Full-grown mouse oocytes spontaneously resume meiosis in vitro when released from their follicular environment. By contrast, growing oocytes are not competent to resume meiosis; the molecular basis of meiotic competence is not known. Entry into M phase of the eukaryotic cell cycle is controlled by MPF, a catalytically active complex comprising p34cdc2 kinase and cyclin B. Incompetent oocytes contain levels of cyclin B comparable to those in competent oocytes, while their level of p34cdc2 is markedly lower; p34cdc2 accumulates abruptly at the end of oocyte growth, at the time of meiotic competence acquisition. We show here that this change in p34cdc2 concentration is not secondary to a corresponding change in the concentration of the cognate mRNA, indicating that translational control may be involved. Microinjection of translatable p34cdc2 mRNA into incompetent oocytes yielded high levels of the protein, but it did not lead to resumption of meiosis. Similarly, microinjection of cyclin B1 mRNA resulted in accumulation of the protein, but not in the acquisition of meiotic competence. By contrast, the microinjection of both p34cdc2 and cyclin B1 mRNAs in incompetent oocytes induced histone H1 and MAP kinase activation, germinal vesicle breakdown, and entry into M-phase including the translational activation of a dormant mRNA. Thus, endogenous cyclin B1 in incompetent oocytes is not available for interaction with p34cdc2, suggesting that a posttranslational event must occur to achieve meiotic competence. Microinjection of either p34cdc2 or cyclin B1 mRNAs accelerated meiotic reinitiation of okadaic acid-treated incompetent oocytes. Taken together, these results suggest that acquisition of meiotic competence by mouse oocytes is regulated at both translational and posttranslational levels.

Growing incompetent mouse oocytes released from follicular cells are unable to spontaneously resume meiosis in vitro. To identify the reasons for meiotic incompetence in these cells, the levels of p34cdc2/cyclin B kinase and p42MAPK between incompetent and competent oocytes were compared. p34cdc2 was present at very low levels in incompetent oocytes and accumulated abruptly at the time of meiotic competence acquisition. By contrast, cyclin B and p42MAPK were present at similar concentrations in both types of oocytes. Okadaic acid induced centrosome phosphorylation and meiotic reinitiation in incompetent oocytes, without inducing an increase in p34cdc2 concentration. However, the p34cdc2 present in incompetent oocytes was activated and all events following germinal vesicle breakdown were induced up to the formation of a metaphase I spindle including p42MAPK activation, sustained increase in p34cdc2 kinase activity, and translational activation of a dormant mRNA. We suggest that a threshold level of p34cdc2 has to be reached for meiotic reinitiation to be spontaneously triggered: competence is restricted at a point preceding MPF activation. Whatever the mechanism involved in this restriction point, i.e., subthreshold concentration of p34cdc2 and/or lack of an activator or presence of an inhibitor, it is bypassed by okadaic acid. Downstream of this point meiosis progresses up to metaphase 1, even though p34cdc2 concentration remains low.

No cleavage (nc) is a maternal effect mutant gene, recessive and sex limited. It affects the eggs laid by homozygous mutant females, independently of the male genotype. Contrary to normal oocytes, following germinal vesicle breakdown (GVBD) during maturation, the transient microtubular array (TMA) is not formed, nor are the meiotic spindles. Cytoplasmic asters with condensed chromosomes are present in the majority of oocytes, as well as microtubular bundles and sometimes cytoplasmic spindle-like asters. These mature oocytes exhibit a disturbance in yolk platelet arrangements. The white spot is rather irregular, and the maturation period is longer than normal. Transfers of cytoplasm from nc mature oocytes into normal stage VI oocytes resulted in abnormal maturation of the normal oocytes. Reciprocal transfers (cytoplasm from normal mature oocytes into nc stage VI oocytes) induce the formation of spindles, usually cytoplasmic; this indicates that the deficiency can be partly rescued. Following fertilization, the nc eggs show neither contraction nor rotation; polyspermy is present in the majority of cases. Even in the same egg, simultaneous spindles and nuclei can be observed, revealing a disturbance in the spatial localization of regulators of the cell cycle. Cytokinesis never occurs. Polyspermy results from the absence of cortical reaction following sperm entry. However, when mature nc oocytes are treated with PMA, they show cortical granule exocytosis and the formation of an altered vitelline envelope. The different factors possibly involved in these anomalies are discussed in relation to cytoarchitectural disorganization of the cell and abnormal cell cycle regulation.