The dentin (or substantia eburnea) composes the bulk of the crown and root of the tooth. Dentin is similar in hardness to |compact bone| tissue, found throughout the skeleton, and features about 30% organic tissue. Like the |enamel|, no cells or vessels penetrate the dentin, so that damage to the dentin (as in the case of a dental cavity) cannot be repaired. In some |teeth|, a secondary dentin may form as an accretive patch over damaged or occluded tooth tissue, however. Though no cells or vessels penetrate the dentin, small tubules do penetrate into the dentin |matrix| from the |pulp| cavity. Within these tubules are housed odontoblasts (tooth-forming cells) in a protoplasmic base. These tubules make the dentinal |matrix| sensitive to contact or temperature changes.
