What is the purpose of a counterstain?

The purpose of a counterstain is primarily to provide contrast to the primary stain, helping to enhance the visibility of different cellular components or tissues under a microscope. In histology, a common primary stain is hematoxylin, which stains nucleic acids and cell nuclei. Eosin is frequently used as a counterstain because it stains the cytoplasm and other tissue components, providing a contrasting color that enables better differentiation and identification of cellular structures.

By staining the cytoplasm with eosin, it allows pathologists and laboratory professionals to observe the overall cellular morphology and anatomy more clearly, making it easier to identify abnormalities or features of interest in tissue samples. This is especially useful in diagnostic settings, such as when examining biopsies.

Focusing on this point illustrates the significance of a counterstain versus other options that refer to enhancing background color, removing excess dye, or fixing tissue, which do not accurately reflect the primary role of counterstains in tissue preparation and visualization.