Ontogeny of Estrogen Receptor-β Expression in Rat Testis**This work was supported by grants from the Swedish Medical Research Council (MFR K98–04P-12596–01A) and the Loo och Hans Ostermans Stiftelse (to G.G.J.M.K.) and a grant from the European Union (EU-PL95–1223; to B.v.d.B., P.T.v.d.S., and J.-A.G.).

The recently discovered estrogen receptor-beta (ERbeta) is expressed in rodent and human testes. To obtain insight in the physiological role of ERbeta we have investigated the cell type-specific expression pattern of ERbeta messenger RNA (mRNA) and protein in the testis of rats of various ages by in situ hybridization and immunohistochemistry. In fetal testes of rats 16 days postcoitum and testes of 4-day-old animals, fetal germ cells (gonocytes) reveal the ERbeta mRNA in their cytoplasm and the ERbeta protein in their nucleus. In testes of 11- and 15-day-old rats, ERbeta mRNA and protein were detected in Sertoli cells and type A spermatogonia. No signal was found in other types of germ cells. In the adult testes, expression of ERbeta mRNA as well as ERbeta protein was found in pachytene spermatocytes from epithelial stages VII-XIV and in round spermatids from stages I-VIII. Low ERbeta expression was observed in all type A spermatogonia, including undifferentiated A spermatogonia, whereas no expression was found in In and type B spermatogonia and early spermatocytes. At all ages, Sertoli cells showed a weak hybridization signal as well as weak immunoreactivity for ERbeta. In adult testes, no ERbeta mRNA or protein was detected in the interstitial tissue, indicating that Leydig cells and peritubular myoid cells do not express ERbeta. The expression of ERbeta in fetal and late male germ cells as well as in Sertoli cells suggests that estrogens directly affect germ cells during testicular development and spermatogenesis.