Characterization of changes in F-actin during maturation of starfish oocytes

Major morphological and mechanical changes occur in the starfish oocyte during maturation. Measurements made by quantitative fluorescence microscopy of fixed specimens stained with saturating levels of rhodamine-phalloidin demonstrated that changes in the level of F-actin in intact oocytes, in the endoplasm, and in the cortex may contribute to these changes. The level of F-actin increased transiently after exposure of oocytes to the maturation inducing hormone, 1-methyladenine (1-MA). This increase correlated with the formation of spikes on the cell surface. The level of F-actin decreased at the time of germinal vesicle breakdown (GVBD), which may account for the decrease in stiffness that occurs at this time. No increase in the level of F-actin was observed during formation of polar bodies, suggesting the existence of a secondary mechanism affecting oocyte stiffness. The changes in the amount of F-actin during oocyte maturation were largest in the cortex. The data also suggested that there are two distinct populations of cortical actin that are regulated both spatially and temporally; these are the F-actin in spikes and nonspike cortical F-actin. Changes in either or both of these populations of cortical actin were induced independently of GVBD by short exposures to 1-MA, induction of maturation with dithiothreitol, and pretreatment of immature oocytes with forskolin before adding 1-MA. Stabilization of F-actin by microinjection of phalloidin had no effect on GVBD. These results suggest that polymerization and depolymerization of actin during maturation are responsible for morphological and mechanical changes in the oocyte. In addition, the data suggest that the regulation of actin polymerization and depolymerization can be dissociated from GVBD.