During the fourth to seventh week, the cloaca divides into urogenital sinus anteriorly and an anal canal posteriorly. The urogenital sinus is characterized by three main regions: the cranial and the largest is the vesical part which gives rise to the urinary bladder, the middle pelvic part is a narrow canal that forms the prostatic and membranous part of the urethra in males, and finally, the flat caudal phallic part which would be pulled ventrally as the genital tubercle grows. Initially, the bladder is continuous with the allantois, however, as the lumen of the allantois is obliterated; the urachus which is a thick fibrous cord remains and connects the apex of the bladder to the umbilicus. The urachus form the median umbilical ligament in adults. |
Distal parts of the mesonephric ducts are incorporated into the dorsal wall of the urinary bladder and contribute to the formation of the connective tissue in the trigone of the bladder. Initially the ureters, which are an outgrowth of the mesonephric ducts enter the bladder separately; however, as the kidneys start to ascend, the orifices of the ureters move further cranially. On the other hand, the mesonephric ducts move closer together and enter the prostatic part of the urethra and become the ejaculatory ducts in males. In females, the distal ends of mesonephric ducts degenerate. The epithelium of the bladder is derived from the endoderm of the urogenital sinus whereas the other layers are derived from the adjacent splanchnic mesenchyme (figure 6).
With the exception of the terminal part of the urethra in males, all urethral epithelium are derived from the endoderm of the urogenital sinus. In males, a solid cord of ectodermal cells grow from the tip of glans penis to join the distal part of the spongy urethra, which was derived from the phallic part of the urogenital sinus. Similar to the urinary bladder, the connective tissue and muscles of the urethra are derived from the splanchnic mesenchyme.
With the exception of the terminal part of the urethra in males, all urethral epithelium are derived from the endoderm of the urogenital sinus. In males, a solid cord of ectodermal cells grow from the tip of glans penis to join the distal part of the spongy urethra, which was derived from the phallic part of the urogenital sinus. Similar to the urinary bladder, the connective tissue and muscles of the urethra are derived from the splanchnic mesenchyme.