Investigation of staining, polarization and fluorescence‐microscopic properties of myoendothelial cells

SUMMARY Electron microscopists have described in vascular endothelial cells filaments which resemble myofibrils. These fibrils cannot be demonstrated with conventional staining techniques. On the basis of previous investigations of relations between dye structure and affinity for muscle fibres, a staining method for demonstration of myoendothelial cells by direct, polarization and fluorescence microscopy has been developed. Carnoy‐fixed paraffin sections were treated consecutively with Kernechtrot, tannic and phosphomolybdic acid and counter‐stained with Levanol fast cyanine 5RN. This procedure stained myoendothelial cells and muscle fibres deep blue, connective tissues yellow and nuclei pink. For polarization and fluorescence‐microscopic studies thiazine red R was substituted for Levanol fast cyanine 5RN and Kernechtrot was replaced by Mayer's acid hæmalum. The light‐microscopic characteristics of fibrils in endothelial cells and in smooth muscle cells were identical. Correlation of light‐microscopic observations with electron‐microscopic data supported the conclusion that the staining procedure indeed demonstrates myofilaments in endothelial cells. Formation of multiple layers of myoendothelial cells was observed in areas of thickening of the intima, while the cells in the deeper layers became indistinguishable from smooth muscle cells. These findings support earlier concepts of the transformation of endothelial cells into intimal muscle cells.