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( Tooth enamel)
Tooth enamel is the hardest and most highly mineralized substance of the body,[1] and with dentin, cementum, and dental pulp is one of the four major tissues which make up the tooth. It is the normally visible dental tissue of a tooth and must be supported by underlying dentin. Ninety-six percent of enamel consists of mineral, with water and organic material composing the rest.[2] 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, the color sometimes has a slightly blue tone. Since enamel is semitranslucent, the color of dentin and any restorative dental material underneath the enamel strongly affects the appearance of a tooth. Enamel varies in thickness over the surface of the tooth and is often thickest at the cusp, up to 2.5&_160;mm, and thinnest at its border, which is seen clinically as the cementoenamel junction (CEJ).[3] Enamel's primary mineral is hydroxylapatite, which is a crystalline calcium phosphate.[4] The large amount of minerals in enamel accounts not only for its strength but also for its brittleness.[5] Tooth enamel ranks 5 on Mohs hardness scale. Dentin, less mineralized and less brittle, 3–4 in hardness, compensates for enamel and is necessary as a support.[4] Unlike dentin and bone, enamel does not contain collagen. Instead, it has two unique classes of proteins called 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 support, among other functions.[6] The basic unit of enamel is called an enamel rod.[4] Measuring 4–8&_160;µm in diameter an enamel rod, formerly called an enamel prism, is a tightly packed mass of hydroxyapatite crystals in an organized pattern.[1] In cross section, it is best compared to a keyhole, with the top, or head, oriented toward the crown of the tooth, and the bottom, or tail, oriented toward the root of the tooth.
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