Steroid production by Xenopus ovarian follicles at different developmental stages

Xenopus ovarian follicles at different developmental stages were compared with respect to their capacity to produce and secrete steroids and to respond to gonadotropic hormones with changes in steroid production. Individual follicles were obtained by treating ovaries with collagenase and were incubated for 10 hr in incubation medium alone or in medium containing a Xenopus pituitary homogenate (FPH, 0.04 pituitary/ml). At 10 hr oocytes were scored for meiotic maturation and ovulation and incubation media and follicles were collected separately and frozen. Both tissue and media were measured for progesterone, testosterone, and estradiol by radioimmunoassay. The smallest follicles (Dumont Stages I and II) secreted very low or nondetectable levels of these three steroids in both the presence and absence of FPH. Medium-sized follicles (Stages III and IV) were active in secreting estradiol and FPH stimulated a twofold increase in the accumulation of estradiol in the incubation medium. In follicles larger than Stage IV, estradiol secretion decreased as follicular size increased. In contrast, large follicles (Stages V and VI) secreted much more testosterone and progesterone than medium-sized follicles and FPH significantly increased the accumulation of these two steroids. Overall, follicular secretion of testosterone was much greater than secretion of progesterone or estradiol. The concentration of the three steroids in the ovarian follicles at 10 hr of incubation followed patterns that were similar in general to the patterns of accumulation of steroids in the incubation medium. Ovine LH, but not FSH, was effective in stimulating secretion of the three steroids and in triggering meiotic maturation of oocytes in Stage VI follicles. Although many of the morphological and biochemical events of oogenesis and folliculogenesis in Xenopus have been described, this is the first report of the developmental pattern of steroid production. The data indicate that growth and development of Xenopus follicles are accompanied by shifts in steroidogenic abilities, with estradiol produced by medium-sized follicles and androgen and progesterone secreted by large follicles.