Which fixative acts as a powerful protein coagulant and enhances staining by leaving tissue receptive to dyes, with pigment removable by iodine and sodium thiosulfate?

• A. Mercuric Chloride
• B. Glutaraldehyde
• C. Formalin Alcohol
• D. 10% NBF

Answer: (A)

Mercury-containing fixatives act as strong protein coagulants, precipitating tissue proteins and forming mercury-protein complexes that bind dyes readily. This creates a dense, dye-friendly matrix that enhances staining intensity and contrast, making the tissue more receptive to dyes. A hallmark of these fixatives is the formation of a mercury-containing pigment within the tissue, which can be removed later by treating with iodine and sodium thiosulfate, clearing the pigment without destroying the tissue. This combination of powerful coagulation and a removable mercury pigment is characteristic of mercuric chloride fixatives, unlike other fixatives that primarily cross-link proteins and don’t produce the same removable pigment.