MOUSE OOCYTES REGULATE GRANULOSA-CELL STEROIDOGENESIS

Recent studies have demonstrated a critical role for the oocyte in pro liferation and differentiation of granulosa cells and expansion of the cumulus oophorus in vitro. The purpose of this study was to determine if steroid production by cumulus granulosa cells was also modulated b y oocytes. Mouse oocyte-cumulus cell complexes (intact) and complexes from which the oocytes were removed microsurgically (oocytectomized; O OX) were cultured for 24 h in the presence or absence of follicle-stim ulating hormone (FSH; 150 ng/ml), testosterone (T; 5x10(-7) M) or both . Oocytectomy had no effect on the ability of cumulus cells to produce progesterone or estradiol in control cultures or in response to T. Ho wever, OOX complexes produced 17- and 36-fold more progesterone than i ntact complexes when cultured in the presence of FSH or FSH + T, respe ctively. Oocyte-conditioned medium (maximum 1 oocyte/2 mul) had no eff ect on progesterone production by intact cumulus complexes, but reduce d the progesterone production by OOX complexes by 75%. This inhibition was directly proportional to the number of oocytes used to condition the medium. Oocytectomy caused a slight decrease (29%) in estradiol pr oduction by complexes in the presence of FSH and T; however, OOX compl exes in oocyte-conditioned medium produced almost twice as much estrad iol as complexes in unconditioned medium. These results indicate that mouse oocytes secrete a factor(s) that inhibits progesterone and stimu lates estradiol production by cumulus granulosa cells.