Department of Earth Sciences, Natural History Museum, London, UK.
Institut Cavanilles de Biodiversitat i Biologia Evolutiva, Universitat de Valencia, Valencia, Spain.
J Anat. 2021 Sep;239(3):704-719. doi: 10.1111/joa.13445. Epub 2021 Apr 25.
Among the cartilaginous fishes (Chondrichthyes), the Holocephali are unique in that teeth are absent both in ontogeny and adult regenerative growth. Instead, the holocephalan dentition of ever-growing nonshedding dental plates is composed of dentine, trabecular in arrangement, forming spaces into which a novel hypermineralized dentine (whitlockin) is deposited. These tissue features form a variety of specific morphologies as the defining characters of dental plates in the three families of extant holocephalans. We demonstrate how this morphology changes through ontogenetic development with continuity between morphologies, through successive growth stages of the dentition represented by the dental plate. For example, rod-shaped whitlockin appears early, later transformed into the tritoral pad, including a regular arrangement of vascular canals and whitlockin forming with increasing mineralization (95%-98%). While the tritoral pads develop lingually, stacks of individual ovoids of whitlockin replace the rods in the more labial parts of the plate, again shaped by the forming trabecular dentine. The ability to make dentine into new, distinctive patterns is retained in the evolution of the Holocephali, despite the lack of teeth forming in development of the dentition. We propose that developmentally, odontogenic stem cells, retained through evolution, control the trabecular dentine formation within the dental plate, and transition to form whitlockin, throughout lifetime growth. Our model of cellular activity proposes a tight membrane of odontoblasts, having transformed to whitloblasts, that can control active influx of minerals to the rapidly mineralizing dentine, forming whitlockin. After the reduced whitloblast cells transition back to odontoblasts, they continue to monitor the levels of minerals (calcium, phosphate and magnesium) and at a slower rate of growth in the peritubate 'softer' dentine. This model explains the unique features of transitions within the holocephalan dental plate morphology.
在软骨鱼类（软骨鱼纲）中，全头鱼目是独一无二的，因为它们在个体发育和成年再生生长中都没有牙齿。相反，全头鱼目的牙齿由不断生长的非脱落牙板组成，由牙本质组成，呈小梁排列，形成空间，其中一种新型超矿化牙本质（惠特洛克林）被沉积。这些组织特征形成了现存全头鱼目三个科的牙板的各种特定形态，作为牙板的定义特征。我们展示了这种形态如何通过牙本质的连续发育阶段和牙板代表的牙本质的连续生长阶段而在个体发育过程中发生变化。例如，杆状的惠特洛克林早期出现，后来转化为三尖齿垫，包括血管通道的规则排列和惠特洛克林的形成，随着矿化程度的增加（95%-98%）。当三尖齿垫向舌侧发育时，由形成的小梁牙本质塑造的单个椭圆形的惠特洛克林堆积物取代牙板更唇侧部分的杆状牙本质。尽管在牙本质发育过程中没有牙齿形成，但全头鱼目中保留了将牙本质形成新的独特模式的能力。我们提出，从发育的角度来看，通过进化保留下来的牙源性干细胞控制牙板内小梁牙本质的形成，并在整个生命生长过程中向形成惠特洛克林转变。我们的细胞活动模型提出了一个紧密的成牙本质细胞细胞膜，已经转化为惠特洛布拉斯细胞，可以控制矿物质快速矿化牙本质的主动流入，形成惠特洛克林。当减少的惠特洛布拉斯细胞转回成牙本质细胞后，它们继续监测矿物质（钙、磷和镁）的水平，并以较慢的速度在管间的“较软”牙本质中生长。这个模型解释了全头鱼目牙板形态内部的独特转变特征。
