Enamel
Main article: Tooth enamel
Enamel is the hardest and most highly mineralized substance of the body and is one of the four major tissues which make up the tooth, along with dentin, cementum, and dental pulp.[7] It is normally visible and must be supported by underlying dentin. Ninety-six percent of enamel consists of mineral, with water and organic material composing the rest.[8] 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 mm, and thinnest at its border, which is seen clinically as the cementoenamel junction (CEJ).[9]

Enamel's primary mineral is hydroxyapatite, which is a crystalline calcium phosphate.[10] The large amount of minerals in enamel accounts not only for its strength but also for its brittleness.[11] Dentin, which is less mineralized and less brittle, compensates for enamel and is necessary as a support.[10] 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 framework support among other functions.[12]

Section through the ivory tusk of a mammothMain article: Tooth (animal)
Teeth vary greatly among animals. Some animals, such as turtles and tortoises, are toothless. Others, such as sharks, may go through many teeth in their lifetime. Walrus tusks are canine teeth that grow continuously throughout life.[75] Dog teeth are less likely than human teeth to form dental caries because of the very high pH of dog saliva, which prevents enamel from demineralizing.[76] Unlike humans whose ameloblasts die after tooth development, rodents continually produce enamel and must wear down their teeth by gnawing on various materials.[77] Horse teeth include twelve premolars, twelve molars, and twelve incisors. The structure of horse teeth is different from human teeth as the enamel and dentin layers are intertwined.[78]