Hematoxylin and eosin stain

Hematoxylin and eosin stain or haematoxylin and eosin stain (H&E stain or HE stain) is one of the principal stains in histology. It is the most widely used stain in medical diagnosis and is often the gold standard; for example when a pathologist looks at a biopsy of a suspected cancer, the histological section is likely to be stained with H&E and termed "H&E section", "H+E section", or "HE section". A combination of hematoxylin and eosin, it produces blues, violets, and reds.

The staining method involves application of hemalum, a complex formed from aluminum ions and hematein (an oxidation product of hematoxylin). Hemalum colors nuclei of cells (and a few other objects, such as keratohyalin granules and calcified material) blue. The nuclear staining is followed by counterstaining with an aqueous or alcoholic solution of eosin Y, which colors eosinophilic structures in various shades of red, pink and orange.

The staining of nuclei by hemalum is ordinarily due to binding of the dye-metal complex to DNA, but nuclear staining can be obtained after extraction of DNA from tissue sections. The mechanism is different from that of nuclear staining by basic (cationic) dyes such as thionine or toluidine blue. Staining by basic dyes occurs only from solutions that are less acidic than hemalum, and it is prevented by prior chemical or enzymatic extraction of nucleic acids. There is evidence to indicate that coordinate bonds, similar to those that hold aluminium and hematein together, bind the hemalum complex to DNA and to carboxy groups of proteins in the nuclear chromatin.

...
Wikipedia