The development of the urinary system is closely related to that of the reproductive tract; initially their excretory ducts open into a common cavity: the cloaca. The sketch primitive kidney is the nefrotomo appearing in the intermediate mesoderm between the somites and the plate side, these structures that originate from the differentiation of mesodermal cells and evolve later in the sketch excretory organ. The excretory organ that appears first in vertebrates is the pronephros. In the process of developing the pronephros degenerates and is replaced by mesonephros formed caudal to it. Although the mesonephros is an organ transient, it reaches a stage fairly sophisticated development and acts as an excretory organ. The tubules of the mesonephros are formed in the intermediate mesoderm around the fourth week of life fetale. Tali tubules merge with the duct of the mesonephros (Wolff's duct), which is the continuation Duct pronephros. The dorsal aorta branches reach the blind ends of the tubules and form the glomeruli. the vessel efferent from each glomerulus form a capillary plexus around a tubule. Although the mesonephros acquire human excretory function around the sixth week of fetal life, the tubules, such as glomeruli, begin to degenerate soon after. The duct of the mesonephros becomes the vas deferens in the male, thus assuming the function of conducted sexual, while completely disappears in the female. After the regression mesonephros, the metanephros appears in the human fetus as the kidney permanent. The metanephros is formed by two different structures: the conduit of the metanephros, ureter or primitive, and tissue metanefrogeno. In the fourth week of fetal life, duct metanephros originates as a bud from the duct of the mesonephros (Wolff's duct) in the vicinity of the outlet of the duct mesonephros same in the cloaca. The ureteric bud is a post equivalent to Level IV lumbar segment. The gem extends dorsocranialmente and its end widens to form the primitive renal pelvis, which quickly shows divisions in the glasses primitive cephalic and caudal. from each chalice primitive stem subdivisions such goblets secondary. From the wall of each cup secondary are generated, for dichotomous division, the collecting tubules. The fabric metanefrogeno also contains aggregates of mesodermal cells lining the end terminals of the terminal branches of the collecting tubules, forming caps, which surround the ampoules (terminal expansion) tubular collectors themselves. From each collecting tubule arched gem a new bulb of the next order, going to join with another vesicle metanefritica. In humans, the formation of nephrons continues until the last stages of intrauterine life. at the end of fetal life, the fabric metanefrogeno disappears, suggesting that there is no formation of nephrons after birth. The blood supply to the metanephros derives from the branches of the renal artery, thus differing from the mesonephros that is sprayed directly from the aorta. referring again the renal tubule primitive, the cephalic part presents an indentation, forming a S-curve Jag or slit S is the site of the future arterial glomerulus. The formation of the renal glomerulus begins with the appearance of mesenchymal cells in the slot of tubule in the shape of S. In the further stage is a progressive organization of the glomerulus kidney. Under the glomerular basement membrane, the mesenchymal cells are transformed into cells endothelial, become flattened and join with other similar cells. Most mesenchymal cells is still in the mail slot and a small amount of mesangial matrix appears between and around esse.Alcune of these begin to differentiate into fiber cells smooth muscle afferent and efferent arterioles. After the second month of fetal life, the metanephros contains glomeruli in various stages of development. It should about a month since the beginning of the formation of the glomerulus to have a well-differentiated glomerulus. The vesicle metanefrica (tubule urinifero primitive), which originates from the tissue metanefrogenico, is the parent structure around the renal tubule. In the course of developing the tubule expands and begins to bending, forming the proximal and distal convoluted parts. The loop of Henle is the segment of the tubule to slower development. The development of the urinary bladder and ureter is intimately connected to the sewer. In the fourth week of fetal life, the cloaca is sepimenta through the crease urorectal into two portions: the rectum, the back, and the urogenital sinus before. With the progress of the development of the urogenital sinus, the ends of the ducts of the metanephros (ureters) are absorbed. In further developing the urogenital sinus differs in two anatomical segments: the channel vesicoureteral from which form the bladder and the first portion urethra, and urogenital sinus from which originates the main portion of the urethra. as a result in late stages of development of the metanephros ducts open into the bladder, while the ducts mesonephros are connected with the urethra. In the fourth month of fetal life, the channel vesicoureteral assumes the characteristic appearance of the bladder Urinary adult and his wall shows well developed layers of smooth muscle cells. The further development of the definitive urogenital sinus, including the portions pelvic and phallic, follows different routes in the male and the female. In the male urethra gives rise to the prostate gland, while in the female gives rise to both urethral glands both paraurethral. Finally the allantois becomes nell'uraco whose lumen is obliterated by fibrosis in the later stages of intrauterine life. In postnatal life the urachus becomes the umbilical ligament average.