SEXUAL-DIFFERENTIATION OF THE VERTEBRATE BRAIN - PRINCIPLES AND MECHANISMS

A wide variety of sexual dimorphisms, structural differences between t he sexes, have been described in the brains of many vertebrate species , including humans. In animal models of neural sexual dimorphism, gona dal steroid hormones, specifically androgens, play a crucial role in e ngendering these differences by masculinizing the nervous system of ma les. Usually, the androgen must act early in life, often during the fe tal period to masculinize the nervous system and behavior. However, th ere are a few examples of androgen, in adulthood, masculinizing both t he structure of the nervous system and behavior. In the modal pattern, androgens are required both during development and adulthood to fully masculinize brain structure and behavior. In rodent models of neural sexual dimorphism, it is often the aromatized metabolites of androgen, i.e., estrogens, which interact with estrogen receptors to masculiniz e the brain, but there is little evidence that aromatized metabolites of androgen play this role in primates, including humans. There are ot her animal models where androgens themselves masculinize the nervous s ystem through interaction with androgen receptors. In the course of ma sculinizing the nervous system, steroids can affect a wide variety of cellular mechanisms, including neurogenesis, cell death, cell migratio n, synapse formation, synapse elimination, and cell differentiation. I n animal models, there are no known examples where only a single neura l center displays sexual dimorphism. Rather, each case of sexual dimor phism seems to be part of a distributed network of sexually dimorphic neuronal populations which normally interact with each other. Finally, there is ample evidence of sexual dimorphism in the human brain, as s ex differences in behavior would require, but there has not yet been a ny definitive proof that steroids acting early in development directly masculinize the human brain. (C) 1998 Academic Press.