Which fixative is best for lipid studies?

• A. Osmium Tetroxide
• B. Formalin
• C. Bouin's solution
• D. Glutaraldehyde

Answer: (A)

Lipid preservation requires a fixative that actually stabilizes lipids rather than dissolving them during tissue processing. Osmium tetroxide does this by reacting with the carbon–carbon double bonds found in unsaturated lipids, forming a stable, insoluble osmium-lipid complex. This not only fixes the lipids in place but also provides dark, electron-dense staining that highlights lipid-rich areas, making their distribution and morphology visible after embedding and sectioning.

In contrast, common fixatives like formalin or Bouin’s solution mainly cross-link proteins and do not protect lipids during dehydration and clearing steps, so lipid components can be lost or disrupted. Glutaraldehyde fixes proteins well but, like formalin, does not adequately preserve lipids on its own.