Development of the bladder
The bladder develops from the upper part of the urogenital sinus (UGS) and is connected with the allantois.
The allantois is obliterated during the development and forms a fibrous cord, the urachus, which following birth becomes the median umbilical ligament.
While the cloaca is being divided, the caudal, originally common part of the mesonephric duct (Wolffian duct) and the ureter anlage is taken up into the upper, postero-lateral wall of the urogenital sinus (future bladder).
The rapid growth of the back wall of the urogenital sinus has the result that the common lowest part of the ureter and the mesonephric duct (Wolffian duct) are both taken up into the bladder wall. Further complicated growth processes have the result that the ureteral orifices and the orifice locations of the mesonephric duct (Wolffian duct) go through a cranio-caudal position exchange during the course of the further development. The ureteral openings appear to migrate thereby in a cranio-lateral direction and the mesonephros orifices appear to be shifted caudo-medially. The triangular zone that is thus created is termed the vesical trigonum. In males, the wolffian duct forms the future deferent duct on both sides.
The trigonum thus originates from the mesoderm while the ventral bladder wall has an endodermal origin. Later, though, the trigonum will be completely covered by endodermal epithelial cells. The smooth musculature of the bladder develops during the 12th week from the splanchnopleural mesoderm, which coats the endoderm on the outside.