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Although the genetic sex of an infant is determined at fertilization, it is not until the seventh week that the gonads can be differentiated. This series of pages will outline development from fertilization to gonadal differentiation
Early in the fourth week of life, the primordial germ cells may be seen in the endoderm around the allantois. These germ cells migrate under the coelomic epithelium, around the gut to the dorsal mesentery.
As these cells are migrating upward, the intermediate mass and its overlying coelomic epithelium are condensing to form an early gonad medial to the mesonephric duct.
By the sixth week, the early gonad has developed projections which divide the gonad into a medulla and cortex. These projections are known as the primary sex cords. By the time these have formed, the primordial germ cells have completed their migration and now lie within the developing gonad. The extent to which the medulla and cortex develop is dependent on the expression (or lack of expression) of SRY.
At the seventh week, sexual differentiation occurs.
