Relationship of the structure of ureteral muscularis to in vitro spontaneous contractions.

We investigated the anatomical structure of the smooth muscle layers of the mid portion of the canine ureter. In transverse sections the inner muscle bundles appear to be oriented longitudinally and the outer muscle bundles appear to be oriented in a circular configuration. In sections cut parallel with the longitudinal axis of the ureter, the inner and outer bundles appear to be oblique and are oriented in a mesh-line manner. In oblique sections, cut parallel with the helically oriented ureteral muscle, bundles appear to be oriented in a lengthwise configuration. These anatomical studies show that canine ureteral smooth muscle is composed of spiral fibers in a mesh structure and that helical muscle strips which include a significant percentage of continuous bundles have a greater capacity to generate spontaneous contractions in vitro than do transverse and longitudinal muscle strips.