Development of Urinary Bladder
During development, the cloaca is divided into a dorsal anorectal canal and a ventral urogenital sinus by the urorectal sinus. The urogenital sinus is divided into three parts:
1. Vesical part: It is the cranial part which is continuous with the allantois and forms the large part of the bladder.
2. Pelvic part: It is the middle narrow part of the sinus which becomes the urethra in the neck of the bladder, and gives rise to the prostatic part of urethra in males and entire urethra in female.
3. Phallic part: It is the caudal part of urogenital sinus and gives rise to the primodium of external genitalia i.e, the penis or the clitoris.
1. Vesical part: It is the cranial part which is continuous with the allantois and forms the large part of the bladder.
2. Pelvic part: It is the middle narrow part of the sinus which becomes the urethra in the neck of the bladder, and gives rise to the prostatic part of urethra in males and entire urethra in female.
3. Phallic part: It is the caudal part of urogenital sinus and gives rise to the primodium of external genitalia i.e, the penis or the clitoris.
Parts of urogenital sinus forming the bladder, available at: http://sprojects.mmi.mcgill.ca/embryology/ug/Kidney_Stuff/Normal_Development/Bladderdev.html
The urinary bladder is formed from the upper end of the vesical part of the urogenital sinus, which is continuous with the allantois. The allantois becomes a thick fibrous cord called the urachus, which is represented by a median umbilical ligament in adults. The trigone of the bladder is formed when the lower ends of the mesonephric duct gets incorporated into the posterior wall of the bladder. The transitional epithelium of the bladder is derived from the endoderm of the urogenital sinus. Other layers of the bladder are derived from the adjacent splanchnic mesenchyme. As the mesonephric ducts get absorbed, the ureters open separately in the bladder. The orifice of mesonephric ducts eventually open into the urogenital sinus below the bladder in males. In females, the caudal ends of the mesonephric ducts degenerate.
Development of bladder from urogenital sinus, available at: http://www.pediatricurologybook.com/embryology_urinary_tract.html
Incorporation of mesonephric duct and formation of trigone of bladder, available at: http://www.ultratwistersgym.com/Resources/Urogenital/Urinary.html
Development of the Urethra
The female urethra is formed from pelvic portion of the urogenital sinus. It develops endodermal outgrowths into the surrounding mesoderm to form the urethral and paraurethral glands. In males, the distal part of the urethra in the glans of the penis arise from a solid cord of ectodermal cells that grows from the tip of the glans penis to meet the spongy urethra which is derived from the phallic part of the urogenital sinus. In males, the epithelium of the urethra proliferates into outgrowths which penetrate the surrounding mesenchyme and these buds then form the prostate gland. In both males and females, the transitional and stratified columnar epithelium lining the urethra is derived from the endoderm of the urogenital sinus. The connective tissue and the smooth muscle of the urethra in both sexes are derived from the splanchnic mesenchyme.