Due to their transparency, tissue samples' cellular and intracellular structures cannot be microscopically examined until dyes color them. Dyes used to stain tissue samples in the histology laboratory for microscopic evaluation are called biological dyes or biological 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 vastly outnumber natural dyes.
"Routine" staining in the histology laboratory uses hematoxylin and eosin (H&E), in which the nuclei of cells are colored purple-blue, while the cytoplasm is colored pink.
The term "special stains" refers to staining methods other than H&E. Special stains provide staining elements, structures, and sometimes microorganisms that cannot be demonstrated by only H&E staining.
