OPTIMIZATION OF SUPEROVULATION IN THE REPRODUCTIVELY MATURE MOUSE

Optimum gonadotropin doses and chronology were established for the ind uction of superovulation in sexually mature hybrid mice (BALB/cBy x C5 7BL/6By). A regime of 12 IU pregnant mares' serum gonadotropin (PMSG), followed 48 hr later by 20 IU human chorionic gonadotropin (hCG) admi nistered 1 hr before the midpoint of the light cycle (1200), gave the maximum ovulatory response. There was no evidence that endogenous lute inizing hormone influenced the superovulation response to exogenous go nadotropins. Fewer than 50% of zygotes reached the blastocyst stage (9 0-93 hr post hCG), with the grentest rate of loss at the two- to four- cell stage. Litter size following superovulation was 19.6 +/- 0.9. The re was no significant difference between the number of blastocysts obs erved and litter size. Similarly, counts of mature follicles in ovarie s prior to hCG stimulation were not significantly greater than the num ber of secondary oocytes that subsequently ovulated. These data indica te that standard superovulation protocols may require fine-tuning to m aximize productivity and confirm that embryo loss is greatest between the first cleavage division and blastocyst formation.