Cell cycle dynamics of maturation-promoting factor during mouse oocyte maturation.

The changes in activity of a cytoplasmic maturation-promoting factor (MPF), capable of inducing resumption of meiosis when injected into starfish oocytes, were examined during mouse oocyte maturation. MPF appeared first at the time of germinal vesicle breakdown (GVBD), and its activity fluctuated along with the meiotic cycle: it reached a peak at the first metaphase and disappeared at the first polar body emission. Then MPF reappeared and reached a peak again at the second metaphase. Thus, a good correlation was found between MPF activity and meiotic metaphase. When mouse oocytes were treated with cytochalasin D, metaphase chromosomes and meiotic spindles were maintained after the first metaphase. In such a case, MPF activity remained at an elevated level after the first peak. Addition of cycloheximide to these cytochalasin-treated oocytes caused diminution of MPF activity, followed by chromosome movement, decondensation and formation of nucleus-like structures. Thus, it is likely that disappearance of MPF activity induces transition from metaphase to the subsequent processes. Further, detailed observation of chromosomes revealed that only the monovalent chromosomes were competent to decondense in response to the disappearance of MPF. Therefore, it was suggested that dissociation of synapsis is necessary for the decondensation of chromosomes.