Dentin () (American English) or dentine ( or ) (British English) () is a calcified tissue of the body and, along with enamel, cementum, and pulp, is one of the four major components of teeth. It is usually covered by enamel on the crown and cementum on the root and surrounds the entire pulp. By volume, 45% of dentin consists of the mineral hydroxyapatite, 33% is organic material, and 22% is water. Yellow in appearance, it greatly affects the color of a tooth due to the translucency of enamel. Dentin, which is less mineralized and less brittle than enamel, is necessary for the support of enamel. Dentin rates approximately 3 on the Mohs scale of mineral hardness. There are two main characteristics which distinguish dentin from enamel: firstly, dentin forms throughout life; secondly, dentin is sensitive and can become hypersensitive to changes in temperature due to the sensory function of odontoblasts, especially when enamel recedes and dentin channels become exposed. Development Prior to enamel formation, dentine formation begins through a process known as dentinogenesis, and this process continues throughout a person's life even after the tooth has fully developed. Events such as tooth decay and tooth wear can also initiate dentine formation. Dentinogenesis is initiated by the odontoblasts of the pulp. Odontoblasts are specialised cells that lay down an organic matrix known as pre-dentine. This pre-dentine is subsequently mineralised into dentine. Mineralisation of pre-dentine begins at the dentino-enamel junction during tooth development and progresses towards the pulp of the tooth. After growth of pre-dentine and maturation into dentine, the cell bodies of the odontoblasts remain in the pulp, along its outer wall, and project into tiny tubules in the dentine. Pre-dentine is composed of 90% type I collagen and 10% non-collagenous proteins (including phosphoproteins, proteoglycans, growth factors, phosphatases such as alkaline phosphatase, and matrix metalloproteinases (MMPs)), and this composition is significantly altered when it is mineralised into dentine. See the Structure section for information about the composition of dentine. Structure Unlike enamel, dentin may be demineralized and stained for histological study. Dentin consists of microscopic channels, called dentinal tubules, which radiate outward through the dentin from the pulp to the exterior cementum or enamel border.<ref name="ross450">{{cite book | vauthors = Ross MH, Gordon IK, Wojciech P | date = 2003 | title = Histology: A Text and Atlas | edition = 4th | page = 450 | isbn = 978-0-683-30242-4}}</ref>