The urogenital sinus is formed by the division of the cloaca by the uro-rectal septum (see partitioning of cloaca in the section on the reproductive system).
For the purposes of clarity, the uro-genital sinus may be divided into three component parts. The first of these is the cranial portion which is continuous with the allantois and forms the bladder proper. The pelvic part of the sinus forms the prostatic urethra and epithelium as well as the membranous urethra and bulbo urethral glands in the male and the membranous urethra and part of the vagina in females. Thirdly, the caudal portion, or definitive uro-genital sinus, forms the penile urethra in males and the vestibule in females.
For the most part, the bladder forms from the cranial end of the urogenital sinus. However, the trigone portion is formed by the caudal ends of the mesonephric ducts. As the bladder expands, the mesonephric ducts begin to become incorporated into the wall of the bladder dragging the ureters along with them. Further growth causes the ureters to eventually have their own opening into the bladder. By the time the kidneys have finished migrating and the bladder has finished growing, the ureters are found at the base of the bladder laterally and the mesonephric ducts have joined together to enter the prostatic part of the urethra. In females, the ducts then degenerate. In males, the ducts form the origin of the ejaculatory ducts.
The bladder is initially continuous with the allantois. Over time, the allantois degenerates to form a cord-like structure, the urachus. The urachus goes from the umbilicus to the apex of the bladder and forms the median umbilical ligament which can be seen in adults. The medial umbilical ligaments may also be seen in adults on both sides of the median umbilical ligament, these are the vestigial remnants of the umbilical arteries.
Formation of the urethrae will be covered within the sections dealing with sexual differentiation.
