Dentin region of a human tooth with canals or dentinal tubules (dental canaliculi), animated coloured scanning electron micrograph (SEM). The dentin tubules (seen in cross section) have been formed by cytoplasmic extensions of the odontoblasts (dentin-producing cells). Dentin is mineralized connective tissue below the tooth enamel surface. It forms the majority of the tooth and the composition is similar to bone. The visible part of a tooth (the crown) consists of enamel covered dentin. Exposed dentin is sensitive to heat, cold and touch because it contains nerve endings. Magnification: x800 when shortest axis printed at 25 millimetres.