MATURATION OF THE BLADDER

After cloacal septation is complete, the primitive urogenital sinus contains three major parts. The most caudal part is known as the definitive urogenital sinus, and it will become the penile and spongy urethra in males, or the vestibule of the vagina in females. The neck, located just proximal to the definitive urogenital sinus, will become the membranous and prostatic urethra in males, or the urethra in females. The bulging area proximal to the neck will become the urinary bladder in both sexes. The allantois, which connects the bladder to the umbilical cord, will regress to form the a thick, epithelial-lined tube known as the urachus, which in turn will further degenerate into a simple fibrous cord known as the median umbilical ligament.

During subsequent weeks, the definitive urogenital sinus continues to undergo structural changes as it becomes the mature bladder. By the tenth week, the endodermal cells become a single layer of cuboidal epithelium. Over subsequent weeks, additional cell layers appear, which begin to assume the characteristics of differentiated urothelial cells. Meanwhile, during the twelfth week, the surrounding splanchnopleuric mesoderm differentiates to form the detrusor muscle, which lines the urothelium. As bladder development proceeds, the mechanical distention associated with urine storage appears to be essential for the development of normal wall compliance.