THE EXIT OF MOUSE OOCYTES FROM MEIOTIC M-PHASE REQUIRES AN INTACT SPINDLE DURING INTRACELLULAR CALCIUM-RELEASE

To study the role of the metaphase spindle during the period of oocyte activation, mouse oocytes were fertilised or activated parthenogeneti cally in the presence or absence of the microtubule inhibitor nocodazo le. In both cases, nocodazole caused the disappearance of the spindle and prevented the passage of the oocytes into interphase. However, the calcium spiking responses of the oocytes were not affected by nocodaz ole, being repetitive after fertilisation and a single spike after act ivation. If, after their activation or fertilisation in nocodazole, oo cytes were later removed from the drug, only those that had been ferti lised progressed into interphase. This progress was associated a with continuing calcium spiking, Moreover, both the spiking and the progres s to interphase could be blocked or reduced in incidence by removal of external calcium or addition of 5,5'-dimethyl BAPTA-AM. Oocytes that had been activated by ethanol in the presence of nocodazole and then r emoved from it, to allow re-formation of the spindle, only progressed into interphase if given a second exposure to ethanol, thereby eliciti ng a second calcium transient. These results show that exit from meiot ic M-phase requires the simultaneous presence of a fully intact spindl e during the release of calcium and that those factors leading to the degradation of cyclin B are only activated transiently. Since cyclin i s being degraded continuously in the metaphase-II-arrested mouse oocyt e and since this degradation is microtubule-dependent, these data sugg est that the superimposition of a high concentration of intracellular calcium is required to tilt the equilibrium further in favour of cycli n degradation if exit from M-phase is to occur.