Embryology of the lower genitourinary tract.

The urinary and genital systems are closely related in their development so that dividing them into separate divisions is often artificial. The lower urinary system begins as a cloaca. The urorectal septum then divides the cloaca into a ventral urogenital sinus and a dorsal hindgut. The ureteral bud forms off the mesonephric duct. The male and female genital systems are the same until the gonad assumes characteristics of the testis or ovary in the eighth week of gestation. It is not until the eleventh or twelfth week that the external genitalia assume specific sexual characteristics. Anomalies of the urethra and bladder can vary from a severe epispadias-exstrophy complex to a minimal Type I urethral valve. Ureteral anomalies include duplications, ectopia, and ureteroceles. An undescended testis is probably the most common anomaly in the male genital tract. Female genital anomalies can include a variety of uterine duplication anomalies. Gonadal abnormalities are often created by a chromosomal defect and are ultimately reflected by abnormal sexual differentiation.