Which fixative is commonly used to preserve lipids in tissues?

• A. Osmium tetroxide
• B. Formalin
• C. Ethanol
• D. Glyoxal

Answer: (A)

Preserving lipids requires a fixative that actually reacts with lipid components rather than just crosslinking proteins. Osmium tetroxide does this by reacting with unsaturated lipids and forming stable osmate lipid complexes, which insolubilize and stabilize the lipid structures such as membranes and myelin. This also provides high electron density, making lipids visible in electron microscopy.

Formalin crosslinks proteins but doesn’t fix lipids well, so lipids are often extracted during processing. Ethanol dehydrates and can dissolve lipids, leading to loss. Glyoxal mainly fixes proteins and isn’t as effective at stabilizing lipids as osmium tetroxide. Thus, the fixative commonly used to preserve lipids in tissues is osmium tetroxide.