Which compound is used as postfixation to fix lipids in electron microscopy processing?

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

Answer: (A)

When processing for electron microscopy, lipids are preserved and made visible by postfixation with osmium tetroxide. Osmium tetroxide reacts with the unsaturated bonds in membrane lipids, forming heavy, electron-dense complexes that stabilize membranes and render them highly contrastive under the electron beam. This lipid-focused fixation step complements initial aldehyde fixation and helps prevent lipid extraction during dehydration and embedding. Other fixatives mainly target proteins or simple dehydration effects and do not provide the same lipid preservation and membrane contrast as osmium tetroxide. Osmium tetroxide is also a heavy metal stain, which contributes to the strong electron density needed to visualize membranes. Handle it with appropriate safety precautions due to its toxicity.