[The morphogenetic reactions of the ectoderm in the early gastrula of the clawed toad to mechanical stretching].

The ventral ectoderm explants of the Xenopus laevis early gastrula were stretched on an elastic substrate 1.5- to twofold for 30 min, then fixed in the stretched state and incubated for up to 6 h. The stretching of explants generated their two-phase active reaction, which was estimated using the criterion of eccentricity of cells and whole explants developed in this study. At the first phase, during the external force application, the explant cells were elongated towards its stretching to a markedly greater extent than the whole explant: eccentricity of the whole cells exceeded that of the whole explants. Cell processes elongated in the same direction appeared, cell contacts and apical cytoskeleton structures developed, and mechanical tensions increased. At the second phase, within 5 h after the stretching, the shape of individual cells, rather than the whole explant, returned to the initial one (approximately isotropic), mechanical tensions decreased, and visible morphogenesis of the explant proceeded: it acquired a complex shape with longitudinal groove and polar thickenings. We conclude that intercalary movements of the cells are present at the second phase of the active explant reaction and morphogenesis is realized at the phase of relaxation of mechanical tensions.