BACKGROUND: Interstitial cells of Cajal (ICC) may be important regulatory cells in gut muscle layers. This study examined ICC within the circular muscle of human small intestine.

METHODS: Surgically resected, uninvolved intestine was studied by light microscopy and electron microscopy.

RESULTS: Muscle lamellae were separated by main septa in continuity with submucosa. Smooth muscle cells ran radially in the septa. Two types of ICC were distinguished. One ICC type had abundant intermediate filaments and smooth cisternae and a discontinuous basal lamina. This ICC type was present in the septa and in the outer third of the circular lamellae. The other ICC type had a complete basal lamina and conspicuous caveolae. This ICC type was observed only in the inner third of the circular lamellae. Both ICC types were close to nerves, but only the latter type formed gap junctions with one another and with muscle cells. Junctions between the two ICC types were not observed.

CONCLUSIONS: The arrangement suggests that ICC and radially oriented muscle cells participate in electrical and mechanical coordination of the circular muscle layer of human small intestine.