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 a lipid composed of three molecules of fatty acids and one molecule of glycerol, which is neutral. Neutral fat is one of the ways of storing energy, and releases energy when oxidized. Under normal conditions, lipid droplets are almost invisible under the light microscope in cells other than adipocytes, and if a large number of lipid droplets appear in the cytoplasm of the cell, that is, lipoatrophy, which is commonly found in hepatocytes, cardiomyocytes, and epithelial cells of the renal tubules. Neutral fat staining is often used Sudan Ⅱ, Sudan Ⅲ, Sudan Ⅳ, Sudan black B, oil red O method, etc. The mechanism of Sudan dye lipid staining is generally considered to be purely physical lipid solubility and adsorption, Sudan dyes due to the solubility in lipids is greater than that in the solubility of organic solvents, so the dye will be stained from the dyeing solution to be stained lipids, so that lipids show the color of the dye.

Sudan Ⅲ staining solution is mainly used to show the fatty degeneration of tissues and organs and the abnormal deposition of lipids, which often occurs in the liver, kidney, heart and other parenchymal organs with fatty degeneration and the appearance of most of the neutral fat droplets in the cells; identification and diagnosis of tumors occurring in the adipose tissues and their nature, the specimen does not use the fixative containing ethanol (10% neutral formalin can be used for fixation, if necessary), and it does not use paraffin section, but frozen section or carbon-wax section is needed. Frozen sections or carbon wax sections are used.