Figure 1.

A genetic model for sex determination, controlled by a balance of antagonistic pathways. In XY gonads, Sry triggers upregulation of Sox9, leading to Sertoli cell commitment and testicular differentiation. Sertoli cell differentiation is a result of the establishment of a positive feedback loop between Sox9 and secretion of Fgf9 (and also PGD₂; not shown), which act in a paracrine manner to recruit additional Sertoli cells. In XX gonads, two independent signaling pathways involving the Rspo1/Wnt4/β-catenin pathway and Foxl2 tilt the balance towards the female side and silence Sox9 and Fgf9. Arrows indicate stimulation; T bars indicate inhibition.