n. The mineralized organic tissue that makes up the bulk of the tooth surrounding the pulp, covered on the root surface by cementum and on the crown surface by enamel. It is pale yellow in colour and is harder than bone but not as hard as enamel or cementum. Dentine is 70% inorganic by weight consisting mainly of calcium hydroxyapatite [Ca10(PO4)6(OH)2] and 20% organic by weight consisting mainly of collagen. The remaining 10% is water. Dentine is made up of many fine parallel tubules (dentinal tubules) extending from the pulpal surface to the amelodentinal junction. Each tubule contains an odontoblast cell lying in a layer on the pulpal surface with the nucleus situated at the pulpal end of each cell. Each odontoblast cell has a process extending along the tubule and is surrounded by intercellular ground substance. Dentine may be divided into intertubular dentine and peritubular dentine. Intertubular dentine is the main product of the odontoblasts constituting the largest volume of the dentine; it consists of a fibrous network of collagen with deposited mineral crystals. The peritubular (intratubular) dentine forms a highly mineralized sheath about 0.5–1.0µ thick around the dentinal tubule consisting mainly of crystals of carbonated apatite together with a small amount of collagen. The peritubular dentine is sensitive to various external stimuli although sensitivity is not uniform in either teeth or individuals. see dentine hypersensitivity. Dentine formation (dentinogenesis) begins at about the 14th week of intrauterine life (late bell stage) when the inner enamel epithelium induces cells at the periphery of the dental papilla to differentiate into dentine-forming columnar odontoblast cells. These cells secrete an unmineralized dentine matrix (predentine) as they retreat towards the pulp. When the predentine reaches a thickness of about 5µ mineralization starts to occur with the formation of spherical areas of calcium hydroxyapatite (calcospherites) which eventually fuse together to form the mineralized dentine layer (primary dentine), the outer layer being known as mantle dentine and the inner layer circumpulpal dentine. With age, more dentine continues to be laid down (secondary dentine) at the pulpal surface of the primary dentine, thus reducing the size of the pulp chamber. In response to caries, the odontoblasts lay down more dentine (tertiary or irregular secondary dentine) which contains fewer tubules and more mineral than primary dentine and appears transparent in ground section. It is therefore also known as transparent or sclerotic dentine. Tertiary dentine may be laid down by primary odontoblasts in response to a mild stimulus (reactionary dentine) or laid down by secondary odontoblasts derived from differentiated pulpal cells (reparative dentine). Porotic dentine in which the dentine becomes porous may occur in conjunction with vitamin C deficiency. See also caries.