Figure 1: Classic View of Sexual Differentiation of the Brain.

Sexual differentiation begins with chromosome complement and is driven by the Sry gene of the Y chromosome directing the formation of a testis from the bipotential gonad. Testosterone production by the resultant testis will commence during late gestation and into the early postnatal period of the rodent and masculinize select regions of the male brain. A separate process of defeminizaton specific to sex behavior removes the capacity for female-like receptive behavior (i.e. lordosis) in males. The female ovary will develop in the absence of signals from the Sry-induced gene expression cascade and will remain quiescent until puberty when cyclical production of estrogens and progestins begins. There is evidence for a sensitive period for feminization that occurs slightly later in development. The masculinizing actions of testosterone and its aromatized by product, estradiol, are considered organizational and are essential for the activational effects of hormones at puberty on various reproductive parameters.