Which fixative is particularly suitable for preserving lipids in tissue samples?

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

Answer: (A)

Preserving lipids requires a fixative that actually stabilizes lipid molecules against the solvents used in processing. Osmium tetroxide does this by reacting with the double bonds in unsaturated lipids, forming osmium-lipid complexes that are insoluble in the dehydration and clearing steps. This “fixes” the lipids in place and also provides strong electron density, making membranes and lipid-rich structures visible, which is why it’s favored when lipid preservation is critical (often in electron microscopy). Other common fixatives mostly fix proteins and are not as protective of lipids; their processing steps involve organic solvents that can dissolve or remove lipids, so lipids aren’t well preserved with them.