# How does OsO_4 (osmium tetroxide) indicate the presence of fat?

Jan 14, 2015

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.