If lipid preservation is a priority for electron microscopy, which fixative is most appropriate to use as a primary fixative?

• A. Osmium tetroxide.
• B. Buffered picric acid-formaldehyde.
• C. Millonig formalin.
• D. Paraformaldehyde.

Answer: (A)

Preserving lipid-rich membranes is essential for high-quality electron microscopy because lipids are easily extracted during processing and membranes need to remain intact to visualize ultrastructure. Osmium tetroxide uniquely fixes and stabilizes lipids by reacting with unsaturated bonds in membranes, turning them into electron-dense complexes that not only preserve the lipid structures but also provide inherent contrast in EM images. This lipid stabilization and membrane visualization is what makes osmium tetroxide the best choice when lipid preservation is the priority, even though aldehyde fixatives like formaldehyde or paraformaldehyde are excellent for preserving overall tissue architecture and proteins but do not protect lipids as effectively. Other options lack this strong lipid-preserving and membrane-contrast capability, so osmium tetroxide stands out for electron microscopic lipid preservation.