ИСТИНА

Studying Cnidaria as sister group of Bilateria is very important for understanding evolutionary processes. Investigation of embryonic development of cnidarian species is necessary for analysis of morphogenesis and conserved developmental mechanisms. Hydroid polyps as widely distributed and available cnidarians are favourite objects of development research. In present work, we analysed the stages of medusoid development and the early embryonic development of athecate Ectopleura larynx. With the help of light and electron microscopy (TEM) and immunocytochemical staining with CLM, we studied development of medusoids starting from early stages of bud formation up to mature medusoids containing developing embryo. Ectopleura larynx belongs to Aplanulata group lacking a planula stage. Embryos develop within medusoids (reduced medusa stage) to actinula larva stage. Two-four oocytes differentiate sequentially from i-cells that accumulate in spadix (reduced manubrium). As oocyte grows and maturates, it detaches from spadix and migrates into the medusoid cavity, while another oocyte becomes visible within spadix. We observed numerous apoptotic nuclei in developing embryo, which maybe be the nuclei of adjacent i-cells phagocytised by growing oocyte. We studied stages of medusary nodule (entocodon) formation, from which smooth and striated muscles derive. Entocodon is formed by ectodermal i-cells proliferation at apex of the developing bud. Due to expression of mesodermal genes in medusary nodule, it is considered as homologous to the third germ layer specific for Bilateria. Thus, the study of cnidarian developmental processes is important for understanding establishment of developmental mechanisms in common ancestor of Cnidaria and Bilateria.