Stimulation of parthenogenesis in mouse ovarian follicles by inhibitors of inosine monophosphate dehydrogenase.
The effects of hormonal priming and inosine monophosphate (IMP) dehydrogenase inhibitors on the meiotic maturation and parthenogenetic activation of mouse oocytes were examined in this study. In the first series of experiments, unprimed mice or mice primed 24 h with equine chorionic gonadotropin (eCG) received injections of the IMP dehydrogenase inhibitors, bredinin (Br) or mycophenolic acid (MA), followed by histological examination at 24 h, 48 h, and 72 h after drug administration. In both treatment groups, oocytes from nonatretic antral follicles were stimulated to undergo germinal vesicle breakdown by 24 h and became parthenogenetically activated as manifested by pronuclear formation and early cleavage divisions. The parthenotes underwent degeneration by 72 h. In the second part of this study, the effects of priming and drug treatment on parthenogenetic activation and subsequent developmental potential in vitro were examined. Mice were primed with eCG, and 24 or 48 h later received injections of Br or MA. Cumulus cell-enclosed oocytes were isolated 21-22 h later and assessed for maturation; those having undergone germinal vesicle breakdown were cultured and subsequently examined for embryonic development. In mice primed for 24 h, but not 48 h, Br and MA stimulated a significant number of oocytes to resume maturation in vivo; these subsequently underwent activation and developed to blastocysts in vitro. In another series of experiments, germinal vesicle-stage oocytes were isolated from primed or unprimed mice and cultured in vitro to permit spontaneous meiotic maturation. Nine percent of mature ova from 24-h-primed mice developed to 2-cell parthenotes; activation in ova from unprimed and 48-h-primed mice was considerably lower. A time-course experiment demonstrated that the extent of parthenogenetic activation in vivo following Br treatment was related to the period of time between drug injection and isolation of ova, the optimal period being 12 h. Neither Br nor MA had a direct activating effect on the oocytes as evidenced by an inability to induce parthenogenesis in vitro. Simultaneous injection of hCG with either Br or MA stimulated ovulation and prevented the parthenogenetic response. These data are consistent with the idea that conditions within the follicle promote parthenogenetic activation when the oocyte matures in the absence of gonadotropin stimulation.