Department of Earth Sciences, Natural History Museum London, London, UK.
Centre for Craniofacial and Regenerative Biology, Oral and Craniofacial Sciences King's College London, London, UK.
J Fish Biol. 2020 Jul;97(1):16-27. doi: 10.1111/jfb.14302. Epub 2020 Apr 26.
All extant holocephalans (Chimaeroidei) have lost the ability to make individual teeth, as tooth germs are not part of the embryonic development of the dental plates or of their continuous growth. Instead, a hypermineralized dentine with a unique mineral, whitlockin, is specifically distributed within a dentine framework into structures that give the dental plates their distinctive, species-specific morphology. Control of the regulation of this distribution must be cellular, with a dental epithelium initiating the first outer dentine, and via contact with ectomesenchymal tissue as the only embryonic cell type that can make dentine. Chimaeroids have three pairs of dental plates within their mouth, two in the upper jaw and one in the lower. In the genera Chimaera, Hydrolagus and Harriotta, the morphology and distribution of this whitlockin within each dental plate differs both between different plates in the same species and between species. Whitlockin structures include ovoids, rods and tritoral pads, with substantial developmental changes between these. For example, rods appear before the ovoids and result from a change in the surrounding trabecular dentine. In Harriotta, ovoids form separately from the tritoral pads, but also contribute to tritor development, while in Chimaera and Hydrolagus, tritoral pads develop from rods that later are perforated to accommodate the vasculature. Nevertheless, the position of these structures, secreted by the specialized odontoblasts (whitloblasts), appears highly regulated in all three species. These distinct morphologies are established at the aboral margin of the dental plate, with proposed involvement of the outer dentine. We observe that this outer layer forms into serially added lingual ridges, occurring on the anterior plate only. We propose that positional, structural specificity must be contained within the ectomesenchymal populations, as stem cells below the dental epithelium, and a coincidental occurrence of each lingual, serial ridge with the whitlockin structures that contribute to the wear-resistant oral surface.
所有现存的全头类（海龙目）都失去了生成个体牙齿的能力，因为牙原基不是牙板胚胎发育或连续生长的一部分。相反，一种超矿化的牙本质具有独特的矿物质，即 whitlockin，专门分布在牙本质框架内，形成赋予牙板独特的、种特异性形态的结构。这种分布的调控必须是细胞性的，牙上皮细胞首先生成最外层的牙本质，然后通过与唯一能够生成牙本质的外间充质组织接触。全头类在其口腔内有三对牙板，上颚两对，下颚一对。在 Chimaera、Hydrolagus 和 Harriotta 属中，同一物种不同牙板之间以及不同物种之间，这种 whitlockin 在每个牙板中的形态和分布都存在差异。Whitlockin 结构包括卵形体、棒状体和三齿垫，在这些结构之间存在着显著的发育变化。例如，棒状体先于卵形体出现，是由于周围小梁牙本质的变化所致。在 Harriotta 中，卵形体与三齿垫是分开形成的，但也有助于三齿的发育，而在 Chimaera 和 Hydrolagus 中，三齿垫是由后来穿孔以容纳脉管的棒状体发育而来的。然而，这些结构的位置，由专门的成牙本质细胞（whitloblasts）分泌，在所有三个物种中似乎都受到高度调控。这些独特的形态是在牙板的口侧边缘建立的，可能涉及到外牙本质。我们观察到，这种外层形成了连续的舌侧嵴，仅在前牙板上出现。我们提出，位置和结构的特异性必须包含在外间充质群体中，因为位于牙上皮细胞下方的干细胞，以及每个舌侧、连续嵴与有助于耐磨损口腔表面的 whitlockin 结构的偶然发生，都必须具有位置和结构特异性。
