Spontaneous oocyte maturation in Rana pipiens: Estrogen and follicle wall involvement

Abstract

AbstractImmature (germinal vesicle stage) Rana pipiens oocytes typically remain arrested in prophase I of meiosis even after extended periods of in‐vitro culture, if not stimulated with hormones. We have, however, sporadically observed “spontaneous” occurrences of oocyte maturation in vitro without the addition of hormones. This study documents some of our observations on this phenomenon and presents experimental results concerning the effects and possible involvement of estrogen and follicle wall components in regulating spontaneous oocyte maturation. Estrogen was found to inhibit spontaneous oocyte maturation (GVBD) in a dose‐dependent fashion. Follicles in which spontaneous maturation was inhibited by estrogen retained their responsiveness (GVBD) to both frog pituitary homogenate (FPH) and progesterone stimulation. Inhibitory effects of estrogen on spontaneous maturation, however, were not reversed following incubation of washed follicles in plain culture medium without added hormones. Possible involvement of progesterone synthesis in spontaneous oocyte maturation was ascertained by simultaneously monitoring endogenous progesterone synthesis and the occurrence of spontaneous GVBD over the course of the maturation process. In spontaneous maturing follicle there was a gradual increase in basal levels of progesterone synthesis that preceded GVBD. Significantly, addition of estrogen abolished both the spontaneous progesterone production and spontaneous oocyte maturation. When FPH was added to follicles exhibiting spontaneous oocyte maturation, progesterone production was augmented and the time course of oocyte maturation was greatly accelerated. Involvement of ovarian components in the maturation process was investigated by selective removal of various follicle layers by microdissection. Removal of follicle epithelium and theca layer (defolliculation) markedly decreased spontaneous and FPH‐induced maturation, whereas removal of the entire follicle wall (denudation) completely blocked it. Our results suggest that both spontaneous and FPH‐induced maturation involve an estrogen sensitive process in the follicle wall. Thus, somatic follicle cells appear to serve as a common mediator for both types of maturation, which are linked by some intrafollicular mechanism involving steroidogenesis. Hence, estrogen may play an important role as an endogenous intrafollicular regulator of oocyte meiotic maturation.