In vitro culture retards spontaneous activation of cell cycle progression and cortical granule exocytosis that normally occur in in vivo unfertilized mouse eggs

We have previously demonstrated that metaphase II-arrested eggs recovered from oviducts at increasing times after hCG administration display a time-dependent spontaneous entry into anaphase, as well as release of cortical granules (CGs) and the associated modifications of the zona pellucida (ZP), a decrease in histone H1 and mitogen-activated protein kinase activities, and the recruitment of maternal mRNAs [Xu et al., Biol Reprod 1997; 57:743-750). These changes are correlated with the time-dependent increase in susceptibility of these eggs to undergo parthenogenetic activation. We report here the effect of culture of ovulated eggs, retrieved 13 or 16 h post-hCG administration and cultured in vitro for various periods of time, on the aforementioned parameters of egg activation and cell cycle resumption. In contrast to extended residence of the eggs in the oviduct, culture in vitro retarded cell cycle events associated with completion of the second meiotic reduction and inhibited CG release and the associated modifications of the ZP, as well as the recruitment of maternal mRNAs. The retardation or inhibition of these changes during in vitro culture resulted in eggs that were less susceptible to parthenogenetic activation than eggs that resided in the oviduct for comparable time periods. Results of these experiments indicate that egg culture in vitro (which likely occurs under suboptimal conditions) inhibits, rather than accelerates, the progression into the interphase-like state as compared to that seen in eggs residing in the oviduct for increasing periods of time. These results also suggest that, for studies focused on in vitro fertilization or egg activation, the ovulated eggs should be placed under appropriate in vitro conditions as soon as possible.