CELL CYCLE-RELATED FLUCTUATIONS IN OOCYTE SURFACE-AREA OF THE ASCIDIAN CIONA-INTESTINALIS AFTER MEIOSIS RESUMPTION

Variations in capacitance or cell surface area were recorded on patch- clamped eggs of the ascidian Ciona intestinalis between the resumption of meiosis and the first mitotic cleavages. The membrane surface area increased within the first minutes after fertilization and then oscil lated in phase with the cell cycles of the two meiotic divisions and f irst mitotic cleavage. With drugs, we generated two opposite situation s (removal and insertion) of artificial variation in capacitance. In u nfertilized eggs, cytochalasin induced a drop in capacitance linked to a decrease in calcium current intensity and specifically disturbed me mbrane removal linked to the first meiotic division cycle. It left una ffected the following cycles, in agreement with previous results that only the first meiosis cycle is microfilament dependent. In fertilized eggs, membrane removal at each cycle was hindered by emetine, an inhi bitor of protein synthesis. The resulting membrane extrusion was obser ved in sections by electron microscopy and was linked to an increase i n calcium current intensity. These fluctuations in surface area never involved the microtubule network, since nocodazole had no effect on an y cycle. The fluctuations of membrane surface area after meiosis resum ption in phase with cell cycles in Ciorna oocytes paralleled the patte rn previously described in the ascidian Boltenia villosa. This may ref lect the mechanism by which the oocyte regulates, with possibly differ ent mediators at each cycle, the connection between cell surface and i nternal membrane networks. This interrelation includes the insertion a nd removal of ion channels necessary to developmental control. (C) 199 4 Academic Press, Inc.