Which fixative fixes neutral lipids by making them insoluble?

• A. Osmium tetroxide
• B. Formalin
• C. Glutaraldehyde
• D. Acetone

Answer: (A)

Neutral lipids are easily lost during tissue processing unless a fixative preserves them by making them insoluble. Osmium tetroxide does this by reacting with the carbon–carbon double bonds in lipid molecules, oxidizing them and forming osmium-lipid complexes that are insoluble in the embedding medium. This not only stabilizes neutral lipids and membranes but also provides strong electron density for visualization under electron microscopy. In contrast, formalin and glutaraldehyde mainly cross-link proteins and don’t prevent lipid extraction, while acetone is a dehydrating solvent that can dissolve lipids rather than fix them. So the fixative that makes neutral lipids insoluble is osmium tetroxide.