What is an advantage of using osmium tetroxide as a primary fixative?

• A. Lipids are rendered insoluble.
• B. It penetrates more slowly than aldehydes.
• C. It prevents electron microscopy altogether.
• D. It causes immediate depolymerization of paraformaldehyde.

Answer: (A)

Osmium tetroxide is advantageous because it fixes lipids by reacting with their unsaturated bonds, making lipid-containing structures insoluble and thus preserving membranes during processing. This lipid stabilization is crucial for electron microscopy, where membranes and lipid-rich organelles need to stay intact and gain contrast from the heavy osmium stain. The other statements don’t describe real advantages: penetration is not necessarily faster than aldehydes, osmapositive—does not prevent electron microscopy, and it does not depolymerize paraformaldehyde.