Fat is chemically fixed and maintained in tissue by which fixative?

• A. Osmium Tetroxide
• B. Formalin
• C. Glyoxal
• D. Bouin Solution

Answer: (A)

Lipids require a fixative that chemically interacts with the lipid molecules to prevent their loss during processing. Osmium tetroxide does exactly this: it reacts with the double bonds in unsaturated lipids and cholesterol, forming stable, osmiophilic products that remain in tissue and also provide strong staining for membranes. This preserves fats in the specimen, which is why osmium tetroxide is used when fat preservation is essential, such as in electron microscopy. In contrast, formalin mainly fixes proteins by cross-linking amino groups and does not effectively preserve fats; Bouin solution and glyoxal are not chosen for lipid preservation. Therefore, the best answer is osmium tetroxide.