Oogenesis, fertilisation and early embryonic development in rats. II: Dose-dependent effects of human chorionic...

A total of 950 female Wistar rats in 81 groups were involved in this study. Different groups of rats were stimulated with PMSG (0, 10 & 20 IU) at diestrus followed, 48-52 hr later, by different doses of HCG (0, 10, 20, 30 & 40) for ovulation induction. The dose-dependent effects of HCG, either with or without the use of PMSG for stimulation of multiple follicular development, on the quality of oocytes and their in vitro fertilisability, quality of Day 2-embryos, viability of pregnancy and status of embryos retrieved on Day 2, 3, 4 or 5 of pregnancy in different subgroups of rats were examined. Results showed that more oocytes and embryos fertilised in vivo were retrieved from rats supraphysiologically stimulated with 20 IU of PMSG. The addition of HCG did not increase the number of ovulated oocytes or Day-2 embryos. In other words, the number of oocytes or embryos produced is dependent on the dose of PMSG administered during diestrus rather than on the dose of HCG given for ovulation induction. Hence, no increase in the amount of HCG is required to effectively ovulate bigger cohort of preovulatory follicles in supraphysiologically stimulated rats. As was shown earlier, in vitro and in vivo fertilisation rates were reduced when higher doses of PMSG were used. Similarly, these rates were reduced when increasing doses of HCG were used in rats not previously stimulated with PMSG. When higher doses of HCG were used in rats stimulated earlier with PMSG (10 and 20 IU), the in vitro but not the in vivo fertilisation rates were further reduced.(ABSTRACT TRUNCATED AT 250 WORDS)