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Dentin is one of the four principal materials forming the tooth: enamel, dentine, cementum and pulp. It encloses the pulp chamber of the tooth. On the crown end it is enclosed by the enamel and on the root side, by cementum. It consists of a mixture of minerals and organic content. The mineral content (70%) consists of hydroxyapatite and calcium phosphate, while organic matter makes up 20%. Water makes up the remaining 10% of dentin. Dentin is porous and contains numerous tubules which extend outwards from the pulp, in the root region up to the cementum, and in the crown region up to the enamel, thus forming the dentiocementum junction (DCJ) and the dentinoenamel junction (DEJ) respectively.

Since Dentin is less mineralized than enamel it is neither hard nor as brittle as the enamel. The average diameter of the tubules is 2.5 micrometer near the pulp, about 1.2 nanometer at mid length, and about 0.9 nanometer near the enamel end. Tubule density is also more near the pulp, reducing by nearly 50% towards the outer end. The tubules contain the dentinal fluid which has a different composition than saliva.

Unlike the enamel, which is translucent, dentin is a yellowish material with a bone like structure. As a result the yellowish color of dentin has an effect on the tooth color. Three types of dentin are in each tooth. Primary dentin is the dominant type extending from the pulp to the enamel in two layers. The outer layer is the mantle dentin which is less mineralized than the inner layer or the circum-pulpal dentin. The latter is formed after the mantle dentin. Fresh dentin is known as pre-dentin and is not mineralized. Secondary dentin has a slower rate of growth and is produced after the root is completely formed. With time, growth of secondary dentin tends to shrink the pulp chamber. Tertiary dentin is formed only in response to external stimuli. Tertiary dentin can further be classified into two types: Reactionary dentin produced in case of a caries attack from an existing odontoblast; and reparative dentin from a fresh one replacing a dead odontoblast.