Spontaneous activation of ovulated mouse eggs: time-dependent effects on M-phase exit, cortical granule exocytosis, maternal messenger ribonucleic acid recruitment, and inositol 1,4,5-trisphosphate sensitivity.

With increasing time after ovulation, mammalian eggs become more sensitive to agonists of activation in vitro or may undergo spontaneous activation in vivo. We have tested the hypothesis that postovulatory eggs undergo time-dependent cell cycle and cytoplasmic changes that result in a partially activated state, accounting for their time-dependent susceptibility to activate. In vivo changes in key activation markers in mouse eggs were quantified at 13, 16, and 22 h post-hCG (1, 4, and 10 h postovulation). Spontaneous activation was first detected at 16 h, with a 20-25% decrease in the activities of histone H1 and mitogen-activated protein (MAP) kinases and with 3% of eggs undergoing both anaphase onset and a partial loss of cortical granules. By 22 h, more than 60% of eggs were in anaphase, H1 and MAP kinase activities had decreased 40-50%, the extent of zona pellucida modification had increased, and proteins normally synthesized after fertilization had appeared. Pronuclear formation in response to inositol 1,4,5-trisphosphate injection increased dramatically from 10% at 13 h to about 40% and 90% at 16 h and 22 h, respectively. The partial decreases (less than those after fertilization) in H1 and MAP kinase activities provide a likely biochemical basis for the increased sensitivity of eggs to agonists, seen over time, that results in pronuclear formation. Also, all of these time-dependent changes caution against the use of mouse eggs > 16 h after hCG administration in studying the mechanism of normal fertilization and have implications for animal and human in vitro fertilization.