Metachromasy is a characteristic change in the color

Color

Color or colour is the visual perceptual property corresponding in humans to the categories called red, green, blue and others. Color derives from the spectrum of light interacting in the eye with the spectral sensitivities of the light receptors...

 of staining carried out in biological tissues

Tissue (biology)

Tissue is a cellular organizational level intermediate between cells and a complete organism. A tissue is an ensemble of cells, not necessarily identical, but from the same origin, that together carry out a specific function. These are called tissues because of their identical functioning...

, exhibited by certain aniline

Aniline

Aniline, phenylamine or aminobenzene is an organic compound with the formula C6H5NH2. Consisting of a phenyl group attached to an amino group, aniline is the prototypical aromatic amine. Being a precursor to many industrial chemicals, its main use is in the manufacture of precursors to polyurethane...

 dye

Dye

A dye is a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and requires a mordant to improve the fastness of the dye on the fiber....

s when they bind to particular substances present in these tissues, called chromotropes. For example, toluidine blue becomes pink when bound to cartilage

Cartilage

Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...

. The absence of color change in staining is named orthochromasy.

Although metachromasy was observed and described since 1875, by Cornil

Victor André Cornil

Victor André Cornil, also André-Victor Cornil was a French pathologist and histologist born in Cusset, Allier.He studied medicine in Paris, earning his doctorate in 1864...

, Ranvier

Louis-Antoine Ranvier

Louis-Antoine Ranvier was a French physician, pathologist, anatomist and histologist, who discovered nodes of Ranvier, regularly spaced constrictions of the myelin sheath, occurying at varying intervals along the length of a nerve fiber.Ranvier was born and studied medicine at Lyon, graduating in...

 and others, it was the German scientist Paul Ehrlich

Paul Ehrlich

Paul Ehrlich was a German scientist in the fields of hematology, immunology, and chemotherapy, and Nobel laureate. He is noted for curing syphilis and for his research in autoimmunity, calling it "horror autotoxicus"...

 (1854-1915) who gave its name and studied it more extensively. The modern understanding why metachromasy occurs was advanced by Belgian histologist Lucien Lison

Lucien Lison

Lucien Alphonse Joseph Lison was a Belgian/Brazilian physician and biomedical scientist, considered the "father of histochemistry" ....

, who studied it between 1933 and 1936 and ascertained its value in the quantitative determination of sulfate ester

Ester

Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

s of high molecular weight. He also studied the metachromasy of nucleic acid

Nucleic acid

Nucleic acids are biological molecules essential for life, and include DNA and RNA . Together with proteins, nucleic acids make up the most important macromolecules; each is found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information...

s.