In vitro estrogen modulation of pituitary and progesterone-induced oocyte maturation in Rana pipiens.

The role of estrogen in the process of hormone-induced meiotic maturation was investigated using in vitro culture of Rana pipiens ovarian follicles and isolated oocytes. Estrogen alone was ineffective in triggering maturation and did not act synergistically when combined with progesterone. Instead, slight inhibitory effects on oocyte maturation were observed when estrogen was added to the culture medium simultaneously with progesterone. Prolonged estrogen pretreatment (1-8 hr) was seen to be increasingly effective in inhibiting progesterone-induced germinal vesicle breakdown (GVBD). Washing follicles after estrogen pretreatment diminished the inhibitory effects, suggesting continued presence of estrogen was necessary for maximal inhibition. Progesterone-induced GVBD was inhibited by estrogen in denuded oocytes (follicle wall removed) indicating that estrogen acts directly on the oocyte. Incorporation of radioactive progesterone into ovarian follicles was not inhibited regardless of the estrogen concentration used. This suggests that estrogen inhibits oocyte maturation by raising intrafollicular threshold to progesterone rather than by suppressing hormone incorporation into ovarian follicles. Next, the action of estrogen on frog pituitary homogenate (FPH)-induced GVBD was examined although both progesterone and FPH-induced GVBD were inhibited by estrogen, the latter was remarkably more susceptible and did not require estrogen pretreatment. Furthermore, increased concentration of progesterone, but not FPH, overcame the inhibitory action of estrogen. The results suggest that estrogen may exert its inhibitory effects on gonadotropin action at more than one site. Estrogen can modulate either the maturation-inducing action of progesterone directly at the oocyte level or the suppression of progesterone production by the follicle cell. Hence, estrogen may play an important role as an endogenous regulator of oocyte meiotic maturation.