Molecular and cellular machinery of gonadal differentiation in mammals.

The direction of gonadal development is established through the predominance of one of two sex-determining pathways in bipotential gonads. When Sry is expressed, the male pathway prevails and a series of prominent changes in morphology and function of the gonad lead to testis differentiation. In the absence of Sry, the female pathway gains control of gonadal development, resulting in a lack of significant changes in the gonad, and as a consequence ovarian development becomes dormant. Several important molecular and cellular processes have been identified as operating during the development of the testis, including testis cord and vasculature formation, as well as testis-specific cell migration. Nevertheless, gene expression analysis has indicated that more genes participate in ovarian than testis development. Ovarian development is actively controlled at the molecular level and seems driven by inhibition of testis-specific processes. Current knowledge concerning genes participating in gonadal differentiation is limited, but it is apparent that this process seems more complicated than previously thought.