Which fixative is known for preserving lipids in tissue processing?

• A. Osmium tetroxide
• B. Formalin
• C. Bouin solution
• D. Carnoy's solution

Answer: (A)

Lipids are best preserved by fixatives that actually react with and stabilize the lipid components rather than dissolve them during processing. Osmium tetroxide does exactly that: it reacts with unsaturated lipid bonds, forming osmiophilic, cross-linked complexes that fix membranes and lipid-rich structures. This chemical fixation preserves lipids and also provides strong membrane contrast, which is why osmium tetroxide is the preferred fixative when lipid preservation is important, especially in electron microscopy.

Formalins (aldehyde-based fixatives) primarily cross-link proteins and do not reliably preserve lipids; lipids can be lost during subsequent dehydration and processing. Bouin’s solution and Carnoy’s solution contain acetic acid and/or alcohol/organic solvents, which tend to extract or dissolve lipids rather than preserve them. So, among common fixatives, osmium tetroxide uniquely stands out for lipid preservation.