General Medical Information

Reproduction Embryology

Joe Leigh Simpson

Primordial germ cells originate in the endoderm of the yolk sac and migrate to the genital ridge to form the indifferent gonad. 46,XY and 46,XX gonads are initially indistinguishable. Indifferent gonads develop into testes if the embryo, or more specifically the gonadal stroma, is 46,XY. This process begins about 43 days after conception. Testes become morphologically identifiable 7 to 8 weeks after conception (9 to 10 weeks gestational or menstrual weeks).