Nipple Differentiation in Fetal Male Rats Treated with an Inhibitor of the Enzyme 5α-Reductase: Definition of a Selective Role for Dihydrotestosterone*

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Abstract
In the rat, androgens are responsible for sexually dimorphic nipple differentiation. Nipples are expressed in the female, while in the male, the nipple anlage regress prenatally. Mammary gland development is present in both sexes. Treatment of pregnant Sprague-Dawley rats from days 12-21 of gestation with the aza-steroid 17.beta.-N,N-diisopropylcarbamoyl-4-aza-5.alpha.-androstan-3-one, a competitive inhibitor of the enzyme 5.alpha.-reductase, resulted in nipple development in male offspring. Additionally, there was feminization of the external genitalia, with urethral displacement to the base of the phallus. The role of androgens in suppression of nipple anlage in the male rat fetus is known. This study, however, suggests for the first time a selective role for 5.alpha.-dihydrotestosterone in regression of the nipple anlage in utero. Thus, 5.alpha.-dihydrotestosterone may be critical not only for masculinization of the external genitalia, but also for inhibition of nipple development in the male rat fetus.

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