Eosins

Eosins are a group of synthetic acidic dyes commonly used in histology and cytology as counterstains. The most widely used members are eosin Y, eosin B, and eosin M, which are all xanthene dyes supplied as water-soluble salts. In routine tissue staining, eosins are typically combined with hematoxylin in Hematoxylin and Eosin (H&E) protocols to provide contrast between cellular components.

Chemically, eosins are anionic dyes, usually used as disodium or tetrasodium salts that contain sulfonate groups for water solubility. They are pH-sensitive, and their staining behavior reflects their affinity for basic components of tissues. As a result, eosins stain acidophilic structures such as cytoplasm, muscle fibers, collagen, and extracellular matrix, giving them pink to red hues.

Staining characteristics can vary among the eosin species, with each producing slightly different shades of pink to red. In practice, the choice among eosins may depend on the desired contrast with hematoxylin, the tissue type, and the specific staining protocol. Eosin dyes are frequently used not only in routine histology but also in cytology preparations and certain immunohistochemistry workflows as a general counterstain.

Handling and safety follow standard laboratory guidelines for dyes: eosins can be irritants and should be handled with appropriate personal protective equipment and proper disposal. The dyes have a long history in microscopy and remain a standard component of many staining protocols due to their reliability and cost-effectiveness.