ACTIVIN STIMULATES SERTOLI-CELL PROLIFERATION IN A DEFINED PERIOD OF RAT TESTIS DEVELOPMENT

The action of activin-A on Sertoli and spermatogonial cell proliferati on during early postnatal life was studied by using in vitro organ cul ture of testis fragments from 9-day-old rats. Activin significantly st imulated H-3-thymidine incorporation into testis fragments cultured fo r 3 days in the presence of FSH, whereas it had no effect in the absen ce of the hormone. This effect was dose dependent in the range 10-200 ng/ml and was specifically inhibited by the activin-binding protein, f ollistatin. The effect of activin upon proliferation of different test icular cells was studied in detail by 5-bromo-2'-deoxy uridine-labelin g fragments at the end of in vitro culture and then determining percen tages of different labeled cells on immunostained histological section s. Concomitant treatment with FSH and activin, but not with FSH or act ivin alone, significantly stimulated Sertoli cell proliferation but ma rkedly depressed that of differentiating type A spermatogonia. In cont rast, proliferative activity of undifferentiated type A spermatogonia was independent of activin, irrespective of the presence of FSR. The e ffect of donor animal age was then investigated by culturing fragments derived from 3- and 18-day-old rats for 3 days. An age-related respon se was evident. Sertoli cell proliferation was stimulated by FSH alone in fragments from 3-day-old rats, activin having no apparent effect a t this age. In contrast, none of the hormones tested either alone or i n combination was effective in 18-day-old animals. These results demon strate that activin acts with FSH in maintaining mitotic potentiality of Sertoli cells in a defined phase of their maturation path, when the ir proliferative activity is approaching the final arrest. These findi ngs suggest that activin may be an important local factor in regulatin g Sertoli cell number and that the mitosis of differentiating spermato gonia subsides during Sertoli cell proliferation.