Staining in the context of "Medical"

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 chromatic (color). Toluidine blue is an example of a partially orthochromatic dye, as it stains nucleic acids by its orthochromatic color (blue), but stains mast cell granules in its metachromatic color (red).

In spectral terms, orthochromasia refers to maintaining the position of spectral peaks, while metachromasia refers to a shift in wavelength, becoming either shorter or longer.

Metachromasia (var. metachromasy) is a characteristic change in the color of staining carried out in biological tissues, exhibited by certain dyes 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 content) when bound to cartilage. Other widely used metachromatic stains include the family of Romanowsky stains 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.

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, 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.

Hematoxylin and eosin stain (or haematoxylin and eosin stain or hematoxylin–eosin stain; often abbreviated as H&E stain or HE stain) is one of the principal tissue stains used in histology. It is the most widely used stain in medical diagnosis and is often the gold standard. For example, when a pathologist looks at a biopsy of a suspected cancer, the histological section is likely to be stained with H&E.

H&E is the combination of two histological stains: hematoxylin and eosin. The hematoxylin stains cell nuclei a purplish blue, and eosin stains the extracellular matrix and cytoplasm pink, with other structures taking on different shades, hues, and combinations of these colors. Hence a pathologist can easily differentiate between the nuclear and cytoplasmic parts of a cell, and additionally, the overall patterns of coloration from the stain show the general layout and distribution of cells and provides a general overview of a tissue sample's structure. Thus, pattern recognition, both by expert humans themselves and by software that aids those experts (in digital pathology), provides histologic information.

A stain is a discoloration that can be clearly distinguished from the surface, material, or medium it is found upon. They are caused by the chemical or physical interaction of two dissimilar materials. Accidental staining may make materials appear used, degraded or permanently unclean. Intentional staining is used in biochemical research, and for artistic effect, such as in wood staining, rust staining and stained glass.

Endospore staining is a technique used in bacteriology to identify the presence of endospores in a bacterial sample. Within bacteria, endospores are protective structures used to survive extreme conditions, including high temperatures making them highly resistant to chemicals. Endospores contain little or no ATP which indicates how dormant they can be. Endospores contain a tough outer coating made up of keratin which protects them from nucleic DNA as well as other adaptations. Endospores are able to regerminate into vegetative cells, which provides a protective nature that makes them difficult to stain using normal techniques such as simple staining and gram staining. Special techniques for endospore staining include the Schaeffer–Fulton stain and the Moeller stain.

Acid-fastness is a physical property of certain bacteria, protozoa, and eukaryotic cells, as well as some subcellular structures, referring to their resistance to decolorization by acids during laboratory staining procedures. Once stained as part of a sample, these organisms can resist the acid and/or ethanol-based decolorization procedures common in many staining protocols, hence the name acid-fast.

Historically, acid-fast stains were thought to stain lipids of the cells based on the observed charectistics of cell staining under a wide range of conditions, although the results were limited by the tools available, however as early as 1959 there were observations of how nucleic acids were acid fast. Dyes such as carbol fuchsin and auramine O penetrate the cell and bind to DNA and RNA, producing characteristic red or yellow-green fluorescence, respectively. The property of “acid-fastness” therefore reflects the organism’s ability to retain these dyes after acid–alcohol decolorization, a feature determined mainly by the integrity and composition of the outer cell wall rather than by any specific lipid chemistry.

Gram stain (Gram staining or Gram's method) is a method of staining used to classify bacterial species into two large groups: gram-positive bacteria and gram-negative bacteria. It may also be used to diagnose a fungal infection. The name comes from the Danish bacteriologist Hans Christian Gram, who developed the technique in 1884.

Gram staining differentiates bacteria by the chemical and physical properties of their cell walls. Gram-positive cells have a thick layer of peptidoglycan in the cell wall that retains the primary stain, crystal violet. Gram-negative cells have a thinner peptidoglycan layer that allows the crystal violet to wash out on addition of ethanol. They are stained pink or red by the counterstain, commonly safranin or fuchsine. Lugol's iodine solution is always added after addition of crystal violet to form a stable complex with crystal violet that strengthens the bonds of the stain with the cell wall.