Lateral to the urogenital folds, another pair of swellings develops, known in the indifferent stage as labioscrotal swellings. In the absence of androgens, they remain largely unfused to become the labia majora. The definitive urogenital sinus gives rise to the vaginal vestibule, into which open the urethra, vagina, and greater vestibular glands.
Clinical Correlations Developmental abnormalities of the urinary and genital systems can be explained and understood by a consideration of female and male embryologic development. Because of the intertwined development of these two systems, abnormalities in one may be associated with abnormalities in the other (25).
Urinary-tract anomalies arise from defects in the ureteric bud, the metanephric blastema, or their inductive interaction with each other.
Renal Agenesis Renal agenesis occurs when one or both ureteric buds fail to form or degenerate, and the metanephric blastema is therefore not induced to differentiate into nephrons. Bilateral renal agenesis is incompatible with postnatal survival, but infants with only one kidney usually survive, and the single kidney undergoes compensatory hypertrophy. Unilateral renal agenesis is often associated with absence or abnormality of fallopian tubes, uterus, or vagina—the paramesonephric duct derivatives.
Abnormalities of Renal Position Abnormalities of renal position result from disturbance in the normal ascent of the kidneys. A malrotated pelvic kidney is the most common result; a horseshoe kidney, in which the kidneys are fused across the midline, occurs in about 1 in 600 individuals and has a final position lower than usual because its normal ascent is prevented by the root of the inferior mesenteric artery.
Duplication of the Upper Ureter and Renal Pelvis Duplication of the upper ureter and renal pelvis is relatively common and results from premature bifurcation of the ureteric bud. If two ureteric buds develop, there will be complete duplication of the collecting system. In this situation, one ureteric bud will open normally into the posterior bladder wall, and the second bud will be carried more distally within the mesonephric duct to form an ectopic ureteral orifice into the urethra, vagina, or vaginal vestibule; incontinence is the primary presenting symptom. Most of the aforementioned urinary abnormalities remain asymptomatic unless obstruction or infection supervenes. In that case, anomalous embryologic development must be included in the differential diagnosis.
Because the early development of the genital system is similar in both sexes, congenital defects in sexual development, usually arising from a variety of chromosomal abnormalities, tend to present clinically with ambiguous external genitalia. These conditions are known as intersex conditions or hermaphroditism and are classified according to the histologic appearance of the gonads.
True Hermaphroditism Individuals with true hermaphroditism have both ovarian and testicular tissue, most commonly as composite ovotestes but occasionally with an ovary on one side and a testis on the other. In the latter case, a fallopian tube and single uterine horn may develop on the side with the ovary because of the absence of local AMH. True hermaphroditism is an extremely rare condition associated with chromosomal mosaicism, mutation, or abnormal cleavage involving the X and Y chromosomes.