Lipid (Lipid) is a general term for neutral fats, lipids and their derivatives, which share the physical property of being insoluble in water and soluble in organic solvents. There are two main types of fat in the human body: 1, storage fat, such as neutral fat, mainly distributed in the subcutaneous, kidney, pancreas and other parts of the body; 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 circumstances, except for adipocytes, other cells can hardly see the lipid droplets under the light microscope, 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 traditional method uses Sudan dyes, recently found that the azo dye oil red O is more suitable for fat staining, oil red O is a very strong lipid solvent and lipid staining agent, it is easier to be combined with triglyceride in the form of small lipid droplets, and the binding power with phospholipids is a little bit poorer.

Neutral fat staining solution (Sudan black B staining method) 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 is not fixed with fixative containing ethanol (if fixation is needed, 10% formalin can be used), or Paraffin section, frozen section or carbon wax section should be used; lipoma, liposarcoma, follicular meningioma, adrenocortical adenoma, etc. are positive after Sudan black staining; renal clear cell carcinoma, ovarian fibroma, etc. are negative after Sudan black staining.