Neutral lipids stained with Oil Red O on a cryostat cut appear what color?

• A. Blue
• B. Red to orange
• C. Green
• D. Purple

Answer: (B)

Neutral lipids stained with Oil Red O in frozen (cryostat) sections appear red to orange because Oil Red O is a fat-soluble dye that specifically binds to neutral lipids such as triglycerides and cholesterol esters. In cryosections, lipids are preserved, so these lipid droplets take up the dye and show up as bright red-orange against a counterstained background (often with hematoxylin to stain nuclei blue). The color blue would come from hematoxylin staining, not from Oil Red O itself; green or purple are not produced by this lipid-specific stain.