摘要
Myelin is an evolutionarily novel and important structure for the proper functioning of the vertebrate nervous system.In the central nervous system (CNS), the myelin sheath is elaborated by
Myelin is an evolutionarUy novel and important structure for the proper functioning of the vertebrate nervous system. In the central nervous system （CNS）, the myelin sheath is elaborated by oligodendrocytes, and is composed of multiple layers of specialized cell membrane wrapping around axons with periodic interruptions at the nodes of Ranvier. The major function of the myelin sheath is to provide ionic insulation to ensure rapid and saltatory conduction of electrical pulses along axons. In addition, myelin provides neurotrophic support for axons, as they become increasingly dependent on myelin-derived signals for survival. Despite the importance of myelin in the functioning of the CNS, oligodendrocytes are particularly susceptible to genetic and environmental perturbations, and demyelination can be triggered by many pathological conditions including traumatic injury, autoimmune disease （multiple sclerosis, MS）, heavy metal toxicity, and hypoxia. Loss of myelin sheaths in the CNS not only results in the compromised conduction of electrical signals, but also causes progressive degeneration of axons and ultimately neuronal loss. Spontaneous myelin repair from immature oligodendrocyte progenitor cells （OPCs） is not effective in demyelinating lesions, due either to the absence of stimulatory developmental signals that are no longer produced in the adult environment, or to the presence of inhibitory factors peculiar to this environment.
