The urinary system

Two components are necessary for the development of an excretory system: a capillary bed from which the filtration product is derived and a tubule to conduct filtration products to the exterior (Fig. 8.3). The tubule begins as a blind-ended cup-like structure invaginating the capillary bed (or glomerulus) and opens into the exterior (via a convergent series of tubes into the urinary bladder). Thus, there is a nephric vesicle (becoming known as either mesonephric or metanephric as differentiation proceeds) and an excretory duct developing from the mesenchymal cells of the intermediate cell mass. The nephric vesicle is the initial part of the tubular system into which the glomerulus invaginates. The tubule opens into the duct (Fig. 8.3) and this pattern occurs at each segmental level. Each tubule lengthens and therefore becomes convoluted, especially so in the metanephros in which the definitive adult nephrons develop.

Fig. 8.3 Transverse sections showing the development of mesonephric tubules between 3 and 5 weeks.

Kidneys and ureters

Three pairs of successive kidneys develop in a craniocaudal sequence. The pronephros develops at the most cranial end of the nephrogenic cord, but because it is non-functional and is only a short-lived structure in humans it will not be considered further.

The mesonephros begins to develop in the lower thoracic and lumbar segments of the embryo. The cavities, which appear in the mesonephros, become the tubules and connect laterally with the mesonephric duct, which also forms from the intermediate mesoderm. The duct lies longitudinally in the embryo draining into the primitive urogenital sinus, the future urinary bladder. The urogenital sinus is closed inferiorly by the cloacal membrane (see Chapters 1 and 7), which is derived from the ectoderm and endoderm that lie in contact with each other to form a closure. In some vertebrate species the mesonephros persists as the adult kidney, though not in humans. Parts of the duct system do persist in humans, however, as components of the reproductive system in the male, as the ductus deferens and the efferent ductules of the testis. In the female the mesonephric duct largely disappears though unimportant remnants may be found between the layers of the broad ligament.

The metanephros (Fig. 8.4) develops at the most caudal end of the nephrogenic cord. The metanephric diverticulum or ureteric bud arises as an outgrowth from the caudal end of the mesonephric duct. The ureteric bud grows into the nephrogenic cord, the tissue of which condenses around it to form the metanephric blastema (Fig. 8.4). The tip of the developing ureter (the ureteric bud) continues to divide to form the collecting ducts and the calyces, each division becoming surrounded by a metanephric blastema. The ureteric bud also forms the definitive adult ureter. The cells in the blastema go on to form the nephrons. These developments are longer and more complex than those of the mesonephros. The blind-ended portion of the tubule, as in the mesonephros, receives an invagination of the glomerular capillaries (Fig. 8.5). The mesonephric duct distal to the ureteric bud is incorporated into the wall of the urinary bladder as the trigone, and also forms the ductus deferens.

Fig. 8.4 The development of the metanephros and its duct system between 5 and 8 weeks. The area enclosed by a box is enlarged in Figure 8.5.

Fig. 8.5 Successive stages of development of metanephric blastema into nephrons.

The kidneys come to occupy their adult lumbar position later in development, apparently having ascended from their pelvic origin (Fig. 8.6). This is because of the differential growth rates of the abdominal walls and the kidneys, and the regression of the enlarged suprarenal glands.

Fig. 8.6 The ascent of the kidneys from the sacral to the lumbar position between 6 and 9 weeks. Note the relatively large size of the suprarenal glands which regress at a later stage.

The metanephric kidney is functional from week 10 and the urine produced at this stage passes into the amniotic fluid. The amniotic fluid is swallowed by the fetus, thus recirculating the water content. However, it is the placenta which removes fetal waste at this stage of development.

Urinary bladder and urethra

The urinary bladder arises after the cloaca has been divided by the urorectal septum. The anterior part is the urogenital sinus, and the posterior component is the anorectal canal (see Chapter 7). In both sexes, the upper part of the urogenital sinus forms the bladder. In males, the pelvic part becomes the prostatic and membranous parts of the urethra and the lower phallic part becomes the penile urethra. In females, the middle and lower parts of the urogenital sinus become the entire urethra. Initially the bladder is continuous with the allantois, but the allantois regresses to form the urachus and then the median umbilical ligament after birth.

Summary box

The UG system is considered as one morphologically because of the common embryological origin and shared elements of the duct systems.

The bulk of the UG system derives from the intermediate mesoderm.

The initial structures that develop in the UG system are associated with excretory functions, some of which lose those functions and become adapted for reproductive roles.

Cavities appear in the intermediate cell mass and differentiation proceeds in a craniocaudal direction, and thus the nephrogenic cord develops along the length of the trunk of the embryo, underlying the urogenital ridges.

The pronephros, mesonephros and then the metanephros develop from the nephrogenic cord.

In each embryonic segment an excretory tubule and a nephric vesicle develop from which the urinary tubules arise. The glomerulus is invaginated into the nephric vesicle.

The metanephroi become the definitive adult kidneys the ureteric bud grows out of the caudal end of the mesonephric duct into the nephrogenic cord, thus forming a metanephric blastema.

Part of the duct of the mesonephros persists as the ductus deferens.

The metanephroi appear to ascend the posterior abdominal wall to achieve their adult position, as a consequence of differential growth rates.

From the tenth week the metanephros becomes functional but it is the placenta that removes fetal waste.

The urinary bladder develops from the urogenital sinus, and is initially continuous with the allantois, the latter regressing to form the urachus and median umbilical ligament after birth.