A unique mineralization mode of hypermineralized pleromin in the tooth plate of Chimaera phantasma contributes to its microhardness

Abstract

Tooth plates of the chimaeroids, holocephalian fishes, are unique dental hard tissues. Unlike the teeth of other animals, the tooth plates are located on the roof of the mouth and in the lower jaw. Their tooth plates consist, to a large extent, of lightly mineralized tissue (osteodentin) and hypermineralized tissue (pleromin). Notably, the mineral phase of pleromin is whitlockite, while that of other animals is apatite. Dietary habits of chimaeroids and wearing features of their tooth plates suggest an extreme hardness of pleromin, but this has never been investigated. We examined the microhardness of the tooth plate of Chimaera phantasma and found that pleromin in the biting region was extremely hard, comparable with the hardness of mature tooth enamel, whereas the hardness of immature pleromin was lower than that of bovine dentin. The hardness of osteodentin, on the other hand, was equivalent to that of bovine dentin and almost the same throughout the tooth plate. Immature pleromin was sparsely packed with oval crystals of whitlockite and, as pleromin matures, the space between crystals was filled with small intercrystalline materials. The maturing process of pleromin could partly contribute to its remarkable hardness and have some implications for designing novel biomaterials.

Ecomorphological, space, and mineral relations of dermal denticles in angular angel shark (Squatina guggenheim)

“…However, we found other additional minerals, such as traces of magnesium and other minerals, consistent with whitlockite, a calcium phosphate so far only reported in the tooth plate of holocephalans (Chondrichthyes) (Johanson et al, 2020; Smith et al, 2019). Whitlockite has a comparable hardness of tooth enamel and has been associated with the hypermineralized portion of the tooth plate in holocephalans (Iijima & Ishiyama, 2020). Additionally, whitlockite is usually found in human kidney stones, and although it presents medium rigidity and porosity, its composition is similar to apatite rocks (Balaji & Menon, 1997).…”

Effects of age on secondary dentine mineralization and resin bonding capacity

“…These characteristics have no impact in the mechanical properties of primary dentine in younger and older age groups, but they signi cantly affect secondary dentine [12]. Elastic modulus, micro-and nano-hardness have been directly related with primary dentine mineralization [13][14][15][16][17]. However, there is limited information on the mineral changes of secondary dentine upon maturation with age, a procedure known to increase the mineral content and the mechanical properties of primary dentine [18].…”

Ontogenetic development of the holocephalan dentition: Morphological transitions of dentine in the absence of teeth

“…Individual teeth have been lost in evolution (Moy‐Thomas, 1936; Patterson, 1965; Lund, 1977; Didier, 1995; Stahl, 1999; Johanson et al, 2020a), and dental plates lack enamel, or enameloid, a cooperatively formed hard tissue between epithelium (ameloblasts) and ectomesenchyme (odontoblasts). As an alternative solution, the hypermineralized dentine, a unique tissue type from its crystal identity (Iijama & Ishiyama, 2020; Ishiyama & Sasagawa, 1984; Smith et al, 2019), provides different degrees of hardness throughout the dental plate. All dentine is derived from embryonic cranial ectomesenchyme (Miletich & Sharpe, 2004), including in fishes (Kundrát et al, 2008), so this holocephalan specialization is part of the repertoire of these cells determining all tissue morphologies.…”

“…Here, the contact between stem odontogenic neural crest cells in pulpal tissue and epithelium of the dental lamina can regulate position, timing and growth of the specialized hypermineralized dentine. As well, this dentine differs in mineral composition, being very high in magnesium (Mg), closest to the mineral whitlockite (Iijima & Ishiyama, 2020; Ishiyama & Sasagawa, 1984). We termed this dentine ‘whitlockin’, with a much lower percentage of hydroxyapatite compared with the less highly mineralized framework trabecular dentine (Smith et al, 2019).…”