Incompetence of preovulatory mouse oocytes to undergo cortical granule exocytosis following induced calcium oscillations

Immature oocytes of many species are incompetent to undergo cortical granul e (CG) exocytosis upon fertilization. In mouse eggs, CG exocytosis is depen dent primarily on an inositol 1,4,5-trisphosphate (IP3)-mediated elevation of intracellular calcium ([Ca2+](i)). While deficiencies upstream of [Ca2+] (i) release are known, this study examined whether downstream deficiencies also contribute to the incompetence of preovulatory mouse oocytes to releas e CGs. The experimental strategy was to bypass upstream deficiencies by ind ucing normal, fertilization-like [Ca2+](i) oscillations in fully grown, ger minal vesicle (GV) stage oocytes and determine if the extent of CG exocytos is was restored to levels observed in mature, metaphase II (MII)-stage eggs . Because IP3 does not stimulate a normal Ca2+ response in GV-stage oocytes , three alternate methods were used to induce oscillations: thimerosal trea tment, electroporation, and sperm factor injection. Long-lasting oscillatio ns from thimerosal treatment resulted in 64 and 10% mean CG release at the Mn and GV stages, respectively (P < 0.001). Three electrical pulses induced mean [Ca2+](i) elevations of approximately 730 and 650 nM in MII- and GV-s tage oocytes, respectively, and 31% CG release in MII-stage eggs and 9% in GV-stage oocytes (P < 0.001), Sperm factor microinjection resulted in 86% C G release in MII-stage eggs, while similarly treated GV-stage oocytes exhib ited < 1% CG release (P < 0.001). Taken together, these results demonstrate a deficiency downstream of [Ca2+](i) release which is developmentally regu lated in the 12 h prior to ovulation, (C) 1999 Academic Press.