Ultrastructure of the urinary tract muscle coat in man. Calices, renal pelvis, pelvi-ureteric junction and ureter.

Muscle coat specimens from human calices, renal pelvis, pelvi-ureteric junction, upper, middle and lower ureter segments were examined under an electron microscope. These specimens were taken from 8 patients who had undergone nephroureterectomy: 6 for localized renal carcinoma and 2 for papillary tumor of the pelvis. Two types of smooth muscle cells were observed, "typical" muscle cells and "special" muscle cells. The latter are rich in agranular endoplasmic reticulum, have few myofilaments and are interconnected in numerous, extended, peculiar contact areas. The ratio between these two types of muscle cells differs as also does their innervation between the various segments examined. On the basis of our findings we propose that the "special" muscle cells perform a "pacemaking" function.