An ultrastructural study on the vitellogenesis in the spotted ray Torpedo marmorata

Abstract

The present investigation strongly suggests that in Torpedo the oocyte growth is not only due to the uptake of exogenous molecules, but also by the oocyte itself and the granulosa cells. The oocyte, starting from the early previtellogenic follicles (see also Mol. Reprod. Dev. 61 (2002) 78), synthesizes large amounts of glycogen. Later, as the oocyte growth goes on, the cytoplasm of granulosa cells progressively bears numerous islets of glycogen, which are also evident inside the intercellular bridges and in the oocyte cortex, suggesting that they may flow from granulosa cells to the oocyte. The contribution of granulosa cells seems to become most relevant during the vitellogenesis. In vitellogenic follicles, both small, intermediate, and pyriform-like cells bear numerous vacuoles containing vitellogenin-like material, suggesting strongly that in Torpedo, differently from other vertebrate species, granulosa cells could be engaged in vitellogenesis. The present investigation does not allow us to know if such a material is due to a transcytosis process and/or is synthesized inside them. The organization of granulosa seems to exclude the possibility that it is transferred to granulosa via transcytosis. On the contrary, granulosa cells, expecially in vitellogenic follicles, display the morphological organization of metabolically active cells, so they could be engaged in vitellogenin synthesis. This interpretation is consistent with the observation that granulosa cells are positively stained by OZI (osmium tetroxide-zinc iodide) and that the same positivity is evident on intercellular spaces, containing vitellogenin-like material, and on nascent yolk globules.