Which fixation step is primarily responsible for stabilizing lipid-rich membranes in electron microscopy?

• A. Osmium tetroxide post-fixation.
• B. Glutaraldehyde primary fixation.
• C. Bouin's solution.
• D. Formaldehyde fixation.

Answer: (A)

Stabilizing lipid-rich membranes in electron microscopy is achieved most effectively by a fixative that targets the lipids themselves. Osmium tetroxide used as a post-fixative does this directly: it reacts with the unsaturated bonds in membrane lipids, forming osmiophilic, electron-dense complexes that harden the lipid bilayers and make them resistant to extraction during dehydration and embedding. This step also enhances contrast of membranes in the final image. In contrast, primary aldehyde fixatives like glutaraldehyde or formaldehyde mainly cross-link proteins to preserve overall cellular structure and are not as effective at stabilizing lipids; Bouin’s solution preserves morphology but isn’t lipid-specific and can complicate lipid preservation. So, the post-fixation with osmium tetroxide is the step that most directly stabilizes lipid-rich membranes.