Due to their transparent nature, the cellular and intracellular structure of tissue samples cannot be microscopically examined until they are colored by dyes. Dyes used to stain tissue samples in the histology laboratory for microscopic evaluation are called biological dyesorbiological stains. Biological dyes can be grouped into the following two categories:
Natural: Dyes that are derived from natural resources. The most important natural dye in the histopathology laboratory is hematoxylin.
Artificial: Dyes that are derived through chemical reactions. Artificial dyes greatly outnumber natural dyes.
"Routine" staining in the histology laboratory is done with the use of hematoxylin and eosin (H&E) in which the nuclei of cells is colored purple-blue, while the cytoplasm is colored pink.
The term "special stains" refers to staining methods other than H&E. Special stains provide a means of staining elements, structures and sometimes microorganisms that cannot be demonstrated with only H&E staining.