightly blue or translucent off-white tone, easily observable on the upper incisors. 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 opaque crystalline form and thus appears whiter than on permanent teeth.
The large amount of mineral in enamel accounts not only for its strength but also for its brittleness. Tooth enamel ranks 5 on Mohs hardness scale (between steel and titanium) and has a Young's modulus of 83 GPa. Dentin, less mineralized and less brittle, 3–4 in hardness, compensates for enamel and is necessary as a support. On radiographs, the differences in the mineralization of different portions of the tooth and surrounding periodontium can be noted; enamel appears lighter than dentin or pulp since it is denser than both and more radiopaque.
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 these 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, among other functions. 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.
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