The effects of norethindrone on the mullerian ducts of the American alligator.

Norethindrone, a reported aromatase inhibitor, has been used to examine the role of estrogens in the unilateral regression of the mullerian ducts that occurs in female chick embryos. The mullerian ducts are embryonic oviducts that regress in most male vertebrates under the influence of the testicular hormone, mullerian inhibiting substance (MIS). The ovaries of the chick also produce MIS during early development, but only the right duct regresses. Based on the finding that the left duct contains significantly more estradiol binding sites than the right duct, it has been proposed that the left duct is protected from the effects of MIS by preferentially binding estradiol from the ovaries. In support of this theory, norethindrone (0.1 and 0.5 mg) injected into the airsac of chick eggs results in regression of the left mullerian duct of female embryos, presumably by blocking the synthesis of estradiol. In the present study, it was hypothesized that crocodilians, because of the common ancestry they share with birds, would respond in a similar manner by exhibiting regression of both mullerian ducts in response to norethindrone. However, the application of norethindrone (0.5 mg) to the chorioallantoic membrane of female alligator embryos in ovo resulted in significant hypertrophy of the ducts, indicating that norethindrone had an estrogenic effect in the alligator rather than acting as an aromatase inhibitor. There was no effect of norethindrone on the gonads.