In the central nervous system oligodendrocytes form the insulating myelin. Such mechanisms may include generation of new neurons, glia, axons, myelin, or synapses. Saltatory conduction provides one advantage over conduction that occurs along an axon without myelin sheaths. The axon is insulated by a myelin sheath. Remyelination looks different from developmental myelination in the structure of the myelin formed. The process of generating myelin is called myelination or myelinogenesis. Research to repair damaged myelin sheaths is ongoing. Axonal degeneration is followed by degradation of the myelin sheath and infiltration by macrophages. Hagfishes and lampreys lack a thymus, spleen, myelin and sympathetic chain ganglia. The recruitment of macrophages helps improve the clearing rate of myelin debris. Dysmyelination is the abnormal formation of the myelin sheath. MBP was initially sequenced in 1971 after isolation from bovine myelin membranes. The primary lipid of myelin is a glycolipid called galactocerebroside. Plasmalogen is the most abundant phospholipid in myelin. PPD is also used as a histological stain for lipids such as myelin.