In Osmium Tetroxide paraffin procedure for fat, what is the appearance of fat after staining?

• A. Fat appears black
• B. Nuclei appear black
• C. Background appears blue
• D. Cells appear green

Answer: (A)

Fat droplets are stained black because osmium tetroxide binds to the double bonds in unsaturated fatty acids, forming a black osmium deposit that intensely colors lipids. In this paraffin method, you’re visualizing fat by its dark, nearly pitch-black appearance against the lighter surrounding tissue. The other options don’t reflect this lipid-specific staining: nuclei would not be black from this stain, the background would not be a general blue, and cells would not appear green from osmium staining.