HA13 is a micrograph of a portion of the wall of the urinary bladder, which is much thicker than the wall of the ureter. The organization of the urinary bladder wall (HA13) is similar to that of the ureter, except that the urinary bladder has three layers of smooth muscle while the ureter has a third layer only in its distal third. These smooth muscle layers are not obvious because the layers are not as regularly oriented as they are in the gut.