To identify lipids, i.e., fats and fatty substances, frozen sections fixed in formaldehyd are generally used. Dying fatty substances is based on the fact that the dyes are more soluble in the tissue lipids than in the solvents in which they are applied.
|Dyes||Reactions / Colours|
|Sudan black||lipids turn blue-black|
|Sudan III||lipids turn red-orange|
|Scarlet||lipids turn red|
|Osmium tetroxide||lipid inclusions turn black|
|Sudan black or Sudan III or Scarlet:||These lipophilic dyes stain lipids black , yellow-red, or red.|
|Osmium tetroxide:||is fat soluble and, by accumulating on the double bond of unsaturated fatty acids, builds reaction products which stain the lipid inclusions black.|