Clocks,oscillation, and temporal delays in morphogenesis

The presence and morphogenetic role of relatively autonomous pacemakers (“clocks”) during ontogeny is discussed. Although autonomous pacemakers in the strict sense seem nonexistent, the study of temporal processes is of great importance for recognizing the fundamental morphogenetic mechanisms. The most important and insufficiently understood questions in this field include the presence of hierarchies of characteristic times and their smoothing during critical developmental periods and also regulation of spatial processes via temporal delays.     

Artificially Applied Tensions Normalize Development of Relaxed Xenopus laevisEmbryos

Relaxation of tensions of the surface of Xenopus laevisembryos at the late blastula stage leads to deep and diverse developmental defects and increased variability in mutual position and volume ratios of the axial rudiments. Here, we demonstrate that the development of such embryos was markedly normalized if the relaxed tensions were restored in one of two ways: (1) isotropic stretching of the blastocoel roof induced by the incubation of relaxed embryos in a hypotonic medium or (2) anisotropic stretching of embryos on two needles. In the latter case, we succeeded in restoring the morphological axis not only after longitudinal stretching but also after transverse stretching, and the new axis had signs of anteroposterior polarity. The role of isotropic and anisotropic tensions in organization of the early amphibian development is discussed.     

On the work of the developmental biophysics laboratory of the embryology department of Moscow State University

The laboratory is engaged in morphomechanics—the study of self-organization of mechanical forces that create the shape and structure of the embryonic primordia. As part of its work, the laboratory described pulsating modes of mechanical stresses in hydroids, identified and mapped mechanical stresses in the tissues of amphibian embryos, and studied morphogenetic reorganization caused by the relaxation and reorientation of tensions. The role of mechanical stresses in maintaining the orderly architectonics of the embryo is shown. Mechano-dependent genes are detected. Microstrains of embryonic tissues and stress gradients associated with them are described. A model of hyper-recovery of mechanical stresses as a possible driving force of morphogenesis is proposed.     

Changes in topology and geometry of the embryonic epithelium of <Emphasis Type="Italic">Xenopus</Emphasis> 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 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°, 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°, 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 straightened 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.