Stimulation of Granulosa Cell Maturation by Follicular Fluid

Antral follicle granulosa cells experience a series of biochemical and morphological changes as the follicles enlarge and mature. These include increased steroidogenic activity (Channing, 1970), increased cellular content of steroidogenic organelles (Rigby et al., 1980a), enhanced responsiveness to the gonadotrophins, and enhanced adenyl cyclase activity (Channing and Ledwitz-Rigby, 1974). Concomitant with follicular enlargement granulosa cell dependence on hormones to stimulate luteinization declines. Granulosa cells from large (6–12 mm) preovulatory, porcine follicles luteinize in vitro without requiring gonadotrophins in the culture media; medium (3–5 mm) follicles require either FSH or LH (Channing, 1970). However, even with estrogen, FSH and LH in the media the granulosa cells of small follicles do not secrete as much progesterone as cells obtained from more mature follicles (Thanki and Channing, 1976). The question remains as to which factors control in vivo maturation of granulosa cells and especially what determines whether an individual follicle will mature or become atretic.