Development of the internal and external genitalia

The development of female internal and external genitalia is largely independent of gonadal influence whereas, in the male, the testes are essential for sexual development. During early fetal life, the pronephros, mesonephros and metanephros develop in chronological order as renal excretory organs. The mesonephros and mesonephric (wolffian) ducts form the male genital tract, while in the female these regress to nonfunctional remnants. The paramesonephric (müllerian) ducts in the female develop into the internal reproductive organs and the upper third of the vagina.

Origin of the ovaries and oocytes

Primordial follicles and oocytes are derived during fetal life and the oogonial stem cell line is lost before birth. In humans, at approximately four weeks of gestation the germ cells arise from the yolk sac and migrate along the hindgut to the genital ridge. The oogonia are able to move by pseudopodial amoeboid movements. Once the oogonia reach the genital ridge, they become associated with cortical cords and lose their motility. From weeks 5 through to 28 the oogonia undergo several mitotic divisions. At the same time, many oogonia are lost by atresia, some in their passage from the yolk sac and others once they have reached the gonad itself. Meiosis starts in approximately week nine and the life cycle of the oocyte is such that it undergoes only part of the first meiotic division entering meiotic arrest at the dictyate stage of prophase 1 (Figure 1.1).