Since exposure to testicular testosterone is essential during normal sexual
differentiation of the brain, it is reasonable to assume that it can cross the
BBB. In normal male brain, this source of testosterone is largely aromatized
to estrogen. It is this estrogen, in turn, that is essential for the ultimate
masculine development of the brain. Estrogen in developing females, on the
other hand, does not resulting masculinization because this supply is bound by
alpha-fetoprotein.
