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, neutral, neutral fat is one of the ways of storing energy, and releases energy when oxidized, under normal circumstances in addition to fat cells, other cells in the optical microscope can hardly see the lipid droplets, if a large number of lipid droplets appear in the cytoplasm of the cell that is, lipoatrophy, commonly found in the hepatocytes, cardiac myocytes, It is common in hepatocytes, cardiomyocytes, renal tubular epithelial cells, etc. Neutral fat staining is often used Sudan Ⅱ, Sudan Ⅲ, Sudan Ⅳ, Sudan black B, oil red O method, Sudan dye lipid staining mechanism is generally believed to be purely physical lipid solubility and adsorption, Sudan dyes due to the solubility in lipids is greater than that in the organic solvent solubility, so the dye dye will be transferred from the dyeing solution to be stained lipids, so that lipids show the color of the dye solution.

Sudan Ⅳ alcohol saturated solution is mainly used to show the fatty degeneration of tissues and organs and the abnormal deposition of lipoid, often occurring in the liver, kidney, heart and other parenchymal organs, fatty degeneration, most of the neutral fat droplets appear in the cells; identification and diagnosis of tumors occurring in the adipose tissue and its nature, the specimen does not use the fixative containing ethanol (such as need to fixation can be used in 10% formalin), and does not use paraffin sectioning, it is necessary to use Frozen sections or carbon wax sections are used.