Genital ducts and external genitalia: development and anomalies.

Human wolffian ducts are considered to be of ectodermal origin. Developing müllerian ducts in the area of the mesonephric ducts grow in contact with wolffian ducts. Vaginal differentiation occurs only if müllerian ducts contact the urogenital sinus, inducing proliferation of the entodermal epithelium and if the caudal ends of the müllerian ducts do not regress. Development of the external genitalia and the role of fetal androgens are discussed. Malformations of the external genitalia are classified pathogenetically as steroid-independent and steroid-dependent. Definitions for steroid-dependent malformations such as ambiguous genitalia, hypospadic genitalia, phallus, hypertrophic and peniform clitoris are presented.