General Medical Information

Physiology of Sexual Differentiation

Claude J. Migeon and Gary D. Berkovitz

The first step in sex differentiation is the determination of genetic sex (XX or XY) at the time of conception. Nonetheless, the external genitalia of the male and female fetus appear to be identical at 6 weeks of gestation. This neutral genitalia includes the genital tubercle, the labioscrotal folds, the urethral folds, and a urogenital sinus. Simultaneously, a variety of bipotential structures develop. The first among these are the gonads, which become apparent as a genital ridge at about 5 weeks. These structures are populated by germ cells, which migrate from the wall of the yolk sac. The fate of these germ cells (whether they develop as oocytes or spermatozoa) will be under the control of the ovary or testis that subsequently will develop from the bipotential gonad. Two sets of ducts appear at about the sixth week of fetal life: the male, wolffian ducts, and the female, müllerian ducts. These structures will give rise later to the internal accessory sex organs.