[Morphomechanical Factors in Gastrulation Process and Differentiation of Embryonic Tissue of Xenopus laevis].

Effect of mechanical stretch on the differentiation of axial anlages and Chordin gene expression was studied in sandwich explants prepared from embryonic tissues of Xenopus laevis at the early gastrula stage in two variants: with dissected or intact dorso-medial region. In the first case, convergent cell movements were suppressed and properly organized axial organs (notochord and somites) were almost completely absent. However, they developed if the explants of such type were artificially stretched in the ventro-dorsal direction. In this case, axial organs elongated in the line of stretching, that is in the direction vertical to their normal orientation. Segmented mesoderm was always in contact with the chord anlage. In situ hybridization revealed that the area of Chordin gene expression was also extended in the direction of stretching. PCR showed that Chordin gene expression in stretched explants with disrupted dorso-medial region was statistically at the same level as in the explants with intact dorso-medial region. At the same time, the corresponding gene expression in unstretched explants with disrupted dorso-medial region was statistically higher. The obtained data indicate that mechanical stretch and associated cell movements are a necessary and sufficient condition for the formation of proper histological structure of axial organs and regulation of Chordin gene expression.

[Changes in topology and geometry of the embryonic epithelium of Xenopus during relaxation of mechanical tension].

The paper presents the results of statistical evaluation of the changes of cellular apex connections, apical angles, and apical indices of ventral cells of the epiectodermal gastrula of Xenopus during the first HC-four hours after the relaxation of mechanical tension. In the unrelaxed epithelium, an overwhelming majority of cells have three apical connections, apical angles close to 120 degrees, and apical indices around one (isodiametric cells); after relaxation, the number of cells with more than three connections, the number of apical angles deviating substantially from 120 degrees, and the percentage of columnar cells with high apical index increase. Apices with more than three connections tend to gather in enclosed groups, forming a smooth line of cell walls. The length and curvature of cell walls with four apical connections significantly exceeds those same indicators for cells with three apical connections. The observed changes in topology and geometry of cells correspond to reconstructions observed during normal morphogenesis. They are considered in terms of the hyper-restoration model of mechanical tension in relaxed epithelial layers.

[Changes in the shape of epithelial embryonic cells of the spur-toed frog upon deformation of the cell layer].

A quantitative study of changes in the shape of cells of double explants of the tectum of gastrule blastocoele of the spur-toed frog within the first four hours after the artificial bending of explants has been performed. It was found that, on the concave (contracted) side of explants, epithelial cells stretch out, and the apical surface of many of these cells contracts, whereas on the convex (stretched) side they remain isodiometric. The maximal difference between the values of the apical index of epithelial cells located on the concave and convex sides was observed after 4 h of the cultivation of explants. It was shown that, by 2 h, the artificially produced curvature of the bent explant increases. The endocytosis on the concave side is more active than on the convex (stretched) side. The results of experiments with the use of inhibitors simulating the behavior of the actomyosin complex showed that the unimpeded functioning of myosin I is more important for the apical contraction and elongation of cells than the proper structural organization of the actin backbone.