Muscular development in the urinary tract.

The structures of the ureter, ureterovesical junction, bladder and sphincteric urethra were investigated histologically in 17 human fetuses (from 3rd to 21st weeks) and 2 neonates, with particular emphasis on their muscular development. The differentiation of ureteral muscle begins from the upper part of the ureter at the 12th week and muscle bundles run spirally. The purely longitudinal intramural ureteral muscle differentiates much later than the upper ureteral muscle and begins to form muscle bundles at the 17th week. The deep periureteral sheath and the trigonal muscle differentiate at an almost similar pace to the intramural ureteral muscle. The detrusor muscle begins to differentiate from the apical dome of the bladder at the 7th week. Distinct superficial periureteral sheath is also seen around the 12th week. The urethral striated muscle is recognized at the 8th week. It starts to develop in the anterior wall of the urethra as a dense cellular accumulation which eventually extends caudolaterally to form a U-shaped figure. The urethral smooth muscle starts its differentiation between the 9th and 11th weeks inside the urethral striated muscle.