Fats are usually preserved best if the tissue is fixed in which fixative?

• A. Osmium tetroxide
• B. Formalin
• C. Bouin's solution
• D. Carnoy fluid

Answer: (A)

Lipids stay in place when the fixative binds to them rather than dissolving them during processing. Osmium tetroxide does this by reacting with the carbon–carbon double bonds in fatty acids, forming stable complexes that fix fats in situ. This not only preserves the lipid-containing structures, such as membranes, but also makes them electron-dense, which is why osmium tetroxide is widely used in lipid- and membrane-rich areas of tissue, especially in electron microscopy.

Other common fixatives focus on fixing proteins and general tissue structure and often involve dehydration and clearing steps with organic solvents. Those steps can remove lipids from the tissue, leading to loss of fat content and poor preservation of fats. Bouin’s solution and Carnoy fluid contain components like picric acid or organic solvents that can extract lipids, while formalin mainly cross-links proteins and does not protect lipids from extraction during processing.

Therefore, for best preservation of fats, a lipid-friendly fixative like osmium tetroxide is the most effective choice.