Chapter 95. The Urinary or Excretory System: The Urinary Bladder and Urethra - Review of Medical Embryology Book

The urogenital sinus and anorectal canal are formed during septation of the entodermal cloaca by the urorectal septum, during weeks 4 to 7. Simultaneously, the distal portions of the 2 mesonephric (wolffian) ducts, undergo complex development

THE UROGENITAL SINUS, according to the position of the opening of the mesonephric ducts, can be divided into 3 distinct zones

The vesicourethral canal or urinary zone: the upper region, above the entrance of the mesonephric ducts, is continuous with the allantois
The middle or pelvic zone
The caudal or genital (phallic) zone: the lower region, below the entrance of the mesonephric ducts, closed by the urogenital membrane

EACH MESONEPHRIC DUCT forms a diverticulum, the ureteric bud, near the end of week 5

The part of the mesonephric ducts between the ureteric buds and the posterior wall of the urogenital sinus enlarges into the ampullae or horns of the urogenital sinus. The mesonephric ducts and the ureters open side by side into these ampullae
Selective development of the posterior wall of the urogenital sinus absorbs the 2 ampullae at about week 7, resulting in the ureters opening separately and directly into the urogenital sinus, just outside the mesonephric ducts

THE POSTERIOR WALL OF THE UROGENITAL SINUS continues to develop, and by week 8 the orifices of the ureters have moved farther cranially and laterally, while those of the mesonephric ducts have remained relatively fixed

Further remodeling of the urogenital sinus causes the ureters to open into the urinary bladder and the caudal end of the mesonephric ducts (future ejaculatory ducts) to open into the urethra (in the male) beneath the bladder
The caudal ends of the ducts, in the female, subsequently degenerate
The portion of the posterior urogenital sinus wall between the openings of the ureters and the mesonephric ducts appears triangular in shape and becomes the future trigone of the bladder (trigonal vesicle of mesonephric origin)

The mesodermal epithelium of the trigone, derived from the mesonephric ducts, is soon replaced by the entodermal epithelium of the urogenital sinus

The urinary bladder: as the bladder forms, the allantois is progressively obliterated and forms the urachus, a thick tube. It becomes the median umbilical ligament after birth
In the male: the vesicourethral canal of the urogenital sinus gives rise to the bladder (vesical) and the upper part of the prostatic urethra (proximal to the ejaculatory ducts). All are derived from entoderm. The lamina propria, smooth muscle, and serosa (adventitia) develop from adjacent splanchnic mesenchyme

MUCOSA of the cranial prostatic urethra, originally mesoderm is replaced by entoderm
THE LOWER PORTION of the prostatic urethra and the membranous urethra are derived from entoderm of the pelvic portion of the urogenital sinus
THE EPITHELIUM of the penile urethra, except for its glandular part, comes from cells of the phallic or genital portion of the urogenital sinus

The epithelium of the glandular part of the penile urethra (terminal or navicular fossa) develops by canalization of an ectodermal cord of cells that extends into the glans from its tip. The connective tissue and smooth muscle develop from adjacent splanchnic mesenchyme

AT THE END OF 3 MONTHS, the epithelium of the cranial part of the urethra proliferates and forms outbuddings that penetrate the surrounding mesenchyme. In the male, these buds form the prostate gland; in the female the urethral and paraurethral glands

In the female: the entodermal vesicourethral canal of the urogenital sinus gives rise to the bladder (vesical) and the entire urethra. The connective tissue and smooth muscle come from the adjacent splanchnic mesenchyme