An Ultrastructural Study of Gametogenesis of the Marine Bivalve Pinna Nobilis (Linnaeus 1758)i. Oogenesis

The ultrastructural stages of female gametogenesis are described in a Mediterranean marine bivalve, Pinna nobiiis. Firstly, the process of oocyte formation is analysed and the formation of the yolk granules is discussed. Secondly the degeneration of oocytes is described and seems to be most common in vitello-genic oocytes. The evolution of the oocyte reserves is particular; they form a dense globulose mass in the acinar lumen which will then appear in the cytoplasm content of auxiliary cells. During early stages of oogenesis the oocyte is completely surrounded by several layers of auxiliary cells. On formation of the viteliine coat around the oocyte, the auxiliary cells are restricted to the stalk and contact is maintained by desmosomes. Interdigitation phenomena are observed. The auxiliary cells appear to play an integral role in development of the oocyte. Their functions are the phagocytosis and intracellular digestion of products originating from oocyte degeneration; this function can permit a transfer of precursors necessary to vitellogenesis; they have also a synthetic activity and they accumulate reserves in their cytoplasm, as glycogen and lipids, which can be employed by vitellogenic oocytes.