The development of the urinary system is closely linked with that of the reproductive system. They both develop from the intermediate mesoderm, which extends on either side of the aorta and forms a condensation of cells in the abdomen called the urogenital ridge. The ridge has two parts: the nephrogenic cord and the gonadal ridge (see Figure 38.2).
Three structures involved in kidney development grow from intermediate mesoderm in an anterior to posterior sequence, termed the pronephros, mesonephros and metanephros.
The pronephros appears in the third week in the neck region of the embryo and disappears a week later. In humans this is a primitive, non‐functional kidney that c ial nephrons joined to an unbranched nephric duct.
Appearing in the fouctional kidney unit, the mesonephros, forms as the pronephros begins to regress (Figures 36.1 and 36.2). The mesonephric ducts (Wolffian ducts) are epithelialined tubes that form in the intermediate mesoderm and extend caudally to the cloaca. They stimulate formation of the mesonephros itself as mesonephric tubules (different to the ducts) from the mesenchyme. The tissue of the mesonephros appears initially as a segmented structure along the mesonephric duct.
Renal corpuscles develop from mesonephric tubules (Bowman’s capsule) and capillaries from the dorsal aortae (glomerulus). At the lateral end the tubules join the mesonephric duct. The duct discharges into the cloaca where the bladder will form. The mesonephros starts to produce urine at about 6 weeks but degenerates almost completely between weeks 7 and 10.
The mesonephric ducts contribute to the ducts of the male reproductive system, but regress in the female foetus (see Chapter 38).
The third renal structure that develops will finally become the adult kidney. It starts to appear at the beginning of the fifth week as a bud from the caudal end of the mesonephric duct, called the ureteric bud (Figures 36.3 and 36.4).
The bud branches and develops into the collecting parts of the adult kidney: the ureter, renal pelvis, calyses and collecting tubules. The bud grows into surrounding intermediate mesoderm and induces the cells in that region (the metanephric blastema) to form a metanephric cap upon the ureteric bud.
As the ureteric bud forms collecting tubules, cells of the metanephric cap form nephrons that link to the collecting tubules. Reciprocal interactions between the buds and the caps initiate and maintain this development (Figure 36.5).
Capillaries grow into the Bowman’s capsule from the dorsal aortae and convolute to form the glomeruli (Figure 36.6). These functional renal units produce urine from week 12 onwards.
The formation of nephrons continues until birth when there are approximately 1 million nephrons in each kidney. Infant kidneys are lobulated because of the branching of the calyces (Figure 36.7), but further growth and elongation of the nephrons after birth pushes out the kidney and the lobulation disappears.
The location of the metanephros changes during development from the level of the pelvis, through growth of the embryo and migration of the kidneys, to the lumbar region. They also rotate medially in ascent. As they ascend, a series of blood vessels from either the common iliac arteries or aorta generate and degenerate to continually supply the kidneys. Usually, the most cranial remain and become the renal arteries.
In week 4 the cloaca is split into the ventral urogenital sinus and the dorsal anal canal by the urorectal septum (see Figure 33.5).
The urogenital sinus can be split into a further three parts. The top part is the biggest and becomes the bladder, the middle part forms the urethra in the female pelvis and the prostatic and membranous urethra in the male (Figure 36.8), and the lowest part forms the penile urethra in the male and the vestibule in the female. The allantois also contributes to the upper parts of the bladder.
The mesonephric ducts become incorporated into the posterior wall of the bladder. The openings of the mesonephric ducts and ureters enter the bladder separately. Remember that the ureters form from the metanephric ducts. The ureters move anteriorly whereas the mesonephric ducts move posteriorly and become the ejaculatory ducts in the male pelvis.
The specialised transitional epithelium of the bladder develops from the endoderm of the urogenital sinus.
The ventral surface of the cloaca (which becomes the urogenital sinus) is continuous with the allantois, which degenerates after birth to form the urachus and eventually the median umbilical ligament (an embryological remnant with no clinical significance). The medial umbilical ligaments are the remnants of the umbilical arteries, which are a little lateral to the urachus.
Incomplete division of the ureteric bud can lead to supernumerary kidneys and, more commonly, supernumerary ureters.
Kidney cysts form when the developing nephrons fail to connect to a collecting tubule in development, or the collecting ducts fail to develop. There are dominant and recessive forms of polycystic kidneys. The recessive form is more progressive and often results in renal failure in childhood.
Balance of fluid in the amnion is vital in the development of the embryo. If urine is not being produced there is a reduction in the amniotic fluid and oligohydramnios develops. This can be a symptom of bilateral renal agenesis, in which both kidneys fail to form. This is lethal. Unilateral renal agenesis generally causes no symptoms.
Accessory renal arteries are quite common, especially on the left and often are only seen during a surgical procedure as they are asymptomatic. They enter the kidney at the superior and inferior poles. Abnormal rotation or location of the kidneys may be found in a patient, and they may fail to ascend into the abdomen. The inferior poles of the left and right kidneys can fuse, forming a horseshoe kidney. In this case the kidney cannot ascend as it gets snagged on the inferior mesenteric artery.
Bladder defects may occur, such as exstrophy in which part of the ventral bladder wall is present outside of the abdominal wall. A urachal cyst, fistula or s the degeneration of the allantois is not completed.