Ultrastructure of Vitellogenic Ovarian Follicles and Morphogenesis of the Oocyte Envelope in the Eurasian Ruffe (Gymnocephalus cernua Linnaeus 1758) (Teleostei, Perciformes).

The ovarian follicle of the Eurasian ruffe (Gymnocephalus cernua) consists of a single oocyte surrounded by follicular cells, a basal lamina, and thecal cells. The growth of the oocyte during vitellogenesis and the ultrastructure and morphogenesis of its envelope are undescribed in this species. To address these issues, vitellogenic follicles were examined. The oocyte cytoplasm is divided into three compartments: the perinuclear compartment, the endoplasm (which contains yolk spheres and lipid droplets), and the periplasm. The components of the Balbiani body, including mitochondria with well-developed cristae in complexes with spherical, thread-like nuage, are located in the periplasm of early vitellogenic oocytes in the vegetal region. The periplasm also contains annulate lamellae, rough endoplasmic reticulum, Golgi apparatus, endocytotic vesicles containing yolk and oocyte envelope precursors, and mitochondria. Advanced vitellogenic oocytes have a large lipid droplet and a multilamellar body in the perinuclear cytoplasm that may be used during embryo development. The oocyte envelope is primary. It derives from precursor proteins that are secreted by hepatocytes, transported in blood plasma to the ovary, and endocytosed. Nascent envelope proteins are then synthesized in the cytoplasm of oocytes and follicular cells and deposited in the perioocytic space. The envelope of advanced follicles consists of three to seven layers, depending on the oocyte hemisphere. The ultrastructure and sequence of deposition of these layers are described and discussed. The follicular cells diversify into a micropylar cell and mainbody cells. The micropylar cell is shaped like a thumbtack, with a flat head and a pin-like projection. Its cytoplasm is divided into four regions with different sets of organelles. The projection acts as a mechanical barrier to the deposition of the oocyte envelope and forms the micropylar canal. The mainbody cells differentiate into bright, and dark cells, both of which are translationally and secretory active.