Using the electron microscope, two types of smooth muscle cell have been recognized in the upper urinary tract of the pig. One type is confined solely to the renal attachment of each minor calix and possesses a number of atypical features. These cells are not grouped into bundles but form a loose meshwork. They frequently branch, are comparatively long with very irregular profiles, and are separated by relatively large amounts of connective tissue although maculae adhaerentes are occasionally observed between neighbouring cells. Dense micronbrils occur in the vicinity of the cells which are surrounded by a partial basal lamina. Groups of axons are seen in the region, many of which contain small dense-cored vesicles. Within the atypical cells, filaments are loosely packed and accumulations of micropinocytotic vesicles are not observed. These features contrast markedly with those of typical muscle cells in other regions of the upper urinary tract where they form tightly packed bundles unrelated to axon profiles. The typical smooth muscle cells are larger and more regular in outline, are surrounded by a complete basal lamina, possess tightly packed filaments, and contain numerous micropinocytotic vesicles and glycogen particles.

Similar modified muscle cells occur in the unicaliceal systems of other species where they are also confined to the proximal end of the urinary tract. It is suggested that the atypical cells are specially adapted to function as spontaneously active “pacemakers” initiating ureteric peristalsis.