Chapter 38

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.

### The Kidneys

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.

###### Figure 38–1.

Schematic of the development of the nephric system. Only a few of the tubules of the pronephros are seen early in the fourth week, while the mesonephric tissue differentiates into mesonephric tubules that progressively join the mesonephric duct. The first sign of the ureteral bud from the mesonephric duct is seen at 4 weeks. At 6 weeks, the pronephros has completely degenerated and the mesonephric tubules start to do so. The ureteral bud grows dorsocranially and has met the metanephric blastema. At the eighth week, there is cranial migration of the differentiating metanephros. The cranial end of the ureteral bud expands and starts to show multiple successive outgrowths (renal calices).

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).

###### Figure 38–2.

The development of the ureteral bud from the mesonephric duct and their relationship to the urogenital sinus. The ureteral bud appears at the fourth week. The mesonephric duct distal to this ureteral bud is gradually absorbed into the urogenital sinus, resulting in separate endings for the ureter and the mesonephric duct. The ...

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