Ruthenium red and violet. II. Fine structural localization in animal tissues

Abstract The inorganic dye, ruthenium red, stains extracellular materials in animal tissues which probably are acidic mucopolysaccharides. It complements other techniques, its advantages being fine grain, high resolution and good contrast. Localization is shown in mouse and rat muscle, heart, lung and intestine, frog cartilage and cells scraped from oral epithelium of human beings. Attention is paid to collagen bundles, the cell/collagen interface and particularly the myotendinal junction, cartilage matrix and agar gel, desmosomes, intestinal microvilli, erythrocytes and vascular endothelium, nerve fibers and the T‐system of striated muscle. Although ruthenium red generally is excluded by plasma membranes, it penetrates giving intracellular density, if the membrane is broken. Even when the cell membrane is intact, exceptions occur with selective staining of the T‐tubules or the sarcoplasmic sacs depending upon the state of contraction of the muscle cell, and with intracellular staining of certain nuclei and epithelial cells. Ruthenium red stains intracellular lipid droplets revealing lamellae, and stains myelin forms grown from crude egg lecithin but cannot penetrate deeply. It is localized in extracellular materials which have an important mechanical function. Its exclusion by cell membranes permits tracing tortuous cellular invaginations and those exceptions to its exclusion invite a comparison of the localization of the dye with the function of the cell.