Ultrastructural and histochemical characterization of special muscle cells in the monkey small intestine.

The ultrastructure, three-dimensional arrangement, and histochemical features of special muscle cells in the monkey small intestine were investigated. The cells formed a special layer separated from the main part by a connective tissue space along the submucosal surface of the circular muscle coat. Scanning electron microscopy using alkali maceration demonstrated this inner sublayer to be a continuous thin sheet consisting of irregularly-shaped muscle cells equipped with many cytoplasmic projections and caveolae. Other ultrastructural features included direct contact with interstitial cells, due to their close association with nerve fibers of the deep muscular plexus. Histochemical examination revealed significant alkaline phosphatase activity and immunoreactivity for vascular smooth muscle alpha actin in these muscle cells, whereas the ordinary circular muscle cells were immunopositive for enteric smooth muscle gamma actin. These findings suggest that the special muscle cells play an important role in regulating the radial stretch of the monkey small intestinal wall.