Kuroda Shunya, Adachi Noritaka, Kusakabe Rie, Kuratani Shigeru
Laboratory for Evolutionary Morphology, RIKEN Center for Biosystems Dynamics Research (BDR), 2-2-3 Minatojima-minami, Chuo-ku, Kobe 650-0047, Japan.
Department of Biology, Graduate School of Science, Kobe University, Kobe, 657-8501, Japan.
Zoological Lett. 2021 Feb 15;7(1):3. doi: 10.1186/s40851-021-00170-2.
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.
脊椎动物的眼外肌（EOMs）负责眼球运动。现代有颌脊椎动物的眼外肌主要由四条直肌和两条斜肌组成，由三条脑神经支配。这种复杂结构以及眼外肌进化上保守模式的形成所涉及的发育机制尚不清楚。软骨鱼类的早期胚胎在头部中胚层发育出三对明显的上皮性体腔，称为头腔（HCs），并且每一个头腔都被认为会分化为眼外肌的一个离散亚群。然而，由于软骨鱼类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。在此，我们为云纹猫鲨胚胎建立了一个卵内操作系统，并用亲脂性荧光染料标记每个头腔的上皮细胞。这个实验系统使我们能够以迄今为止最高的细节程度和可重复性追踪眼外肌的细胞谱系。我们证实头腔确实是眼外肌的原基，但表明鲨鱼眼外肌的形态模式并不完全依赖于头部中胚层的早期模式，头部中胚层沿着简单的前后轴短暂地呈现为三分头腔。此外，我们发现其中一个头腔产生眼外肌的肌腱祖细胞，这在我们之前对头肌的认知中是一种特殊情况；与头肌相关的肌腱通常被认为源自颅神经嵴（CNC）细胞，这是脊椎动物头部间充质的另一个来源。基于种间比较，提示直肌两端的发育环境存在显著差异，并且这种差异在有颌脊椎动物中被认为在进化上是保守的。我们提出，发育中的眼外肌环境中的间充质界面（头部中胚层与颅神经嵴）对于确定眼外肌直肌成分的近远轴过程是必需的。
