Ultrastructural studies on developing follicles of the spotted ray Torpedo marmorata

Light and ultrastructural investigations on sub-adult and adult sexually mature females, demonstrates that in Torpedo marmorata folliculogenesis starts in the early embryo and that the two ovaries in the adult contain developing follicles of various sizes and morphology. Initially, the follicle is constituted by a small oocyte, surrounded by a single layer of squamous follicle cells. The organization is completed by a basal lamina and, more externally, by a theca, that at this stage is composed by a network of collagen fibers. As the oocyte growth goes on, during previtellogenesis and vitellogenesis, the organization of the basal lamina and of the oocyte nucleus does not change significantly. The basal lamina, in fact, remains acellular and constituted by fibrils intermingled in an amorphous matrix; the nucleus always shows an extended network of chromatin due to the lampbrush chromosomes, and one or two large nucleoli. By contrast, the granulosa (or follicular epithelium), the ooplasm, and the theca cells significantly change. The granulosa shows the most relevant modifications becoming multi-layered and polymorphic for the progressive appearance of intermediate and pyriform-like cells, located respectively next to the vitelline envelope, or spanning the whole granulosa. The appearance of intermediate cells follows that of intercellular bridges between small follicle cells and the oocyte so that one can postulate that, as in other vertebrates, small cells differentiate into intermediate, and then pyriform-like cells, once they have fused their plasma membrane with that of the oocyte. Regarding the ooplasm, one can observe as in previtellogenic follicles, it is characterized by the presence of intermediate vacuoles containing glycogen, while in vitellogenic follicles by an increasing number of yolk globules. The theca also undergoes significant changes: initially, it is constituted by a network of collagen fibers, but later, an outermost theca esterna containing cuboidal cells and an interna, with flattened cells, can be recognized. The role of the different constituents of the ovarian follicle in the oocyte growth is discussed.