Effect of activin A on in vitro development of rat preantral follicles andlocalization of activin A and activin receptor II

Preantral follicles (140-160 mum) were isolated mechanically from the ovari es of 10-day-old rats and cultured in groups of four to six for 6 days in m edium containing 0, 1, 10, or 100 ng/ml of activin A. Activin stimulated (P < 0.05) the growth of preantral follicles in a dose-dependent fashion and enhanced the proliferation of preantral, oocyte-free follicular cells. Furt hermore, treatment with activin induced the majority of follicles to form a n antrum-like structure and helped to maintain the ultrastructure of these follicles during culture. Activin A also induced further changes characteri stic of follicle and oocyte maturation, such as the elongation of granulosa cells contacting the oocyte and migration of the cortical granules to the oocyte cortex. In addition, gene expression for activin and activin recepto r type II (ActR II) was demonstrated in both the oocytes and the somatic ce lls using the reverse transcription-polymerase chain reaction, and immunohi stochemical studies demonstrated the presence of activin and ActR II protei ns in the somatic tissue and, especially, the oocytes of these follicles. I t is concluded that, in vitro, activin A stimulates the growth of rat prean tral follicles and promotes antrum formation. Furthermore, because activin A and ActR 11 are synthesized within preantral follicles, intrafollicular a ctivin likely plays an important role in early follicular development.