Immunoelectron microscopical characterization of the epithelioid type of smooth muscle cells in human glomus organs.

The wall structure of arterio-venous anastomoses in human glomus organs was studied by immunohistochemistry and immunoelectron microscopy. Smooth muscle cells of the epithelioid type I and of the dense type II could be found in the media. The immunohistochemical study confirmed the immunopositivity of both smooth muscle cell types for alpha-smooth muscle actin, vimentin, and smooth muscle myosin. All smooth muscle cells also stained positively for caveolin, a recently described structural protein of microvesicles present in selected epithelial and nonepithelial cells. The immunoreactivity for cathepsin D, however, was much higher in the type I cells than in the type II cells. Immunoelectron microscopy revealed that type I cells contain loose arrays of alpha smooth muscle actin positive microfilaments, sometimes arranged in small bundles, whereas the dense medial smooth muscle cells of the type II have tightly packed actin filaments. Only type I cells contained cathepsin D positive lysosomes. The data suggest that two types of phenotypic variants of vascular smooth muscle cells in the human arterio-venous anastomosis exist: a more "synthetic" type I cell and a more contractile type II cell.