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Lipids are biomolecules having a diverse group of organic compounds that include fatty acids, waxes, phospholipids, glycolipids, and sterols. These compounds are insoluble or poorly soluble in water due to the presence of long hydrocarbon chains in their structures. They are easily soluble in organic solvents such as benzene, ether or chloroform. Lipids perform a diverse range of functions within the cells of living organisms, such as 1) acting as a storage form of metabolic fuel or energy, 2) being an integral component of the biological membrane, 3) signaling molecule, and 4) acting as a protective coat in bacteria, plants, insects, and invertebrates (protects from harsh environment). Because of its role in the living organisms, lipids and its derivatives have been an area of interest in many research laboratories to understand the various functional processes of living organisms.
This article will introduce some histochemical techniques that are being used in laboratories worldwide to understand the histochemistry of lipids. But, before we discuss the histochemical methods for lipid demonstration, it’s essential to learn the pretreatments that are done to maintain the chemical nature and structural integrity of the cells or tissues.
This includes freezing, sectioning, and fixation of the tissues. To demonstrate lipids, use fresh frozen tissues. After sectioning, the process of demonstration should be quickly performed to prevent the auto-oxidation of lipids that can change its physicochemical properties.
For fixation, use formaldehyde mixed with calcium chloride (calcium immobilizes the phospholipids). But not all types of lipids demonstration require fixation, as some of the lipids can be demonstrated without performing any fixation step. How different types of lipids differ in their fixation is explained below:
- For lipids like triglycerides, lecithin, sphingomyelin, unsaturated lipids, cholesterol esters, and plasmalogen phospholipids, after cryostat sectioning, no post-fixation is required.
- For lipids like free fatty acids, glycolipids, and phospholipids, staining of all lipids by the Sudan black method require post-f