What is the fixative for Paraffin processed fat?

• A. Osmium tetroxide
• B. 10% NBF
• C. Bouin
• D. Zenkers

Answer: (A)

Preserving fats in tissue requires a fixative that interacts with the lipids themselves, not just the surrounding proteins. Osmium tetroxide fixes unsaturated lipids by reacting with their carbon–carbon double bonds, forming osmium-containing lipid complexes that are insoluble in the solvents used during paraffin processing. This renders fat structures resistant to extraction during dehydration and clearing, allowing fat to be demonstrated and preserved in paraffin-embedded tissue.

In contrast, standard fixatives like 10% neutral buffered formalin mainly cross-link proteins and do not protect lipids from being dissolved during processing. Bouin’s and Zenker’s fixatives are not lipid-specific and can produce artifacts or promote lipid loss. Because osmium tetroxide specifically safeguards lipids, it is the best choice for paraffin-processed fat, though it is highly toxic and used only in specialized protocols.