As indicated above, the urinary bladder develops from the endoderm of the urogenital sinus, which gives rise to the lining epithelium of the bladder. However, the thick muscular coat and connective tissue elements of the bladder wall develop from adjacent splanchnic mesoderm.
The apex of the developing urinary bladder is continuous with the allantois that passes up along the ventral body wall and extends into the umbilical cord. As already described in chapter 20,the umbilical part of the allantois degenerates and disappears. However, the abdominal part is transformed into a thick fibrous cord termed the urachus. After birth the urachus is known median umbilical ligament. This ligament passes from the apex of the urinary bladder to the umbilicus.
As the urinary bladder develops, the distal parts of the mesonephric ducts are absorbed into its dorsal wall. As a result, the mesonephric ducts and ureters come to open independently into the posterior wall of the developing bladder. As the kidneys ascend, the ureteric openings shift superolaterally (ie, the orifices of the two ureters move cranially and away from each other). The result of this process is that, finally, ureters enter the bladder wall obliquely through the base of the urinary bladder. While the ureteric openings are shifting in a cranial direction, the orifices of the mesonephric ducts migrate inferomedially (ie, the openings of mesonephric ducts shift in a caudal direction and, also, toward each other). Finally the two mesonephric ducts open into the pelvic part of the urogenital sinus. In the male, the caudal ends of the mesonephric duct become 'the ejaculatory ducts. In the female, the mesonephric ducts degenerate.
When the process of absorption of the distal portions of the mesonephric ducts and shifting of the orifices of ureters and mesonephric ducts has been completed, a triangular area can be recognized in the posteroinferior wall of the bladder, which lies between the ureteric openings and the internal urethral orifice. This triangular area is called trigone of the' urinary bladder. Because this part of the bladder wall is derived from the mesoderm of the absorbed mesonephric ducts, temporarily it is an island of mesodermal epithelium admist the general endodermal lining of the urinary bladder. However, the mesodermal epithelium of the trigone is later replaced by endodermal epithelium that spreads over
the trigone, so that finally the interior of the bladder is completely covered by the endodermal epithelium. During the second trimester the epithelial lining of the urinary bladder becomes transitional in type.