ИСТИНА

Holothuroidea is a class of echinoderms, whose body plan is strikingly different from all other Echinodermata. The amount of information about holothurians reflects their economic significance. Commercially important species have been studied in great detail, however little is known about structure and ecology of many other holothurians. The anatomy and ultrastructure of gonad of holothuria Chiridota laevis are studied using light microscopy, cLSM, SEM, and TEM. The gonad is a single organ consisting of elongated and ramified tubules of variable length and diameter. Proximally gonad tubules unite into a basal sac, which connects the gonoduct that opens by gonopore. The most distal sac-like parts of tubules are called acini. According to our new results, the wall of different gonad parts has different ultrastructure. In all parts, the wall consists outer visceral epithelium, inner germinative epithelium, and the genital haemal sinus cavity between two cellular layers. In acini and basal sac, the wall contains muscle cells, which are located under the visceral epithelium and form longitudinal, circular, and diagonal musculature. In gonoduct, the visceral epithelium is formed by typical podocytes. The hemal sinuses is strongly developed in gonoduct, whereas is represented by thin layer in acini and basal sac. The inner epithelium of the gonoduct exhibits ultrastructural features of apocrine secretion. Many specific features of the gonad organization, which are described for the first time in this report, correlate with specific type of gametogenesis that is still poor studied in sequential hermaphroditic holothurians. Work is supported by Russian Science Foundation (14-50-00029).