The urinary tract develops from the 3rd week of the embryonic period from the intermediate mesoderm as well as from the urogenital sinus.
The kidneys develop from the 4th week in three steps:
As a first one, a cranial anlage, the pronephros, forms that then later atrophies in the 8th week and is never active functionally. It is followed by a further anlage from the intermediate mesoderm, the mesonephros, that is formed between the 6th and 10th weeks, but is only transitory, and the anlage of the definitive kidneys, the metanephros. They develop from a metanephric anlage (mesodermal origin) and the ureter anlage (that has its origin in the caudal part of the wolffian duct).
The urine-excreting part of the kidneys, the nephron, mainly arises from the metanephric anlage (glomerulus, proximal, intermediate and distal tubules), while the rest of the upper urinary tract (collecting ducts, calices, renal pelvis and ureter) develop from the ureter anlage.
The lower urinary tract differentiates from the cloaca between the 5th and 8th weeks in that it becomes subdivided by the urorectal septum. The ventral part of the cloaca forms the primary urogenital sinus, out of which the urethra forms in the lower part and the bladder in the upper part. The ureter anlage discharges into the upper posterior wall of the urogenital sinus. In males, the wolffian duct remains present and forms a connection to the genital tract in the lower part of the urogenital sinus.
The numerous induction mechanisms between ureter anlage and metanephric mesenchyma during the development of the renal system, as well as the ascent of the kidneys, originating at the level of the sacrum and moving up to the diaphragm at the end of the development, make it possible for a large number of abnormalities to arise. Many remain asymptomatic whereas others are not compatible with survival.