ONTOGENY AND LOCALIZATION OF AN ONCOSTATIN M-LIKE PROTEIN IN THE RAT TESTIS - ITS POSSIBLE ROLE AT THE START OF SPERMATOGENESIS

Oncostatin M (OSM), a member of the interleukin 6 family of cytokines, was found to be highly expressed in the late fetal and early neonatal rat testis, as well as in the maturing and adult testis, Two differen t forms of OSM were observed, one of M, 22,000 and the other of M, 36, 000, In the prepubertal rat testis [19 days post coitum, 8 days post p artum (dpp), and 15 dpp], the form with the higher molecular weight pr evailed, whereas in the maturing testis (30 dpp, 45 dpp, and 12 weeks post partum), a shift toward the lower molecular weight form was obser ved, as well as a decrease in its relative amount. By immunohistochemi stry on testicular sections, OSM-specific immunostaining was observed in the interstitial tissue at every age studied, In contrast, OSM immu noreaction was localized in the Sertoli cells exclusively around the s tart of spermatogenesis, being strongest at 3 dpp, In vitro studies re vealed that neonatal Sertoli cells produce OSM. The possible role of O SN1 at the start of spermatogenesis was investigated by using a cocult ure of Sertoli cells and gonocytes isolated from newborn rats, OSM sig nificantly increased the survival of both Sertoli cells and gonocytes in a dose-dependent manner. The proliferative activity of the Sertoli cells was not affected by OSM, whereas that of gonocytes was increased by almost 60% after 6 days of culture, Comparison of the effect of OS M on these cocultures with other members of the interleukin 6 family o f cytokines demonstrated that this factor is more potent than leukemia inhibitory factor or ciliary neurotrophic factor, On the basis of the se findings, it can be concluded that OSM is present in the rat testis , and it is likely to play an important role at the start of spermatog enesis.