Vertebrate extraocular muscles (EOMs) function in eye movements. The EOMs of modern jawed vertebrates consist primarily of four recti and two oblique muscles innervated by three cranial nerves. The developmental mechanisms underlying the establishment of this complex and the evolutionarily conserved pattern of EOMs are unknown. Chondrichthyan early embryos develop three pairs of overt epithelial coeloms called head cavities (HCs) in the head mesoderm, and each HC is believed to differentiate into a discrete subset of EOMs. However, no direct evidence of these cell fates has been provided due to the technical difficulty of lineage tracing experiments in chondrichthyans. Here, we set up an in ovo manipulation system for embryos of the cloudy catshark Scyliorhinus torazame and labeled the epithelial cells of each HC with lipophilic fluorescent dyes. This experimental system allowed us to trace the cell lineage of EOMs with the highest degree of detail and reproducibility to date. We confirmed that the HCs are indeed primordia of EOMs but showed that the morphological pattern of shark EOMs is not solely dependent on the early pattern of the head mesoderm, which transiently appears as tripartite HCs along the simple anteroposterior axis. Moreover, we found that one of the HCs gives rise to tendon progenitor cells of the EOMs, which is an exceptional condition in our previous understanding of head muscles; the tendons associated with head muscles have generally been supposed to be derived from cranial neural crest (CNC) cells, another source of vertebrate head mesenchyme. Based on interspecies comparisons, the developmental environment is suggested to be significantly different between the two ends of the rectus muscles, and this difference is suggested to be evolutionarily conserved in jawed vertebrates. We propose that the mesenchymal interface (head mesoderm vs CNC) in the environment of developing EOM is required to determine the processes of the proximodistal axis of rectus components of EOMs.

中文翻译：

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鲨鱼头腔的发育命运揭示了中眼皮对眼外肌腱祖细胞的贡献

脊椎动物眼外肌（EOM）在眼球运动中起作用。现代颌骨脊椎动物的EOM主要由四个直肌和两个由三个颅神经支配的斜肌组成。这种复合物的建立以及EOM的进化保守模式所基于的发展机制尚不清楚。软骨鱼类早期胚胎在头部中胚层中形成三对明显的上皮腔，称为头腔（HC），并且每个HC都可以分化为EOM的离散子集。但是，由于软骨鱼类中谱系追踪实验的技术难度，尚未提供这些细胞命运的直接证据。在这里，我们为多云的猫鲨Scyliorhinus torazame的胚胎建立了一个卵内操纵系统，并用亲脂性荧光染料标记了每种HC的上皮细胞。该实验系统使我们能够追踪EOM的细胞谱系，具有最高的详细程度和可重复性。我们确认HCs确实是EOMs的原基，但表明鲨鱼EOMs的形态模式不仅取决于头部中胚层的早期模式，后者沿简单的前后轴瞬时显示为三方HCs。此外，我们发现其中一种HC产生EOM的肌腱祖细胞，这是我们先前对头部肌肉的了解中的一种例外情况。一般认为，与头部肌肉相关的肌腱来源于颅神经neural（CNC）细胞，而后者是脊椎动物头部间质的另一种来源。根据种间比较，提示直立肌两端之间的发育环境存在显着差异，并且该差异在有颚脊椎动物中被认为是进化保守的。我们建议，在发展EOM的环境中需要间充质界面（头部中胚层与CNC）确定EOM的直肌成分的近前轴过程。