ИСТИНА

The diversity of developmental processes and anatomical structures of the representatives of the main groups of cnidarians is enormous. It is accepted that the anthozoans are the earliest branching cnidarian lineage. In this case, the question of interest is why the polypoid stage in the Medusozoa clade has a much simpler organization compared to that of Anthozoa? We compared the embryonic and early postembryonic development of anthozoan (using literature data on Nematostella vectensis) and Aurelia aurita, a representative of the scyphozoans. Development of A. aurita was studied by light and electron microscopy and immunocytochemistry and confocal laser scanning microscopy. Cleavage of the A. aurita embryo ends with the formation of a coeloblastula. During gastrulation the blastopore lip involutes following the archenteron rudiment. However, the blastopore closes, and the blastopore lip integrates into the oral ectoderm at the posterior pole of the planula. One can distinguish the anterior vacuolated, middle granulated, and posterior vacuolated partitions of the entoderm of the competent planula. At the onset of planula metamorphosis, an ectodermal invagination at the oral pole of the planula fuses with the upper part of the middle partition of the entoderm forming the lining of the gastral cavity of the primary polyp. The oral part of the gastral cavity corresponds to the manubrium cavity of the primary polyp, and has an ectodermal internal lining. The posterior partition of entoderm is consumed in the formation of the entodermal axis of the first pair of tentacles. The middle entoderm partition participates in the formation of the entoderm of the second pair of tentacles. At the base of the tentacles, the entodermal partitions of both pairs of tentacles come into contact. Along this boundary, a bundle of epithelio-muscular cells, originating from the oral disc’s outer ectoderm, is formed, which connects the ectoderm of the oral disc with the ectoderm of the lateral wall of the calyx of the primary polyp. The location of muscle bundles corresponds to the places of septa formation of the primary polyp. Most of the septa surface, looking into the gastral cavity, is formed by overgrowth of the inner ectodermal lining of the polyp manubrium. Analyses of A. aurita early development revealed that the development and organization of scyphozoan polyp represent a modified version of the development and organization of the coral polyp. The project was supported by RFBR, grant #19-04-01131-a.