Due to their transparent nature, the cellular and intracellular structures in tissue samples cannot be microscopically examined until they are colored by dyes. Dyes used to stain tissue samples for microscopic analysis in the clinical histology laboratory 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 greatly outnumber natural dyes.