Dentin, also spelled dentine, is a yellowish tissue that makes up the bulk of all teeth. It is harder than bone but softer than enamel and consists mainly of apatite crystals of calcium and phosphate. Dentin is the main supporting structure of the tooth and is the second hardest tissue in the body after enamel. It is 70% mineral and acellular, as hydroxyapatite crystals, and 30% organic as water, collagen, and mucopolysaccharide. The main structure of dentin is the dentinal tubule, which extends from the external surface to the pulp. There are approximately 30,000 - 40,000 tubules per square millimeter, which can transmit pain to the pulp if the dentin is exposed. Dentin is capped by a crown made of highly mineralized and protective enamel, and in the root, it is covered by cementum. Dentin is best known for its occurrence in teeth, but in early vertebrates, it was an important part of the dermal skeleton that covered most of the body, and it persists today in a few taxa such as the coelacanth.