Metachromasy

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Metachromasy (var. metachromasia) is a characteristic change in the color of staining carried out in biological tissues, exhibited by certain aniline dyes when they bind to particular substances present in these tissues, called chromotropes. For example, toluidine blue becomes pink when bound to cartilage. The absence of color change in staining is named orthochromasy.

The underlying mechanism for metachromasia is the presence of polyanions within the tissue. When these tissues are stained with a concentrated basic dye solution, such as toluidine blue, the dye molecules are close enough to form dimeric and polymeric aggregates. The absorption properties of these aggregations differ from those of the individual nonaggregated dye molecules. Cell and tissue structures that have high concentrations of ionized sulfate and phosphate groups—such as the ground substance of cartilage, heparin-containing granules of mast cells, and rough endoplasmic reticulum of plasma cells—exhibit metachromasia. Therefore, toluidine blue will appear purple to red when it stains these components.

Although metachromasy was observed and described since 1875, by Cornil, Ranvier and others, it was the German scientist Paul Ehrlich (1854-1915) who gave its name and studied it more extensively. The modern understanding why metachromasy occurs was advanced by Belgian histologist Lucien Lison, who studied it between 1933 and 1936 and ascertained its value in the quantitative determination of sulfate esters of high molecular weight. He also studied the metachromasy of nucleic acids.

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