Some morphological and hormonal aspects of ovulation and superovulation in the rat

In the first experiment, mature female Wistar rats, displaying a regular 4-day oestrous cycle, were killed in succession every 2 or 3 h on the day of pro-oestrus and oestrus until the time of ovulation. In the second experiment, immature female Wistar rats (aged 24 days) were injected s.c. with 30 IU pregnant mare serum gonadotrophin (PMSG) and 56 h later with 20 IU human chorionic gonadotrophin (hCG). They were killed in groups at 0, 24, 48, 56 and 57 h, and then every 2 h until 72 h. Excised ovaries were homogenized and analysed for steroid content or they were submitted to a routine histological procedure. The cyclic and PMSG/hCG-treated rats exhibited some similarities and differences in the general pattern of steroid content. Either a presumptive endogenous LH surge or administration of hCG resulted in an increase in the ovarian androgen concentration which preceded a rise in progesterone; the progesterone peak, in turn, was accompanied by a fall in the amount of androgens and oestradiol. However, in comparison with cyclic rats, superovulated animals displayed a significantly higher ovarian androgen level for a prolonged period; ovarian oestradiol concentration was also raised while the progesterone content was much lower. Histological analysis revealed large differences between the ovaries of superovulated and cyclic rats, especially with regard to the maturing follicles. The majority of PMSG/hCG-derived follicles showed hypertrophied theca interna and degenerated or luteinized granulosa. A large number of preovulatory follicles did not ovulate. These results clearly indicate that PMSG/hCG-induced follicles are not equal to the follicles developing during a normal oestrous cycle. This should be taken into consideration when using superovulated animals in experiments.