Disruption of ovarian development in alligator embryos treated with an aromatase inhibitor

It has been suggested that sex differentiation in vertebrates is steroid hormone dependent, that estrogens play a critical role in ovarian differentiation, and that male sex differentiation will occur in the absence of estrogens. Using the model of the alligator in which sex can be manipulated by incubation conditions (eggs incubated at a constant temperature of 30° produce 100% females, and at 33° produce 100% males), a series of experiments using antiestrogens, antiandrogen, estradiol-17β, dihydrotestosterone (DHT), and aromatase inhibitors were performed. Test substances were injected into alligator eggs prior to gonadal sex differentiation and the eggs were incubated at male or female temperatures until just before expected date of hatching. Gonads were excised and the sex was verified histologically. Control embryos injected with vehicle produced the expected sex: females at 30° and males at 33°. Estradiol in eggs at 33° (male temperature) produced 100% females and did not alter female development in eggs at 30°. Antiandrogen, DHT, and a steroid antiestrogen had no discernible effect in either the 30° or the 33° eggs at the doses tested. The aromatase inhibitors aminoglutethimide and 4-hydroxyandrostenedione caused a moderate disruption of ovarian development and had no apparent effect on testicular development. The nonsteroidal aromatase inhibitor, Ciba Geigy 16949A, caused inhibition of ovarian development in all treated embryos. The Mullerian ducts did not appear to be affected by this treatment, or by any of the other treatments, and the gonads did not appear masculinized. We conclude that estrogen appears to be necessary for normal ovarian development, but that inhibition of estrogen synthesis alone is insufficient to cause masculinization. Likewise, exogenous androgens appear unable to masculinize embryonic gonads. The evidence suggests that testicular differentiation in amniote vertebrates is dependent on factors other than androgens or level of estrogens.