Remyelination in the central nervous system and the peripheral nervous system.

Remyelination in the PNS is efficient, quick, and consistently found in all demyelinating diseases. Schwann cell proliferation in response to demyelination is rapid and prolific, and the numbers of Schwann cells generated are in excess of those required for adequate remyelination. This cell poses no limit to regenerative potential, and it can divide and remyelinate following numerous repetitive episodes. The Schwann cell generally has easy access to the denuded axons. The limiting factor to remyelination is the persistence of the demyelinating agent, be it directed at the myelin or secondarily through axonal disease. CNS remyelination differs in some respects. Although it has now become clear that it may occur in a variety of clinical and experimental situations, it is slower and often less complete than in the PNS. The limiting factors here include the nature of the demyelinating process, the regenerative potential of the oligodendrocyte, and the accessibility of the latter to the demyelinated axon. It is apparent that the oligodendrocyte is capable of some proliferation, but the time frame in which this can occur is more constrained than for the Schwann cell. Nevertheless the demonstration that the process occurs leads to increasing hope that clinically useful remyelination may be encouraged in the future either by more carefully controlling the extent of demyelination or by finding ways of stimulating oligodendrocyte proliferation and access to the axon.