Fig. 3. Comparison of the trigone in humans and mice. (A) A section through the human trigone at the level of the intramural ureter stained for smooth muscle alpha actin (brown). Black arrows point to the intramural muscle fibers. (B) A section through a newborn mouse showing the trigone stained for smooth muscle alpha actin (green) and the urothelium stained for uroplakin (red). The yellow arrows point to the longitudinal ureteral muscle fibers that encircle the intramural ureter. (C) Section through a human trigone showing the intramural path of the ureter and its surrounding thin layer of fibers (black arrows). (D). Section through the mouse trigone at birth showing the path of the intramural ureter, stained for uroplakin (red) to reveal the urothelium and smooth muscle alpha actin (green). The yellow arrows point to the longitudinal muscle fibers associated with the intramural ureter. Magnification: x20.