The treatment of biological specimens with dyes (stains) to colour part of the structure so as to make details more clearly visible through a microscope. Most laboratory stains are synthetic organic dyestuffs, which are absorbed by or bind to particular types of tissue. They are applied by immersing the specimen in a solution of the dye. Nonvital staining is the colouring of dead tissue; vital staining is the staining of living tissue without harming or killing the cells.
Most stains are organic salts consisting of a positive and negative ion. In acid stains the colour comes from an organic anion (negative ion). Such stains (e.g. eosin) tend to colour the cytoplasm of cells. In basic stains the colour comes from an organic cation (positive ion). Such stains, e.g. haematoxylin, tend to colour the nuclei. Neutral stains are mixtures of acid and basic stains and are used to stain both nucleus and cytoplasm. An example is Leishman's stain, which is made by mixing the acid stain eosin with the basic stain methylene blue in alcohol. Materials in cells can be described as acidophilic if they are receptive to acid dyes; basophilic if receptive to basic dyes; and neutrophilic if receptive to neutral dyes. Certain dyes need an additional substance (a mordant) to bind them to the tissue. The combination of the dyestuff and the mordant is called a lake. Iron alum, for instance, is used as a mordant for haematoxylin. In its absence, the tissue will not take up the stain. Various complex techniques are used for staining different types of tissue for light microscopy. Counterslaining (or double staining) is the application of two stains in sequence so as to colour different parts of the specimen. For example, safranin is often counterstained with light green. Overstaining occurs when too much stain is taken up by the specimen. In such cases, part of the stain is removed by a solvent - a process known as differentiation.
By analogy with stains for the light microscope, electron stains are substances used with specimens for electron microscopy. They act by hindering the transmission of electrons (i.e. they are 'electron dense'). Examples are lead citrate, uranyl acetate (UA), and phosphotungstic acid (PTA). See also histochemistry
, metachromatic stain
, negative staining
, permanent stain
, temporary stain
, vital stain