Optimization of superovulation in the reproductively mature mouse.

Optimum gonadotropin doses and chronology were established for the induction of superovulation in sexually mature hybrid mice (BALB/cBy x C57BL/6By). A regime of 12 IU pregnant mares' serum gonadotropin (PMSG), followed 48 hr later by 20 IU human chorionic gonadotropin (hCG) administered 1 hr before the midpoint of the light cycle (1200), gave the maximum ovulatory response. There was no evidence that endogenous luteinizing hormone influenced the superovulation response to exogenous gonadotropins. Fewer than 50% of zygotes reached the blastocyst stage (90-93 hr post hCG), with the greatest rate of loss at the two- to four-cell stage. Litter size following superovulation was 19.6 +/- 0.9. There was no significant difference between the number of blastocysts observed and litter size. Similarly, counts of mature follicles in ovaries prior to hCG stimulation were not significantly greater than the number of secondary oocytes that subsequently ovulated. These data indicate that standard superovulation protocols may require fine-tuning to maximize productivity and confirm that embryo loss is greatest between the first cleavage division and blastocyst formation.