Effects of a Superovulatory Dose of Pregnant Mare Serum Gonadotropin on Ovarian Function, Serum Estradiol, and Progesterone Levels and Early Embryo Development in Immature Rats12

Abstract

Reduced fertility is frequently associated with excessive follicle stimulation following the use of exogenous gonadotropins to induce ovulation. To investigate the mechanisms involved, a superovulatory dose of pregnant mare serum gonadotropin (PMSG, 40 IU) was administered to 76 30-day-old rats (0800-0900 h, day 0). Control rats [68] received 4 IU, a dose which induces normal numbers of ovulations followed by normal pregnancy. Some rats were killed at .apprx. 1200 h on day 2. The remainder were caged with mature fertile males on day 2, scored for the occurrence of mating on the morning of day 3, and killed at about 1200 h on days 3, 4, 5 or 7 (days 1, 2, 3 or 5 of pregnancy, respectively). Corpora lutea (CL) and large, nonluteinized follicles were counted, and oviducts and uteri flushed to determine number, location, and stage of development of embryos. Ovarian and uterine weight, the accumulation of fluid in the uterus and ampulla, the ovarian content of estradiol, and serum levels of estradiol and progesterone were also recorded. Mating and fertilization occurred in most superovulated and control rats. Mean numbers of CL observed in control rats killed on days 2, 3, 4, 5 and 7 were 0, 10.7, 10.5, 11.6 and 11.5, respectively. The corresponding means for CL plus luteinized follicles in superovulated rats were 55, 65, 85, 85 and 112, respectively. Ovaries from superovulated rats also contained, on average, 20.8, 27.5, 18.5, 13.6 and 12.3 large, nonluteinized follicles on days 2, 3, 4, 5 and 7, respectively; 10.4 such follicles were observed in controls on day 2 only. Mean numbers of ova or embryos recovered on days 2, 3, 4, 5 and 7, respectively, from oviducts of controls were 0, 8.3 (1-cell embryos), 8.7 (2-cell embryos), 8.9 (2- 3-. and 4-cell embryos), and 0, and from the oviducts of superovulated rats, 15.1 (unfertilized ova), 38.4 (1-cell embryos plus degenerate ova), 17.9 (2-cell embryos plus degenerate ova), 6.6 (2-, 3-, and 4-cell embryos plus degenerate ova), and 0. Flushings of uteri yielded 9.4 blastocysts from controls on day 7 only, and only an occasional embryo from superovulated rats on days 4 and 5, but none on day 7. Superovulated rats ovulated over a prolonged period, presumably on the nights of days 1 and 2. Mean serum estradiol levels (pg/ml .+-. SEM [standard error of the mean]) observed in control and superovulated rats on days 2, 3, 4, 5 and 7, respectively, were 93 .+-. 5 vs. 230 .+-. 10, 34 .+-. 3 vs. 134 .+-. 19, 31 .+-. 2 vs. 201 .+-. 33, 34 .+-. 2 vs. 98 .+-. 10, and 37 .+-. 3 vs. 49 .+-. 3. The corresponding serum progesterone levels (ng/ml .+-. SEM) were 2 .+-. 1 vs. 29 .+-. 3.9 .+-. 3 vs. 74 .+-. 5.22 .+-. 3 vs. 152 .+-. 11, 43 .+-. 3 vs. 200 .+-. 15, and 61 .+-. 4 vs. 220 .+-. 18, respectively. Apparently embryo loss following superovulation is due to excessive estrogenic stimulation of the genital tract, probably arising from large follicles which persist after normal ovulation occurs.