Ultrastructural study of oogenesis in the African mussel, Perna perna (Bivalvia: Mytilidae)

Transmission electron microscopy of female gonads in Perna perna (Mytilidae) was carried out to study the fine structure of the different developmental stages during oogenesis and to examine the functions of the follicular cells. The gonad is composed of acini, each containing oogoniae and oocytes surrounded by follicular cells, and of an interacinar connective tissue with vesicular cells. Groups of primary oogoniae, surmounted by secondary oogoniae, were found along the inner wall of each acinus. Previtellogenesis of oocytes was characterized by a great increase of these cells in volume, the accumulation of numerous organelles, and the formation of the first yolk granules at the end of this phase. The period of vitellogenesis involved both autosynthetic and heterosynthetic pathways, and was marked by the accumulation of cortical granules and of yolk granules. Numerous lipid droplets (two types), several inclusions resulting from the heterosynthetic uptake of exogenous substances by pinocytosis, and droplets showing two types of electron-lucent materials in the cytoplasm of mature oocytes could also be observed. In the atretic ovocytes, a vacuolization progressively developed in their cytoplasm, with the subsequent rupture of the vitelline membrane and the release of oocyte remnants in the lumen of the acinus. At the end of vitellogenesis, the follicular cells became detached from the oocyte and contained numerous lipid droplets and glycogen inclusions. In P. perna, the formation of female gametes was mostly similar to the oogenesis described in other species of bivalvia. The only difference concerned the composition of the yolk in the mature oocyte, as it was constituted of several endogenous substances (lipoproteins mainly) and also of exogenous materials intaken by the oocyte via pinocytosis