One tissue, two fates: Molecular genetic events that underlie testis versus ovary development

The pivotal point of vertebrate sex determination is the development of the gonad into a testis or ovary, which governs phenotypic sex through the production of hormones. The identification of Sry, the genetic switch that controls testis development in mammals, fuelled the race for the discovery of downstream pathways. Comparative analyses of XY versus XX gonadogenesis in both mouse and human genetic mutants have uncovered a burgeoning network of intra- and extra-cellular pathways. Here, we review the old and new players that are involved in the initial steps of testis and ovary development.