TESTICULAR EXPRESSION OF INHIBIN AND ACTIVIN SUBUNITS AND FOLLISTATININ THE RAT AND HUMAN FETUS AND NEONATE AND DURING POSTNATAL-DEVELOPMENT IN THE RAT

Inhibins, activins, and follistatins are all believed to play roles in the regulation of FSH secretion by the pituitary and in the paracrine regulation of testis function. Previous studies have resulted in conf licting data on the pattern of expression of the inhibin/activin subun its, and little information on expression of follistatin during fetal/ neonatal life. We have made use of new, highly specific monoclonal ant ibodies and fixed tissue sections from fetal, neonatal, and adult rats , and limited amounts of fetal and neonatal human testis, to undertake a detailed immunocytochemical study of the pattern of expression of t hese regulatory proteins. In the rat, positive immunostaining for the alpha-subunit of inhibin (alpha) was first detectable on day 14.5 post coitum (p.c.), the first day on which the testis could be morphologic ally distinguished ham the ovary. During fetal life, the alpha-immunos taining was most prominent in the fetal Leydig cells. In Sertoli cells , alpha-immunostaining was slightly stronger on days 14.5 and 15.5 p.c . compared with 16.5-20.5. After birth, alpha-immunostaining remained intense in fetal Leydig cells but declined following their replacement with their adult-type counterparts; in contrast, alpha-subunit increa sed in Sertoli cells immediately after birth. Immunostaining with anti bodies specific to beta B-subunit showed a similar pattern to that of the alpha-subunit, except that positive immunostaining was first detec table on day 16.5 p.c., 2 days later than immunostaining for the alpha -subunit. The pattern of beta B-immunostaining in postnatal samples pa ralleled that of the alpha-subunit. Immunostaining using antibodies ag ainst the beta A-subunit did not produce any significant reaction prod uct in any sample. Follistatin was undetectable in the fetal rat testi s but appeared in the Leydig cells immediately after birth and its exp ression remained intense throughout postnatal development and in adult testis. No evidence was obtained for expression of either the inhibin /activin subunits or follistatin in the germ cells, peritubular myoid cells, or other interstitial cells in any of the sections examined. In the human fetal testis, both alpha- and beta B-subunits were immunode tectable at 16, 18, and 24 weeks gestation in Sertoli and Leydig cells , with stronger immunostaining in Sertoli cells at 24 weeks. Postnatal ly at 4 months, immunoexpression of the beta B-subunit was no longer d etectable, whereas the alpha-immunostaining became weaker but was stil l present in both Sertoli and Leydig cells. No positive immunostaining for beta A-subunit or follistatin was detectable at any time point st udied. In conclusion, we have shown that, in the rat testis, the major ity of inhibin alpha-subunit and inhibin/activin beta B-subunit is imm unolocalized to the fetal-type Leydig cells during fetal/neonatal life but, following birth, immunoexpression in the Sertoli cells of both s ubunits increases markedly while follistatin is immunodetectable only postnatally.