HOW DOES DAILY TREATMENT WITH HUMAN CHORIONIC-GONADOTROPIN INDUCE SUPEROVULATION IN THE CYCLIC HAMSTER

Daily s.c. injection of 2.0 IU hCG per day, begun on Day 1 of the cycle (estrus), results in hamsters ovulating 20.7 +/- 0.7 eggs instead of the normal number of 13.3 +/- 0.5 (SEM). This is associated with a reduced rate of follicular atresia so that more of the 10 developing follicles per ovary (large preantral stages) normally recruited on Day 1 of the cycle mature and go on to ovulate. The hCG-treated follicles were larger than control follicles, but contained similar amounts of DNA/follicle; increased size of the antral cavity accounted for their greater size. Moreover, DNA synthesis was significantly reduced in the hCG follicles on Days 2 and 4. Thecal vascularity as judged by the number of red blood cells retained in the theca or microsphere uptake by follicles indicates that on Day 2, thecal blood flow was significantly lower in the hCG-treated animals than in controls. On the other hand, after hCG treatment begun on Day 1, serum levels and in vitro incubation of individual follicles revealed that on Day 2 and beyond, androstenedione (A) and estradiol (E2) levels were elevated. After hCG treatment, the elevated serum E2 correlated with reduced serum 1,H on Days 3 and 4 whereas FSH was unaffected. To study in vitro steroid accumulation, the 10 largest follicles (the developing follicles) were dissected from alternate left and right ovaries from control and hCG-treated animals and incubated individually, and their histology was then compared with the steroid profiles. Accumulation of A and E2 was significantly greater in the hCG-treated follicles than in controls in a 1-h basal incubation and after the addition of 50 ng LH. Progesterone accumulation usually did not differ between the control and hCG-treated follicles. Early stage 1 atretic follicles (judged by histology) were still capable of producing A and E2 in vitro, comparable to control follicles; but, as atresia progressed, the follicles synthesized only progesterone. This is consistent with the temporal pattern previously observed in a model of induced follicular atresia in the hamster [Greenwald, Biol Reprod 1989; 40:175-181]. It is concluded that superovulation resulting from hCG injections is due to thecal production of androgens from follicles normally destined for atresia. For the untreated cyclic hamster, the critical time for thecal androgen production is the first 2 days of the cycle. The aromatizable androgens are then converted into estrogens, which in turn may maintain the microenvironment of the antral cavity, which is essential for viability of the granulosa cells. It is postulated that in rodents the undifferentiated theca producing less androgen than normal is the precipitating factor in inducing atresia of antral follicles, although other possibilities cannot be discounted.