Metachromasia

METACHROMASIA, a term introduced by Ehrlich to denote a phenomenon consisting of the fact that a dye, homogeneous in its chemical properties, stains various tissue elements with different nuances; thus, methyl violet, while staining tissues in a diffuse violet-blue tone, simultaneously stains amyloid deposits in these tissues a pink-red or cherry-red color; thionine, while staining tissues a violet-blue color, simultaneously stains mucus and cartilage a red color. Such dyes are called metachromatic, and the corresponding tissue substances that stain metachromatically are called chromotropic. It is essential that the dyes themselves represent something chemically uniform; therefore, phenomena where different nuances in staining are obtained merely due to the admixture of some other dye cannot be classified as metachromasia; thus, iodine green sometimes stains amyloid red only because this dye often contains an admixture of methyl violet, which possesses metachromasia. Metachromatic dyes include iodine, triphenylmethane organic compounds with one or more methylated amino groups (dahlia, gentian violet, methyl violet, crystal violet), thiazines (thionine, toluidine blue, methylene azure, the amino groups of which are weakly or not at all methylated), and oxazines (e.g., cresyl violet). The most important metachromatic dyes are blue-violet, and the corresponding nuances are reddish. Among red dyes, safranin and neutral red (with an orange nuance of chromotropic substances) possess metachromasia. Analysis of the phenomenon of metachromasia has shown that the nuance itself (in a red or orange tone) depends on the color of the free base of the dye; thus, the base of thiazines and oxazines is red, and the base of neutral red is orange; therefore, methylene blue, the free base of which is also blue, does not produce metachromasia. However, the essence of metachromasia has not yet been fully clarified; there are indications of the importance of the medium in which the dye is dissolved (dispersion capabilities, temperature of the solvent). Among chromotropic substances, the most important are amyloid, mucus, the ground substance of cartilage, and the granulation of mast cells. To detect metachromasia, aqueous solutions of the corresponding dyes are usually used, and preparations are preserved either in water or in a levulose solution; prolonged exposure of the material to alcohol usually greatly reduces chromotropicity.