ABSTRACT
The urogenital system develops from the intermediate mesoderm. During folding of the embryo in the horizontal plane, this mesoderm is carried ventrally; then, a longitudinal ridge of mesoderm, the urogenital ridge, forms on each side of the dorsal aorta. This structure will eventually give rise to parts of the urinary and genital tract. The part of the urogenital ridge giving rise to the urinary system is the nephrogenic cord; the part giving rise to the genital system is the genital ridge [1]. The urinary system begins to develop prior to the genital system. Three regions of excretory kidney develop in the human embryo: the pronephros, mesonephros, and metanephros. These arise sequentially from the intermediate mesoderm beginning at days 19-21 of development. The pronephros and mesonephros degenerate, while the metanephros will develop into the adult kidney. The metanephros begins to develop during the fifth week of development and starts to be functionally active five weeks later. Initially, it consists of two cell types: the epithelium of the ureteric bud and the mesenchyme of the metanephric blastema. A series of reciprocal interactions between these two structures causes the ureteric bud to repeatedly branch to form the ureter, the renal pelvis, the calices, and the collecting tubules. Concurrently, the mesenchyme, induced by the adjacent ureteric bud branch tips, undergoes a transformation to form the nephrons. The first glomeruli develop by eight-nine weeks, and the nephrogenesis continues in the cortex of the fetal kidney until 34-36 weeks. The bladder develops from the urogenital sinus, and the first stage of bladder development involves division of the cloaca into the anorectal and urogenital regions. Initially, the bladder is in continuity with the allantois, but eventually this
structure regresses, converting into a thick fibrous cord, the urachus, which connects the bladder with the umbilicus. The mature human kidney contains about a million nephrons, consisting of specialized segments, including glomeruli, proximal tubules, the loop of Henle, and the distal tubule, connected to the tree-like collecting duct system, intimately associated with the vascular supply.
