Hydroxyapatite Cooperates\nwith Whitlockite to Form\nthe Whitlockite-Based Hypermineralized Hard Tissue of Chimaeroid Tooth\nPlates

Extant Chimaeroid fish (subclass: Holocephali) have unique\ndental\norgans, i.e., three pairs of tooth plates, instead of separate teeth\nlike other animals. The mineral component of the hypermineralized\ntooth plate was reported as whitlockite (WH: Ca<sub>9</sub>MgHPO<sub>4</sub>(PO<sub>4</sub>)<sub>6</sub>), as for two species of Chimaeroid.\nGiven the fact that the mineral phase of vertebrate hard tissues is\nhydroxyapatite (HAp: Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>OH),\nWH-tissue is a characteristic found only in these species. Therefore,\nfurther study about the mineral of the hypermineralized tooth plate\nis required. Our present study of the tooth plate of adult and juvenile Chimaera phantasma and adult Hydrolagus\nmitsukuri confirmed that low-crystalline HAp formed\nas an initial phase. As the fish grew, WH became predominant, but\nHAp kept forming as a minor phase. The rates of WH and HAp were different\nby age and species. WH was synthesized biomimetically, and the growth\nprocess was compared with that of WH in the Chimaeroid tooth plate.\nAnalysis of a series of products indicated that both Chimaeroid and\nsynthetic WH were formed by the process similar to each other. Furthermore,\nSEM observation suggested that the fibrous substance in the matrix\ncould be related to the formation of the WH-H tissue.