No abstract

The aim of this study was to investigate whether GH and insulin-like growth factor I (IGF-I) are involved in preantral folliculogenesis and, if so, to clarify the relationship between GH/IGF-I and activin/follistatin (FS) systems in immature female mice. Ovaries were obtained from 11-day-old mice, and preantral follicles, 100-105 microm in diameter, were mechanically isolated and selected for culture. Ten preantral follicles per well were cultured with different quantities and combinations of additives as follows: no additives (control), recombinant human FSH (rhFSH), IGF-I, recombinant human GH (rhGH), activin A, and recombinant human FS (rhFS). Mean diameters of the follicles were measured daily, and estradiol and immunoreactive inhibin levels in the cultured medium were assayed by RIA on day 4. rhGH showed stimulatory effects on the follicular diameter and the secretion of estradiol and immunoreactive inhibin. These effects were augmented by the presence of IGF-I and activin A. IGF-I alone did not show any stimulatory effect. The addition of rhFSH to activin A or to rhGH and activin A promoted preantral follicular growth and hormone production. On the other hand, GH- or activin-stimulated follicular growth was suppressed by rhFS in a dose-dependent manner. These results indicate that activin A and rhGH may play an important role in controlling earlier phases of follicular development during the infantile period, which is considered to be gonadotropin independent.

Activin A is a gonadal protein originally isolated from follicular fluid and is recognized as a local regulator of granulosa cell differentiation. Whether activin A promotes folliculogenesis, however, still remains unclarified. The present study was designed to elucidate the effect of activin A on follicular growth in in vitro follicle culture systems. Preantral follicles, 100-120 microm in diameter, were mechanically isolated from BDF1 hybrid immature mice (11 days old) and adult mice (8 weeks old), then cultured for 4 days in a serum-free medium supplemented with activin A (100 ng/ml), FSH (100 mIU/ml), and a combination of both. Follicular diameter was measured daily, and the amount of estradiol and inhibin released at day 4 was determined by RIA. Preantral follicles collected from immature mice showed a significant increase in diameter when cultured with activin A or both activin A and FSH. FSH alone showed no significant effect on the diameter of follicles from immature mice. In contrast, the diameter of preantral follicles from adult mice significantly increased in response to FSH. Activin A did not stimulate growth of follicles from adult mice, and more interestingly, blocked the effect of FSH. The inhibitory action of activin A was in part restored by co-culture with follistatin (100 ng/ml). These results indicate that activin A is folliculogenetic in the prepubertal mouse, but not in adults.

The purpose of the present study was to investigate 1) whether activin A can cause primary follicles to become dormant at the resting stage, and 2) the role of the secondary follicle on follicular growth of primary follicles. Preantral follicles (100-120 microm in diameter) harvested from adult mice and cultured in in vitro follicle culture system showed a significant increase in size and estrogen and inhibin secretion in response to FSH, but the administration of activin A blocked the effect of FSH. Withdrawal of activin A not only restored the follicular response to FSH but also enhanced the effect of FSH, indicating that the action of activin A is to cause small preantral follicles to become dormant at the preantral stage. To investigate the role of secondary follicles in early folliculogenesis, small preantral follicles were cocultured with secondary follicle (300-350 microm in diameter) in the presence of FSH. The secondary follicle showed a significant increase in follicular diameter as a result of stimulation by FSH, but the small preantral follicles did not increase in size. After removal of the secondary follicle, however, the small preantral follicles commenced follicular growth, indicating that the growth of small preantral follicles is suppressed by the secondary follicle. Administration of the activin binding protein follistatin caused a significant increase in follicular diameter of both small preantral and secondary follicles as a result of stimulation by FSH. These results have suggested that secondary follicles cause primary follicles to become dormant at the resting stage by secreting activin.

Both transformation growth factor-beta (TGFbeta) and activin belong to the TGFbeta superfamily, and each receptor is structurally related. We have shown that the action of activin A on folliculogenesis is different in immature and adult mice, so it is of interest to study whether TGFbeta has such an action on follicular development. The effect of TGFbeta on folliculogenesis was studied in isolated preantral follicles from immature, adult, and diethylstilbestrol (DES)-primed immature mice and was compared with that of activin A. TGFbeta caused a significant increase in follicular diameter and estradiol and immunoreactive inhibin secretion in adult mice in a dose-related manner, but did not affect the size of preantral follicles from immature mice. Activin A, on the other hand, caused a significant increase in the size of follicles from immature mice, but did not change the size of preantral follicles from adult mice. TGFbeta enhanced the effect of FSH, whereas activin A completely blocked the action of FSH on preantral follicles from adult mice. Such a specific action of TGFbeta and activin A was age dependent because preantral follicles obtained from 28-day-old mice, compared with those from 11- and 56-day-old mice, showed an intermediate reaction to TGFbeta and activin A. DES pretreatment of 11- and 28-day-old mice caused an enhanced response to FSH, but this response was completely inhibited by TGFbeta. These results indicate that both TGFbeta and activin A have proliferative action and cytodifferentiative action on granulosa cells, but the action of each is age dependent and opposite in direction. In conclusion, although both TGFbeta and activin A belong to the same family, and each receptor is structurally related, both share a specific role in early folliculogenesis before and after puberty.