How does #OsO_4# (osmium tetroxide) indicate the presence of fat?
OsO₄ turns fats black.
The OsO₄ molecule is tetrahedral and therefore non-polar. It is highly soluble in nonpolar solvents such as fats.
A paraffin section of tissue is soaked in a solution of OsO₄.
The OsO₄ dissolves in the fats, and it also reacts with the double bonds in unsaturated triglycerides.
The sample is then washed to remove excess OsO₄ and treated with formalin.
The formalin reduces any osmium in the tissue to a black solid.
All lipids appear as black or grey in the slide.
Here's a typical example.