Bipotential gonads, which appear in the mouse at around E10.5, may adopt either male or female fate and are the first stage of reproductive organ development. The gonadal ridge continues to grow and form primary sex cords (ovigerous/testis cords) within it.
Development of a testis (male) or an ovary (female) is triggered by the expression and proper function of Sry, the testis-determining gene. The primary sex cords in the male will eventually mature to become the sperm-nurturing seminiferous tubules. In the female, the primary sex cords will contribute to nurturing tissue of developing ova.
The gonad consists of germ cells and somatic cells.
The primordial germ cells (PGCs) do not arise within the genital ridge. The precursors of the PGCs originate in the mouse epiblast at E6–6.5. The PGCs migrate from the base of the allantois via the gut mesentery and populate the bipotential gonad at around E10.5-11.5. The somatic cells originate from mesonephros and coelomic epithelium.