Induction of Structural Alterations in the Preovulatory Amphibian Ovarian Follicle by Hormones

After in vitro incubation amphibian ovarian follicles undergo structural and cellular alterations independent of ovulation in response to progesterone and frog pituitary hormones. Separation of the follicular wall from the oocyte is observed after fixation of hormonetreated, but not control, follicles. In unfixed follicles detachment of the follicle cells from the exterior surface of the vitelline membrane and a separation of the oocyte from the interior surface of the vitelline membrane were produced by the hormones. Detachment from both surfaces of the vitelline membrane appeared to occur simultaneously and occurred close to the time of germinal vesicle breakdown within the follicular oocyte. Dissection of an intact acellular vitelline membrane from unovulated ovarian follicles was, therefore, possible. Steroids which were effective stimulators of germinal vesicle breakdown also produced membrane separation, whereas estrogenic hormones were ineffective in stimulating either process. Frog pituitary hormones were less effective than progesterone in producing these changes; however, in both cases, the types of changes were identical. Structural alterations produced occurred earlier in response to progrsterone than to the frog pituitary extracts. Puromycin and cycloheximide prevented these hormone-induced changes and this suggests that the structural alteration of the ovarian follicles involves the synthesis of protein(s). These results suggest that structural alterations over the entire surface of the ovarian follicle as well as follicle rupture are necessary prerequisites for ovulation to occur and that hormones are the natural mediators of these events. The relevance of these structural changes to follicular functions other than ovulation are also discussed.