Immunohistochemical observation of foci of muscle actin-positive tumor cells in meningiomas

. Objective-It has been suggested that muscle actin is expressed in some meningiomas. The purpose of this study was to examine this hypothesis in a large number of meningiomas and to examine possible explanations for this expression. Design.-Sixty-five meningiomas were examined using immunohistochemistry for muscle actin. Meningiomas with foci of muscle actin-positive cells (n = 6) were further examined using monoclonal antibodies for desmin, epithelial membrane antigen, cytokeratin, and von Willebrand factor. These specimens were also stained for reticulin. Setting.-All tissues were obtained from surgical biopsy specimens at a large academic medical center. Main Outcome Measures.-Meningioma cells were considered to express muscle actin if the expression was shown to localize specifically in meningothelial cells and whorls that were also positive for epithelial membrane antigen. Results.-seventeen of 65 meningiomas were shown to contain muscle actin-positive cells. Scattered cells were positive in 11 cases, and in the remaining six cases we observed foci of muscle actin-positive cells, which had a propensity to form cellular whorls. These foci expressed epithelial membrane antigen, but stains for desmin, cytokeratin, von Willebrand factor, and reticulin were negative. Conclusions.-Our findings suggest that meningioma cells elaborate muscle actin without showing differentiation into specific cell types.