Eosin

  Eosin is a fluorescent red dye resulting from the action of bromine on fluorescein. It can be used to stain cytoplasm, collagen and muscle fibers for examination under the microscope.

There are actually two very closely related compounds commonly referred to as eosin. Most often used is eosin Y (also known as eosin Y ws, eosin yellowish, Acid Red 87, C.I. 45380, bromoeosine, bromofluoresceic acid, D&C Red No. 22); it has a very slightly yellowish cast. The other eosin compound is eosin B (eosin bluish, Acid Red 91, C.I. 45400, Saffrosine, Eosin Scarlet, or imperial red); it has a very faint bluish cast. The two dyes are interchangeable, and the use of one or the other is a matter of preference and tradition.

Eosin is most often used as a counterstain to haematoxylin in H&E (haematoxylin and eosin) staining. H&E staining is one of the most commonly used techniques in histology. Tissue stained with hematoxylin and eosin shows cytoplasm stained pink-orange and nuclei stained darkly, either blue or purple. Eosin also stains red blood cells intensely red. Eosin is an acidic dye and shows up in the basic parts of the cell, ie the cytoplasm. Hematoxylin however is a basic dye and shows up in the acidic part of the cell. For example the nucleus, where nucleic acids (DNA and RNA) are concentrated.

Structures that stain readily with eosin are termed eosinophilic.

Eosin Y is a tetrabromo derivate of fluorescein. Its CAS number is [17372-87-1] and its SMILES structure is O=C5C(Br)=C2O C1=C(Br)C([O-]) =C(Br)C=C1C(C4=C (C([O-])=O)C=C C=C4)=C2C=C3Br.

Eosin B is a dibromo dinitro derivate of fluorescein. Its CAS number is [548-28-3] and its SMILES structure is O=C5C(Br)=C2O C1=C(Br)C([O-]) =C([N+]([O-])=O) C=C1C(C4=C(C([O-]) =O)C=CC=C4)=C2 C=C3[N+]([O-])=O.

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Recipes for Histology

5% Aqueous Eosin Y

Formula:

• 5 g Eosin Y
• 100 mL Distilled water

Dissolve Eosin Y crystals in water by gentle heating. Cool and Filter. Thymol crystals may be added to prevent formation of molds.

Eosin, Stock Alcoholic Solution

Formula:

• 1 g Eosin Y
• 20 mL Distilled water
• 95% Alcohol

Dissolve Eosin Y in the water by gentle heating. Cool and add Alcohol.

For use, one part of the stock solution is usually diluted with three parts of 80% Alcohol. Addition of 0.5 mL Glacial Acetic Acid (HOAc) for every 100 mL of stain will usually give a deeper red stain to the tissue.

Etymology

The name Eosin comes from Eos, the Greek word for 'dawn' and the name of the Greek Goddess of the Dawn.[1]

See also

• Hematoxylin
• H&E stain

References

• Jocelyn H. Bruce-Gregorios, M.D.: Histopathologic Techniques, JMC Press Inc., Quezon City, Philippines, 1974.