At birth, the genital and urinary systems are related only in the sense that they share certain common passages. Embryologically, however, they are intimately related. Because of the complex interrelationships of the embryonic phases of the two systems, they are discussed here as five subdivisions: the nephric system, the vesicourethral unit, the gonads, the genital duct system, and the external genitalia.
The nephric system develops progressively as three distinct entities: pronephros, mesonephros, and metanephros.
The pronephros is the earliest nephric stage in humans, and it corresponds to the mature structure of the most primitive vertebrate. It extends from the 4th to the 14th somites and consists of six to ten pairs of tubules. These open into a pair of primary ducts that are formed at the same level, extend caudally, and eventually reach and open into the cloaca. The pronephros is a vestigial structure that disappears completely by the 4th week of embryonic life (Figure 2–1).
Schematic representation of the development of the nephric system. Only a few of the tubules of the pronephros are seen early in the 4th 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 6 weeks, the pronephros has completely degenerated and the mesonephric tubules start to do so. The ureteral bud grows dorsocranially and has met the metanephrogenic cap. At the 8th week, there is cranial migration of the differentiating metanephros. The cranial end of the ureteric bud expands and starts to show multiple successive outgrowths. (Adapted from several sources.)
The mature excretory organ of the larger fish and amphibians corresponds to the embryonic mesonephros. It is the principal excretory organ during early embryonic life (4–8 weeks). It too gradually degenerates, although parts of its duct system become associated with the male reproductive organs. The mesonephric tubules develop from the intermediate mesoderm caudal to the pronephros shortly before pronephric degeneration. The mesonephric tubules differ from those of the pronephros in that they develop a cuplike outgrowth into which a knot of capillaries is pushed. This is called Bowman's capsule, and the tuft of capillaries is called a glomerulus. In their growth, the mesonephric tubules extend toward and establish a connection with the nearby primary nephric duct as it grows caudally to join the cloaca (Figure 2–1). This primary nephric duct is now called the mesonephric duct. After establishing their connection with the nephric duct, the primordial tubules elongate and become S-shaped. As the tubules elongate, a series of secondary branchings increase their surface exposure, thereby enhancing their capacity for interchanging material with the blood in adjacent capillaries. Leaving the glomerulus, the blood is carried by one or more efferent vessels ...