For Oil Red O staining, what would be an appropriate control slide?

• A. Fatty Liver cut at 10 microns
• B. Normal Liver tissue
• C. Muscle tissue
• D. Adipose tissue in paraffin block

Answer: (A)

Oil Red O specifically stains neutral lipids, and lipids are preserved only in frozen (cryosection) tissue because routine paraffin processing removes them. For a reliable positive control, you want a tissue with abundant lipid droplets that will clearly take up the stain. A fatty liver cryosection cut at about 10 microns provides numerous lipid droplets that will appear red with Oil Red O, confirming the staining worked and that lipids are present. In contrast, normal liver would have far fewer lipids and might not reliably show staining, muscle contains little fat, and adipose tissue in a paraffin block has had its lipids extracted, so it wouldn’t stain. Thus, fatty liver in a frozen 10‑μm section is the best control.