Myelin (Myelin Sheath) is a membrane wrapped around the axons of nerve cells, i.e. myelin sheath consists of myelin cells and cell membranes, the plasma membrane of the nerve membrane cells is a multilayered lipid bilayer structure formed by helical winding along the axis of the axon, and the myelin sheaths are covered with Langfei's knots, which can make nerve impulses jumping and transmitting. Myelin staining has some significance in pathological diagnosis, the pathological changes of myelin sheath are divided into early, middle and late stages, the coloring is darker in the early stage; the myelin sheath in the middle stage of the lesion is degenerated to form lipid droplets, which can be shown by lipid staining, and in the late stage, it is completely collapsed and cleared by phagocytes, so there is no longer a positive result of myelin sheath.

Many diseases can cause changes in myelin, myelin staining solution (metachromatic acid 2R method) can show whether the myelin sheath is intact, degeneration, necrosis and repair under pathological conditions, which is meaningful to the pathological diagnosis and study of nerve tissue, myelin sheaths are dark green, demyelinated fibers are not colored.