Interstitial cells in deep muscular plexus of canine small intestine may be specialized smooth muscle cells

The fine structures and properties of cells between the inner and outer circular muscle layers in the canine small intestine were studied by transmission electron microscopy (TEM), immunocytochemistry, and scanning electron microscopy (SEM). A nerve plexus (deep muscular plexus) supported by enteroglial cells, fibroblasts around blood vessels, macrophages, and thin and branched cells previously identified as interstitial cells of Cajal was observed. The interstitial cells of the deep muscular plexus (IC-DMP) were rich in mitochondria, dense bodies, and caveolae, and they were closely associated with nerve fibers. The IC-DMP had incomplete basal laminae. These cells also had numerous interconnecting gap junctions, and they also formed gap junctions with the surrounding smooth muscle cells of the outer circular muscle layer. IC-DMP were rich in myofilaments, which were primarily actin thin filaments, but myosin thick filaments, identified with anti-myosin light-chain antibodies, were also apparent. IC-DMP and circular smooth muscle cells both expressed immunoreactivity to anti-smooth muscle actin antisera, but these two types of cells differed in their intermediate filament proteins: IC-DMP featured vimentin immunopositive filaments, and circular smooth muscle cells featured desmin immunoreactivity. SEM showed that IC-DMP had thin and flat cell bodies with numerous branching processes. These cells came into close contact with nerve fibers and circular smooth muscle cells. The findings that IC-DMP cells contained myosin thick filaments and were immunopositive for anti-smooth muscle actin suggest that they may be more properly categorized as a type of smooth muscle cell.