Lipid (Lipid) is a general term for neutral fats, lipids and their derivatives, whose common physical properties are insoluble in water and soluble in organic solvents (e.g., ethanol, ether, etc.). There are two main types of body fat: 1, storage fat, such as neutral fat, mainly distributed in the subcutaneous, kidney, pancreas and other parts; 2, structural fat, such as lipids (phospholipids, glycolipids, cholesterol, etc.), mainly distributed in the cell. Neutral fat (Neutral fat) is made up of three molecules of fatty acids and one molecule of glycerol composed of lipids, neutral, neutral fat is one of the ways to store energy, in the oxidation of the release of energy, neutral fat staining is often used in Sudan Ⅱ, Sudan Ⅲ, Sudan Ⅳ, Sudan black B, oil red O method, etc., the traditional method of the use of Sudan dyes, and recently found that azo dyes, oil red O, is more suitable for the staining of fat, the azo dye. Oil red O is a very strong lipid solvent and fat dye, easier to combine with triglycerides in the form of small lipid droplets, and phospholipids with a slightly poorer binding power, the staining principle is generally considered to be a physical solution effect or adsorption, by solution effect to make the fat staining, the dye in the intracellular lipids solubility is greater than that of the original solvent, so the dye in the staining of the dye from the organic solvents into the lipids and make the fat staining.