Ovulation response and fertilization failure in immature rats induced to superovulate

Abstract

Immature female rats (75 g body wt, aged 29 days) were injected with 4 or 40 IU PMSG [pregnant mare''s serum gonadotropin] on day -2 and were killed at intervals between 1800 h on day -2 and 0900 h on day 1. Control animals (4 IU) ovulated between 0030 and 0530 h on day 1, while the number of ova recovered from superovulated rats (40 IU PMSG) increased slowly between 0600 h on day -1 and 2400 h on day 0 and markedly between 2400 on day 0 and 0600 on day 1. Similarly treated rats were caged overnight on day 0 with males of proven fertility and killed between 1400 and 1600 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 while 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. Apparently, 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 factror.