ИСТИНА

Animals demonstrate the extreme diversity of morphogenetic processes involved in gastrulation. Among them,cell ingression based on the process of EMT is one of the most widespread morphogenetic movements. In the embryo of reptiles, birds and most mammals, cells ingress through the primitive streak and generate mesoderm and endoderm. Cell ingression provides mesoderm internalization in many insects, skeletogenic mesenchyme formation in sea urchins. The process of cell ingression may significantly vary between species. It is known that the process of EMT is highly dynamic and the cells undergoing EMT "move through a spectrum of intermediary phases" between full epithelial and full mesenchymal states (Nieto et al., 2016). The objects of our study are cnidarians belonging to the class Hydrozoa: Clytia hemisphaerica, Rathkea octopunctata, and Obelia longissima. In all these species, gastrulation is based on the unipolar cell ingression. The endoderm is formed through mass EMT at the future oral pole of an epithelial blastula. To understand the species-specific features of cell ingression, we have characterized in detail successive cell morphology changes during EMT by electron microscopy and confocal imaging. These changes accompany bottle cell formation followed by ingression in the oral domain and cohesive migration of ingressed cells towards the aboral pole inside blastocoel. We have found that spatial and temporal differences in the dynamic of EMT (e.g. number and location of involved cells, timing of intercellular contact reduction, and order of cell shaping events) are responsible for the species - specific embryonic morphology and might underlie evolutionary changes of cnidarian gastrulation. (Supported by Federal Project of IDB RAS 0108-2019-0003).