THE DEVELOPING XENOPUS OOCYTE SPECIFIES THE TYPE OF GONADOTROPIN-STIMULATED STEROIDOGENESIS PERFORMED BY ITS ASSOCIATED FOLLICLE CELLS

As a response to gonadotropin, amphibian ovarian follicles primarily s ynthesize and secrete estradiol-17 beta (E2) during vitellogenesis and progesterone (P) when fully grown. Stage IV (vitellogenic) and stage VI (full-grown) ovarian follicles from Xenopus laevis, as well as inte rmediate sizes, were used to explore this change in steroidogenesis. O ptimum steroidogenesis occurred in both stage IV and stage VI follicle s exposed for 6 h to 20 IU human chorionic gonadotropin/mL. Although t he total amounts of steroid found were about the same, the E2/P ratios ranged from 26 to 35 for intact stage IV follicles, but only 0.02-0.0 3 for intact stage VI follicles. Steroid-producing follicle cells were isolated from stage IV and stage VI follicles by non-enzymatic proced ures, were washed and were tested for steroidogenic activity in the ab sence of oocytes. In both cases, P was the predominant steroid produce d (E2/P = 0.004-0.04), so the presence of stage IV, but not stage VI, oocytes appears to be necessary for E2 production as a response to gon adotropin. Octanol had no significant effect on the E2/P ratio of inta ct stage IV follicles. Dissected oocyte/follicle cell preparations fro m stage IV follicles were also periodically challenged with gonadotrop in over 72 h, during which time most follicle cells detached from the oocyte and formed a monolayer over the bottom of the culture dish. The relatively high E2/P ratios for such preparations showed no significa nt change when stimulated with gonadotropin at various times over the 72 h, as long as the medium was not replaced. We conclude that the est rogenic effect of stage IV oocytes is most likely mediated by a secret ory product rather than by gap junctions or by cell contact. Because t he X. laevis oocyte has been shown to be a self-differentiating cell, the steroidogenic shift that occurs in developing ovarian follicles ap pears to be fundamentally regulated by the growing oocyte as it underg oes a physiological change rather than by different gonadotropins.