The musculature of the human renal calices, pelvis and upper ureter.

The musculature of the proximal part of the human upper urinary tract has been examined using light and electron microscopy. Two morphologically and histochemically distinct types of smooth muscle cell have been identified within the walls of the renal calices and pelvis. One type is rich in non-specific cholinesterase and appears identical in fine structure to the smooth muscle of the ureter. The second type is devoid of non-specific cholinesterase and possesses a number of unusual fine structural features. Cells of the latter type form a thin sheet of caliceal muscle which extends across the renal parenchyma to interconnect with adjacent minor calices. Similar cells extend away from the renal substance to form a discrete layer confined to the inner aspect of the muscle coat, the remainder of which consists of 'typical' smooth muscle cells arranged into compact bundles. The inner layer of 'atypical' cells is present in the walls of the major calices and pelvis but ceases in the pelvi-ureteric region. It is proposed that these 'atypical' cells are capable of spontaneous contractility and perform a pacemaker function which is responsible for the initiation of ureteric peristalsis.