Youn B W, Malacinski G M
J Embryol Exp Morphol. 1981 Dec;66:1-26.
Detailed SEM observations of the changes in cellular morphology, arrangements, and contacts that occur during the process of somite formation were made in two species of urodele amphibians, Ambystoma mexicanum and Pleurodeles waltlii, and one species of anuran amphibian, Rana sphenocephala. After fixation, embryos were fractured transversely, horizontally, and parasagittally, and the intrasomitic cellular arrangement pattern was examined with the SEM. It was found that Ambystoma and Pleurodeles embryos followed exactly the same development sequence in rosette formation and myoblast fusion. Rana somites did not, however, appear to form rosettes. Those myotomal cells underwent fusion immediately after a few segmentations occurred. Patterns of cellular rearrangement were also described during urodele rosette formation at the time of somite segmentation and during myoblast fusion. Extensive changes in cell shape and orientation appeared to occur during those processes. When cells changed their orientation, they often exhibited a triangular configuration. Probable roles of these triangular-shaped cells in rosette formation and myoblast fusion are discussed. During the initial period of myoblast or myotomal cell fusion, cells first send out specialized cell processes and then establish their cell-cell contacts. The establishment of such contacts eventually leads to tight membrane appositions and fusion. Since myoblast fusion appeared to occur between two cells which were tandemly arranged in a rosette, the origin of multi-nuclearity in the fused cells is discussed. Finally, comparative analyses of the pattern of somite formation and subsequent muscle development were made between different species of amphibians. The possibility is discussed that patterns of somitogenesis may provide useful indicators for determining how different families of amphibians evolved.
对墨西哥钝口螈和沃氏肋突螈这两种有尾两栖动物以及斯氏湍蛙这种无尾两栖动物在体节形成过程中细胞形态、排列和接触的变化进行了详细的扫描电子显微镜观察。固定后，将胚胎横向、水平和矢状旁切开，并用扫描电子显微镜检查体节内细胞排列模式。发现钝口螈和肋突螈胚胎在玫瑰花结形成和成肌细胞融合方面遵循完全相同的发育顺序。然而，湍蛙的体节似乎不形成玫瑰花结。那些肌节细胞在进行几次分裂后立即发生融合。还描述了有尾类动物在体节分割时玫瑰花结形成和成肌细胞融合过程中的细胞重排模式。在这些过程中，细胞形状和方向似乎发生了广泛变化。当细胞改变方向时，它们通常呈现三角形结构。讨论了这些三角形细胞在玫瑰花结形成和成肌细胞融合中的可能作用。在成肌细胞或肌节细胞融合的初始阶段，细胞首先发出特化的细胞突起，然后建立细胞间接触。这种接触的建立最终导致紧密的膜贴合和融合。由于成肌细胞融合似乎发生在玫瑰花结中串联排列的两个细胞之间，因此讨论了融合细胞中多核性的起源。最后，对不同种类两栖动物的体节形成模式和随后的肌肉发育进行了比较分析。讨论了体节发生模式可能为确定两栖动物不同科的进化方式提供有用指标的可能性。
