Urinary system

The urinary system comprises the kidneys, ureters, bladder and urethra.
Kidneys

The intermediate mesoderm lies between the segmented paraxial mesoderm (somites) and the unsegmented lateral mesoderm (lateral plate). Between the 9th (corresponds to the segment Th1) and the 27th somite it gives rise to the nephrogenic cord. This forms the primordium of the kidneys and the excretory ducts. In most animals, it is possible to distinguish pronephros, mesonephros (middle kidney) and metanephros (metanephric kidney or definitive kidney). However, a pronephros cannot be observed in human embryos. The primordium of the mesonephros extends from the 9th up to the 23th/24th somite (corresponding to approximately L3). The primordium of the metanephros (metanephrogenic blastema) is centred around the ureteric bud, which sprouts from the mesonephric duct at the level of the first sacral segment. It extends from the 24th up to the 27th somite.