20.4 Lower urinary system



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. Fig. 24 - Development of the bladder  Legend

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A
B		 Primary urogenital sinus
Mesonephric duct (Wolffian duct)
Ureter anlage
Common drainage urinary
passages
Pelvic limb of the urogenital sinus

Part of the genital tract
Part of the urinary tract

Fig. 24
Dorsal view of the entry level of the mesonephric canal (common extremities of the mesonephric duct (Wolffian duct) and the ureter) while it is incorporated into the dorsal wall of the bladder.



Animation

Reversal of the orifice locations of the ureter and mesonephric duct (Wolffian duct) in the bladder wall

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.



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