Hydroxyapatite Cooperates with Whitlockite to Form the Whitlockite-Based Hypermineralized Hard Tissue of Chimaeroid Tooth Plates

Extant Chimaeroid fish (subclass: Holocephali) have unique dental organs, i.e., three pairs of tooth plates, instead of separate teeth like other animals. The mineral component of the hypermineralized tooth plate was reported as whitlockite (WH: Ca9MgHPO4(PO4)6), as for two species of Chimaeroid. Given the fact that the mineral phase of vertebrate hard tissues is hydroxyapatite (HAp: Ca5(PO4)3OH), WH-tissue is a characteristic found only in these species. Therefore, further study about the mineral of the hypermineralized tooth plate is required. Our present study of the tooth plate of adult and juvenile Chimaera phantasma and adult Hydrolagus mitsukuri confirmed that low-crystalline HAp formed as an initial phase. As the fish grew, WH became predominant, but HAp kept forming as a minor phase. The rates of WH and HAp were different by age and species. WH was synthesized biomimetically, and the growth process was compared with that of WH in the Chimaeroid tooth plate. Analysis of a series of products indicated that both Chimaeroid and synthetic WH were formed by the process similar to each other. Furthermore, SEM observation suggested that the fibrous substance in the matrix could be related to the formation of the WH-H tissue.