Nagashima Hiroshi, Sugahara Fumiaki, Watanabe Keisuke, Shibata Masahiro, Chiba Akina, Sato Noboru
Division of Gross Anatomy and Morphogenesis, Niigata University Graduate School of Medical and Dental Sciences, Niigata, Japan.
Division of Biology, Hyogo College of Medicine, Nishinomiya, Hyogo, Japan.
J Anat. 2016 Oct;229(4):536-48. doi: 10.1111/joa.12502. Epub 2016 Jun 9.
In fish, the pectoral appendage is adjacent to the head, but during vertebrate evolution a long neck region emerged via caudal relocation of the pectoral appendage. The pectoral appendage is comprised of endochondral portions, such as the humerus and the scapula, and a dermal portion, such as the clavicle, that contributes to the shoulder girdle. In the search for clues to the mechanism of the caudal relocation of the pectoral appendage, the cell lineage of the rostral lateral plate mesoderm was analyzed in chickens. It was found that, despite the long neck region in chickens, the origin of the clavicle attached to the head mesoderm ranged between 1 and 14 somite levels. Because the pectoral limb bud and the endochondral pectoral appendage developed on 15-20 and 15-24 somite levels, respectively, the clavicle-forming region corresponds to the embryonic neck, which suggests that the relocation would have been executed by the expansion of the source of the clavicle. The rostral portion of the clavicle-forming region overlaps the source of the cucullaris muscle, embraces the pharyngeal arches caudally, and can be experimentally replaced with the head mesoderm to form the cucullaris muscle, which implies that the mesodermal portion could have been the head mesoderm and that the clavicle would have developed at the head/trunk boundary. The link between the head mesoderm and the presumptive clavicle appears to have been the developmental constraint needed to create the evolutionarily conserved musculoskeletal connectivities characterizing the gnathostome neck. In this sense, the dermal girdle of the ganathostomes would represent the wall of the branchial chamber into which the endochondral pectoral appendage appears to have attached since its appearance in evolution.
在鱼类中，胸鳍附肢与头部相邻，但在脊椎动物进化过程中，胸鳍附肢通过向尾侧移位，出现了一个长的颈部区域。胸鳍附肢由软骨内部分（如肱骨和肩胛骨）和真皮部分（如锁骨）组成，锁骨对肩带的形成有贡献。为了寻找胸鳍附肢尾侧移位机制的线索，对鸡的头侧侧板中胚层的细胞谱系进行了分析。结果发现，尽管鸡有长的颈部区域，但附着于头部中胚层的锁骨起源于第1至14体节水平之间。由于胸鳍肢芽和软骨内胸鳍附肢分别在第15 - 20和15 - 24体节水平发育，形成锁骨的区域对应于胚胎颈部，这表明这种移位可能是由锁骨来源的扩展执行的。形成锁骨区域的头侧部分与颈头肌的来源重叠，向尾侧包围咽弓，并且可以通过实验用头部中胚层替代以形成颈头肌，这意味着中胚层部分可能曾经是头部中胚层，并且锁骨可能在头/躯干边界处发育。头部中胚层与假定锁骨之间的联系似乎是创建具有颌口类动物颈部特征的进化保守的肌肉骨骼连接所需的发育限制。从这个意义上说，颌口类动物的真皮肩带可能代表鳃腔的壁，自软骨内胸鳍附肢在进化中出现以来，它似乎就附着于此。
