{{inline citations needed, date=November 2024
Metachromasia (var. metachromasy) is a characteristic change in the
color
Color (or colour in English in the Commonwealth of Nations, Commonwealth English; American and British English spelling differences#-our, -or, see spelling differences) is the visual perception based on the electromagnetic spectrum. Though co ...
of
staining
Staining is a technique used to enhance contrast in samples, generally at the Microscope, microscopic level. Stains and dyes are frequently used in histology (microscopic study of biological tissue (biology), tissues), in cytology (microscopic ...
dye
Juan de Guillebon, better known by his stage name DyE, is a French musician. He is known for the music video of the single "Fantasy
Fantasy is a genre of speculative fiction that involves supernatural or Magic (supernatural), magical ele ...
s when they bind to particular substances present in these tissues, called chromotropes. For example, toluidine blue becomes dark blue (with a colour range from blue-red dependent on
glycosaminoglycan
Glycosaminoglycans (GAGs) or mucopolysaccharides are long, linear polysaccharides consisting of repeating disaccharide units (i.e. two-sugar units). The repeating two-sugar unit consists of a uronic sugar and an amino sugar, except in the case o ...
content) when bound to
cartilage
Cartilage is a resilient and smooth type of connective tissue. Semi-transparent and non-porous, it is usually covered by a tough and fibrous membrane called perichondrium. In tetrapods, it covers and protects the ends of long bones at the joints ...
. Other widely used metachromatic stains include the family of
Romanowsky stains
Romanowsky staining is a prototypical staining technique that was the forerunner of several distinct but similar stains widely used in hematology (the study of blood) and cytopathology (the study of diseased cells). Romanowsky-type stains are use ...
that also contain thiazine dyes: the white cell nucleus stains purple, basophil granules intense magenta, whilst the cytoplasm (of mononuclear cells) stains blue, which is called the ''Romanowsky effect''. The absence of color change in staining is named
orthochromasia
In chemistry, orthochromasia is the property of a dye or stain to not change color on binding to a target, as opposed to ''metachromatic'' stains, which do change color. The word is derived from the Greek '' orthos'' (correct, upright), and chr ...
.
The underlying mechanism for metachromasia requires the presence of polyanions within the tissue. When these tissues are stained with a concentrated basic dye solution, such as toluidine blue, the bound dye molecules are close enough to form dimeric and polymeric aggregates. The light absorption spectra of these stacked dye aggregates differ from those of the individual monomeric 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. This depends on the charge density of the negative sulfate and carboxylate anions in the glycosaminoglycan (GAG). The GAG polyanion stabilizes the stacked, positively charged dye molecules, resulting in a spectral shift as the conjugated double bond π-orbitals of adjacent dye molecules overlap. The greater the degree of stacking, the greater the metachromatic shift. Thus,
hyaluronic acid
Hyaluronic acid (; abbreviated HA; conjugate base hyaluronate), also called hyaluronan, is an anionic, nonsulfated glycosaminoglycan distributed widely throughout connective, epithelial, and neural tissues. It is unique among glycosaminog ...
, lacking sulphate groups and with only moderate charge density, causes slight metachromasia; chondroitin sulfate, with an additional sulfate residue per GAG saccharide dimer, is an effective metachromatic substrate, whilst heparin, with further N-sulfation, is strongly metachromatic. Therefore, toluidine blue will appear purple to red when it stains these components.
The metachromatic properties of dimethylmethylene blue, a thiazine dye closely related to toluidine blue, have been exploited to assay glycosaminoglycans extracted from cartilage and other connective tissues. The absorption peak shifts from about 630 nm (red absorption, therefore blue colour) to about 530 nm in the presence of GAG. Humbel and Etringer's original assay was developed by others to create a stable and widely used dimethylmethylene blue reagent.
Although metachromasia was observed and described since 1875, by Cornil, Ranvier and others, it was the German scientist
Paul Ehrlich
Paul Ehrlich (; 14 March 1854 – 20 August 1915) was a Nobel Prize-winning German physician and scientist who worked in the fields of hematology, immunology and antimicrobial chemotherapy. Among his foremost achievements were finding a cure fo ...
(1854-1915) who gave its name and studied it more extensively. The modern understanding of metachromasia was advanced by Belgian histologist Lucien Lison, who studied it between 1933 and 1936 and ascertained its value in the quantitative determination of sulfate
ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
s of high
molecular weight
A molecule is a group of two or more atoms that are held together by Force, attractive forces known as chemical bonds; depending on context, the term may or may not include ions that satisfy this criterion. In quantum physics, organic chemi ...
. He also studied the metachromasia of
nucleic acid
Nucleic acids are large biomolecules that are crucial in all cells and viruses. They are composed of nucleotides, which are the monomer components: a pentose, 5-carbon sugar, a phosphate group and a nitrogenous base. The two main classes of nuclei ...
s. More recently, Karlheinz Toepfer published in 1970 spectral shifts with increasing concentration of the thiazine dyes that matched the spectra of dye:heparin mixtures, showing clearly that metachromasia, corresponding to the colour of stained cartilage, could be reproduced by high concentration of the dye alone in solution. Hence, proximity of the dye molecules was the key parameter in defining metachromasia. Another example of metachromatic dye (fluorochrome) is acridine orange. Under certain conditions it stains single-stranded nucleic acids fluorescing red (red luminescence) while when interacts with double stranded nucleic acids gives green fluorescence.Darzynkiewicz Z, Kapuscinski J. (1990)“Acridine Orange, a Versatile Probe of Nucleic Acids and Other Cell Constituents.” Chapter in: Flow Cytometry and Sorting. Melamed MR, Mendelsohn M & Lindmo T (eds), Alan R. Liss, Inc., New York. pp. 291-314. ISNBM 0-471-56235-1