Developmental patterns of the testicular response to experimental modifications of circulating androgen levels in the fetal rabbit.

The circulating level of free androgens in fetal and newborn rabbits was reduced by active immunization of mothers against testosterone (T) or was increased by injecting dihydrotestosterone (DHT) into the mothers. After immunization, about 100% of the circulating T and DHT in fetuses was bound. After maternal injection of DHT, the circulating level of this hormone in fetuses was increased 2- to 120-fold. The effects of these treatments were evaluated by determining testicular levels of T, a physiological index of circulating gonadotropin. From 20--23 days of gestation, testicular T content was modified neither by circulating antibodies nor by an increase of blood DHT. The same overloads of circulating DHT, which were ineffective between 20--23 days, significantly reduced testicular T content between 25--29 days. Testicular T content was significantly increased in newborns from immunized mothers. These results suggest that the appearance of the negative feedback action of circulating androgens takes place relatively late, at 24 or 25 days, after differentiation of the genital tract.