ИСТИНА

Gametogenesis is usually used not only for understanding of reproductive biology, but for phylogenetic analysis as well. Holothuroidea is a class of echinoderms, whose oogenesis was described briefly for several commercially important species, most of which are dioecious. At the same time, oogenesis of other holothurians, especially in sequential hermaphroditic species, is still poor studied. The oogenesis of holothuria Chiridota laevis is studied using light microscopy, SEM, and TEM. Oogenesis occurs in the most distal parts of the gonad, in acini. The inner layer of acini is represented by germinative epithelium, which gives rise to primordial germ cells and accessory cells. The accessory cells form a follicle around developed oocytes. In C. laevis, each follicle can contain more than one oocyte that is unusual feature, which has never been described in holothurians before. Between follicle cells and oocytes there are no any specialized contacts, which may provide the transport of nutriments. The surface of vitellogenic oocyte forms thin rare microvilly exposed into narrow peryoocyte space. In mature oocyte, the cytoplasm adjacent to the voluminous space of haemal sinuses differs significantly in its absence of yolk and presence of numerous transparent vesicles of different sizes. The presence of vesicles may indicate the endocytosis. The late vitellogenic oocyte is covered by yolk envelope, which contacts the basal lamina of follicle cells. In general, well-developed synthetic apparatus together with presence of microvilli and endocytosis indicate an autosynthetic vitellogenesis. C. laevis has follicular autosynthetic vitellogenesis, which is described in holothurians for the first time.