ИСТИНА

Cnidarians display almost the full range of existing gastrulation modes, which can be modified and combine with each other in different ways. In the development of colonial hydroid Dynamena pumila the unique way of gastrulation can be observed. Morular delamination is the principal mode of gastrulation in D. pumila and proceeds through several steps. At the first step of gastrulation, epithelial sheet fragments start to form simultaneously at the several regions of the embryo surface. At the second step, epithelial sheet fragments join and fuse via active planar cell movements, when cells from different epithelization regions form contacts with each other through filopodia and get closer, contracting them. Convergence of epithelial fragments lead to the formation of multiple toroidal-like structures. At the last step of gastrulation, holes of toroidal surfaces close basically according to the same mechanism as the merging of epithelial sheet fragments earlier. Upon tori closure, initially outer cells forming the bottom of torus passively submerge inside the embryo, where they may contribute to the endoderm. Some surface cells of the embryo take a bottle-like shape during morula and early gastrula stages. Perhaps, these cells migrate inside the embryo and join the endoderm population. In this case, it can be assumed that ingression is one of the mechanisms of gastrulation in D. pumila. Cells also can get inside, when some outer cells divide perpendicularly to the surface of the embryo at the morula stage, which looks similar to the mechanism of cellular delamination. Mechanisms of epithelial sheet fragments merging and tori closure resembles the processes occurring during wound healing in embryonic and adult tissues and epithelium formation in the embryogenesis of higher Metazoa. Thus, it is assumed that in the center of the epithelial tori the integrity of the epithelial layer is broken, and it should be restored during their “healing”. In sum, gastrulation in D. pumila proceeds in an apolar fashion by a mixed delamination, probably accompanied by multipolar cell ingression. The study was supported by grant from RFBR No 17-04-01988а.