Development of the human bladder and ureterovesical junction.

The urinary bladder collects urine from the kidneys and stores it until the appropriate moment for voiding. The trigone and ureterovesical junctions are key to bladder function, by allowing one-way passage of urine into the bladder without obstruction. Embryological development of these structures has been studied in multiple animal models as well as humans. In this report we review the existing literature on bladder development and cellular signalling with particular focus on bladder development in humans. The bladder and ureterovesical junction form primarily during the fourth to eighth weeks of gestation, and arise from the primitive urogenital sinus following subdivision of the cloaca. The bladder develops through mesenchymal-epithelial interactions between the endoderm of the urogenital sinus and mesodermal mesenchyme. Key signalling factors in bladder development include shh, TGF-β, Bmp4, and Fgfr2. A concentration gradient of shh is particularly important in development of bladder musculature, which is vital to bladder function. The ureterovesical junction forms from the interaction between the Wolffian duct and the bladder. The ureteric bud arises from the Wolffian duct and is incorporated into the developing bladder at the trigone. It was previously thought that the trigonal musculature developed primarily from the Wolffian duct, but it has been shown to develop primarily from bladder mesenchyme. Following emergence of the ureters from the Wolffian ducts, extensive epithelial remodelling brings the ureters to their final trigonal positions via vitamin A-induced apoptosis. Perturbation of this process is implicated in clinical obstruction or urine reflux. Congenital malformations include ureteric duplication and bladder exstrophy.