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, etc. The mechanism of Sudan dye lipid staining is generally considered to be purely physical lipolysis and adsorption, Sudan dyes in lipids due to the solubility is greater than that of the organic solvent solubility, so the dye will be transferred from the dyeing solution to the stained lipids, so that lipids show the color of the dyeing solution.

Neutral fat staining solution (Sudan IV staining method) is mainly used to show fatty degeneration of tissues and organs and abnormal deposition of lipids, which often occurs in liver, kidney, heart and other parenchymal organs, with the appearance of most of the neutral fat droplets in the cells; identification and diagnosis of tumors occurring in adipose tissues and their nature, the specimen does not use the fixative containing ethanol (if fixation is required, 10% formalin may be used), nor does it use Paraffin sections, frozen sections or carbon wax sections are used.