Aromatase activity in the developing rabbit brain.

The formation of 17beta-[3H]estradiol from [1,2,6,7-3H]testosterone was assessed in placenta and central nervous system tissues from rabbit embryos that varied in age from 13-28 days of gestation. In the fetal brain, significant rates of aromatase activity were limited exclusively to the forebrain, and the highest rates of activity (approximately 0.5 pmol/h/mg protein) were found in the diencephalon both male and female embryos between days 19 and 25 of gestation. These rates of aromatase activity are second only to the fetal ovary when expressed per mg protein; moreover, forebrain is the only tissue in the male embryo capable of synthesizing significant amounts of estrogens in vitro. When projected to the whole organ, the capacity of the diencephalon for aromatization exceeds the capacity of the fetal ovary approximately 9-fold. Placental aromatase activity was high (2.1 pmol/h/mg protein) on day 13 but fell to a level approximately 20-fold lower by day 19 of gestation. These findings indicate the potential importance of the forebrain as a source of estrogens during embryogenesis.