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EMBRYOLOGY OF THE GENITOURINARY TRACT
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A basic understanding of genitourinary embryology facilitates learning many aspects of urology. Embryologically, the genital and urinary systems are intimately related. Associated anomalies of the two systems are commonly encountered.
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The kidneys pass through three embryonic phases (Figure 38–1): (1) The pronephros is a vestigial structure without function in human embryos that, except for its primary duct, disappears completely by the fourth week. (2) The pronephric duct gains connection to the mesonephric tubules and becomes the mesonephric duct. While most of the mesonephric tubules degenerate, the mesonephric duct persists bilaterally; from where it bends to open into the cloaca, the ureteral bud grows cranially to interact with the metanephric blastema. (3) This forms the metanephros, which is the final phase. The metanephros develops into the kidney. During cephalad migration and rotation, the metanephric tissue progressively enlarges, with rapid internal differentiation into the nephron and the uriniferous tubules. Simultaneously, the cephalad end of the ureteral bud expands and divides within the metanephros to form the renal pelvis, calices, and collecting tubules.
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Subdivision of the cloaca (the blind end of the hindgut) into a ventral (urogenital sinus) and a dorsal (rectum) segment is completed during the seventh week and initiates early differentiation of the urinary bladder and urethra. The urogenital sinus receives the mesonephric duct and absorbs its caudal end, so that by the end of the seventh week, the ureteral bud and mesonephric duct have independent openings. The ureteral orifice migrates upward and laterally. The mesonephric duct orifice moves downward and medially, and the structure in between (the trigone) is formed by the absorbed mesodermal tissue, which maintains direct continuity between the two tubes (Figure 38–2).
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