Fig. 1. The trigone is the site of the anti-reflux mechanism. (A). Schematic of the trigone at the bladder base and its connections with the ureters showing the intramural ureter segment that is normally compressed to prevent back-flow of urine to the ureters and kidneys. (B) Schematic showing compression of the intramural ureter. (C) A detailed representation of the trigone, which is thought to be composed of ureteral fibers that enter the bladder via Waldeyer's sheath, fan out across the base to form the inter-ureteric ridge and extend down toward the apex to form Bell's muscle. (D) A vibratome section from an adult mouse stained for uroplakin (red) to reveal the urothelium, and for smooth muscle alpha actin (green) to reveal smooth muscle. (E) Opened bladder showing the trigone in an adult Hoxb7-Gfp mouse. The ureter orifices (yellow) are located at the base of the trigone. (F) High magnification of the ureter orifice, showing its eyelet shape at the point it opens into the urothelium (red, uroplakin). Magnification: x100 in D,E; x200 in F.