Figure 1. Current model of human gonad and adrenal development between Carnegie Stage 13 (CS13) and Fetal Stage 3 (F3).

The bipotential gonad and adrenal gland both arise from the adrenogonadal primordium around CS13. Sex determination in the bipotential gonad occurs between CS17 and CS23 when transient SRY expression promotes the upregulation of genes, including SOX9, in the developing testis. Sex differentiation starts with the onset of steroidogenesis around CS23, which results in development of the external genitalia and phallic growth (right panel). Ovary-specific genes are thought to suppress male pathway specification. The ovary also supports germ cell expansion and entry into meiosis. The fetal adrenal cortex develops into definitive and fetal zones, producing steroid hormones and adrenal androgens. Chromaffin cells derived from the sympathetic nervous system migrate into the developing adrenal gland, merging later to form the adrenal medulla.