Possible mechanisms in the rearrangement of non-yolk cytoplasmic materials during maturation of theXenopus laevis oocyte.

Using the monoclonal antibody (MoAb) Xa5B6 as probe, the authors examined the mechanisms of cytoplasmic rearrangement occurring during maturation of theXenopus oocyte. The antigen molecules recognized by the MoAb are arranged in radial striations of the oocyte cytoplasm. The radial striations were disorganized in vitro by progesterone treatment, and the antigen molecules were uniformly distributed, predominantly in the animal hemisphere. Even when the germinal vesicle was mechanically removed or when germinal vesicle breakdown was suppressed in a K+-free medium, progesterone induced a disorganization of the radial striations. This progesterone-induced disorganization was inhibited by the protein synthesis inhibitor cycloheximide. When full-sized oocytes were treated with cytochalasin B, the radial striations were also disorganized, but the antigen molecules did not disperse into the large mass. Colchicine treatment had little effect. Antigen molecules were no longer arranged in radial striations and were completely dispersed when the oocyte was simultaneously treated with both drugs. These results indicate that the two compartments in the oocyte cytoplasm, the yolk-free cytoplasm and yolk column, are organized by different types of cytoskeletal system. It is also suggested that the maturation-promoting factor (MPF) activated during progesterone-induced maturation disrupts these cytoskeletal systems and disorganizes the radial striations.