The kidneys, renal collecting system, and ureters derive from the longitudinal mass of mesoderm (known as the nephrogenic cord) found on each side of the primitive aorta.
Table 5.6 Development of Genital and Urinary Tracts by Embryologic Age
This process gives rise to three successive sets of increasingly advanced urinary structures, each developing more caudal to its predecessor.
The pronephros, or “first kidney,” is rudimentary and nonfunctional; it is succeeded by the “middle kidney,” or mesonephros, which is believed to function brieﬂy before regressing. Although the mesonephros is transitory as an excretory organ, its duct, the mesonephric (wolffian) duct,is of singular importance for the following reasons: 1.
It grows caudally in the developing embryo to open, for the first time, an excretory channel into the primitive cloaca and the “outside world.” 2.
It serves as the starting point for development of the metanephros, which becomes the definitive kidney.
3.
It ultimately differentiates into the sexual duct system in the male.
4.
Although regressing in female fetuses, there is evidence that the mesonephric duct may have an inductive role in development of the paramesonephric or m¨ullerian duct (22).
The ureteric buds, which sprout from the distal mesonephric ducts, initiate the development of the metanephros; these buds extend cranially and penetrate the portion of the nephrogenic cord known as the metanephric blastema. The ureteric buds begin to branch sequentially, with each growing tip covered by metanephric blastema. Ultimately the metanephric blastema form the renal functional units (the nephrons), whereas the ureteric buds become the collecting duct system of the kidneys (collecting tubules, minor and major calyces, renal pelvis) and the ureters. Although these primitive tissues differentiate along separate paths, they are interdependent on inductive inﬂuences from each other—neither can develop alone.
The kidneys initially lie in the pelvis but subsequently ascend to their permanent location, rotating almost 90 degrees in the process as the more caudal part of the embryo in effect grows away from them. Their blood supply, which first arises as branches of the middle sacral and common iliac arteries, comes from progressively higher branches of the aorta until the definitive renal arteries form; previous vessels then regress. The definitive kidneys become functional in the late 7th to early 8th weeks of gestation.
The cloaca forms as the result of dilation of the opening to the fetal exterior. During the 7th week of gestation, the cloaca is partitioned by the mesenchymal urorectal septum into an anterior urogenital sinus and a posterior rectum. The bladder and urethra form from the most superior portion of the urogenital sinus, with surrounding mesenchyme contributing to their muscular and serosal layers. The remaining inferior urogenital sinus is known as the phallic or definitive urogenital sinus. Concurrently, the distal mesonephric ducts and attached ureteric buds are incorporated into the posterior bladder wall in the area that will become the bladder trigone. As a result of the absorption process, the mesonephric duct ultimately opens independently into the urogenital sinus below the bladder neck.
The allantois, which is a vestigial diverticulum of the hindgut that extends into the umbilicus and is continuous with the bladder, loses its lumen and becomes the fibrous band known as the urachus or median umbilical ligament. In rare instances, the urachal lumen remains partially patent, with formation of urachal cysts, or completely patent, with the formation of a urinary fistula to the umbilicus (23).