Fatty liver disease is a serious condition in cats. When a body is undernourished or starved, the liver must metabolize fat in the body into useable energy to sustain life. However, cat livers are not efficient at processing fat so much of it is stored as lipid droplets in hepatocytes.
This tissue section is from a cat with fatty liver disease. Osmic acid (osmium tetroxide) is a selective stain for unsaturated fatty acids and lipoproteins (such as myelin). Most of the hepatocytes contain one or more black-stained lipid droplets.
In normal liver, lipid droplets occur mostly in hepatic stellate (or Ito) cells found in the the perisinusoidal space (or space of Disse).
(Osmium tetroxide is also a commoningly used fixative in electron microscopy.)