Johanson Zerina, Boisvert Catherine, Maksimenko Anton, Currie Peter, Trinajstic Kate
Department of Earth Sciences, Natural History Museum, Cromwell Road, London, SW7 5BD, United Kingdom.
Australian Regenerative Medicine Institute (ARMI), EMBL Australia Building 75, Level 1 Monash University, Clayton, Victoria, 3800, Australia.
PLoS One. 2015 Sep 4;10(9):e0135138. doi: 10.1371/journal.pone.0135138. eCollection 2015.
The synarcual is a structure incorporating multiple elements of two or more anterior vertebrae of the axial skeleton, forming immediately posterior to the cranium. It has been convergently acquired in the fossil group 'Placodermi', in Chondrichthyes (Holocephali, Batoidea), within the teleost group Syngnathiformes, and to varying degrees in a range of mammalian taxa. In addition, cervical vertebral fusion presents as an abnormal pathology in a variety of human disorders. Vertebrae develop from axially arranged somites, so that fusion could result from a failure of somite segmentation early in development, or from later heterotopic development of intervertebral bone or cartilage. Examination of early developmental stages indicates that in the Batoidea and the 'Placodermi', individual vertebrae developed normally and only later become incorporated into the synarcual, implying regular somite segmentation and vertebral development. Here we show that in the holocephalan Callorhinchus milii, uniform and regular vertebral segmentation also occurs, with anterior individual vertebra developing separately with subsequent fusion into a synarcual. Vertebral elements forming directly behind the synarcual continue to be incorporated into the synarcual through growth. This appears to be a common pattern through the Vertebrata. Research into human disorders, presenting as cervical fusion at birth, focuses on gene misexpression studies in humans and other mammals such as the mouse. However, in chondrichthyans, vertebral fusion represents the normal morphology, moreover, taxa such Leucoraja (Batoidea) and Callorhinchus (Holocephali) are increasingly used as laboratory animals, and the Callorhinchus genome has been sequenced and is available for study. Our observations on synarcual development in three major groups of early jawed vertebrates indicate that fusion involves heterotopic cartilage and perichondral bone/mineralised cartilage developing outside the regular skeleton. We suggest that chondrichthyans have potential as ideal extant models for identifying the genes involved in these processes, for application to human skeletal heterotopic disorders.
头环椎是一种整合了轴骨骼两个或更多前椎骨多个元素的结构，形成于颅骨后方紧邻处。它在化石类群“盾皮鱼纲”、软骨鱼纲（全头亚纲、鳐总目）、硬骨鱼纲海龙目内趋同演化形成，并且在一系列哺乳动物类群中也有不同程度的出现。此外，颈椎融合在多种人类疾病中表现为一种异常病理状态。椎骨由轴向排列的体节发育而来，因此融合可能是由于发育早期体节分割失败，或者是由于后期椎间骨或软骨的异位发育。对早期发育阶段的研究表明，在鳐总目和“盾皮鱼纲”中，单个椎骨正常发育，只是后来才并入头环椎，这意味着体节分割和椎骨发育正常。在这里我们表明，在全头亚纲的澳洲银鲛中，也会出现均匀且规则的椎骨分割，前部单个椎骨先单独发育，随后融合形成头环椎。直接在头环椎后方形成的椎骨元素会通过生长持续并入头环椎。这似乎是脊椎动物中的一种常见模式。对出生时表现为颈椎融合的人类疾病的研究，主要集中在人类和其他哺乳动物（如小鼠）的基因错误表达研究上。然而，在软骨鱼类中，椎骨融合代表正常形态，此外，白斑鳐属（鳐总目）和澳洲银鲛属（全头亚纲）等类群越来越多地被用作实验动物，并且澳洲银鲛的基因组已经测序并可供研究。我们对早期有颌脊椎动物三大类群头环椎发育的观察表明，融合涉及在正常骨骼之外异位发育的软骨和软骨膜骨/矿化软骨。我们认为，软骨鱼类有潜力成为识别参与这些过程的基因的理想现存模型，可应用于人类骨骼异位疾病。
