Ovulation response and fertilization failure in immature rats induced to superovulate

Immature female rats (75 g body wt, aged 29 days) were injected with 4 or 40 i.u. PMSG on Day -2 and were killed at intervals between 18:00 h on Day -2 and 09:00 h on Day 1. Control animals (4 i.u.) ovulated between 00:30 and 05:30 h on Day 1 whereas the number of ova recovered from superovulated rats (40 i.u. PMSG) increased slowly between 06:00 h on Day -1 and 24:00 h on Day 0 and markedly between 24:00 on Day 0 and 06:00 on Day 1. Similarly treated rats were caged overnight on Day 0 with males of proven fertility and killed between 14:00 and 16:00 h on Day 1. A significantly lower percentage of normal 1-cell ova was recovered from the superovulated rats compared to control animals (71.6 and 98.5%) and of these 1-cell ova a lower percentage was fertilized (69.7 and 99.1%). In the control group all mated animals had a high proportion of ova fertilized whereas 26% of superovulated rats had none or a very low proportion fertilized. In the control animals there was evidence of sperm penetration and pronucleus formation; in superovulated rats significantly fewer ova had pronuclei than were penetrated. These results suggest that reduced fertility of superovulated immature rats is due to complete or partial failure of fertilization in some animals. The extended period during which ovulation occurs may be a contributory factor.