Which fixative makes lipids insoluble?

• A. Osmium Tetroxide
• B. Formalin
• C. Bouin's
• D. Acetone

Answer: (A)

Lipids become insoluble when a fixative that directly reacts with them is used. Osmium tetroxide does just that: it reacts with the unsaturated bonds in lipids, converting them into insoluble, osmiophilic compounds that remain in place through dehydration and clearing. This preserves lipid-rich membranes and makes lipids resistant to subsequent solvent extraction, which is essential for maintaining lipid structure during processing.

Formalin fixes proteins by cross-linking amino groups and does not adequately immobilize lipids, so they can be lost during processing. Bouin’s is a protein fixative that can also interact with lipids but is not specialized for preventing lipid solubility, and acetone is a solvent that dissolves lipids rather than fixing them.