The clinical use of diethylstilbestrol (DES) by pregnant women has resulted in an increased incidence of genital carcinoma in the daughters born from these pregnancies. Also, in the so-called DES-sons abnormalities were found, mainly, the presence of Mü llerian duct remnants, which indicates that fetal exposure to DES may have an effect on male sex differentiation. Fetal regression of the Mü llerian ducts is under testicular control through anti-Mü llerian hormone (AMH). In male mice, treated

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... utero with DES, the Mü llerian ducts do not regress completely, although DES-exposed testes do produce AMH. We hypothesized that incomplete regression in DES-exposed males is caused by a diminished sensitivity of the Mü llerian ducts to AMH. Therefore, the effect of DES on temporal aspects of Mü llerian duct regression and AMH type II receptor (AMHRII) messenger RNA (mRNA) expression in male mouse fetuses was studied. It was observed that Mü llerian duct regression was incomplete at E19 (19 days post coitum), upon DES administration during pregnancy from E9 through E16. Furthermore, analysis of earlier time points of fetal development revealed that the DES treatment had clearly delayed the onset of Mü llerian duct formation by approximately 2 days; in untreated fetuses, Mü llerian duct formation was complete by E13, whereas fully formed Mü llerian ducts were not observed in DES-treated male fetuses until E15.