Fertilization in vitro of cumulus-enclosed mouse oocytes: effect of timing of the ovulatory HCG injection.

Human chorionic gonadotropin (HCG) may be injected to time ovulation and plan oocyte retrieval for clinical in vitro fertilization (IVF). Neither clinical nor experimental data on effects of subtle alterations in timing of the HCG injection on oocyte fertilizability in vitro were available. We induced follicular development in immature hybrid mice with an injection (4 IU) of pregnant mare serum gonadotropin (PMSG). In the first series of experiments, HCG was given 42, 46 or 50 hours later. We collected cumulus-enclosed, oviducal oocytes for IVF using capacitated mouse sperm 13 hours after the last HCG injection. The fertilization incidence (mean, three experiments) fell as the PMSG-HCG interval was reduced (50 hours, 64%; 46 hours, 40%; 42 hours 24%), but this could be explained by a corresponding increase in spontaneous oocyte activation caused by prolonging the HCG-oocyte collection interval. In the second series of experiments, the latter was fixed at 13 hours; the PMSG interval was altered by staggering the initial PMSG injection. No spontaneous activation occurred, but oocytes collected after "early" HCG injection still showed significantly lower fertilization incidences (42 hours, 36%; 46 hours, 46%) compared with the 50-hour injection (66%). The possible clinical implication of this finding is discussed.