Different cytoskeletal organization in two maturation stages of Discoglossus pictus (Anura) oocytes: thickness and stability of actin microfilaments and tropomyosin immunolocalization

In Discoglossus pictus oocytes, the germinative area (GA) contains long and irregular microvilli where actin microfilaments are located. In the egg, the funnel-shaped dimple that originates by invagination of the GA is present. In the dimple both microvilli and microfilament bundles have a very orderly appearance. This report extends previous observations (Campanella and Gabbiani, Gamete Res 3:99-114, 1980) and shows that GA microfilaments are thinner (36 A average) than dimple microfilaments (60 A average), as measured in ultrathin section. Moreover, the interfilament distance is smaller in GA bundles than in the dimple bundles. To get an insight into actin organization in oocytes and eggs, we used an actin-depolymerizing factor (ADF) in which cryostat sections were incubated prior to immunofluorescent staining with antiactin antibodies. The microfilaments of the GA microvilli and partially of the oocyte cortex are resistant to ADF when compared to those in the dimple and the rest of the egg cortex. We also investigated immunocytochemically the presence of tropomyosin and found that this protein is localized in the dimple and in the cortex of oocytes and eggs but is absent in the GA.