Remyelination within the CNS: do schwann cells pave the way for oligodendrocytes?

Schwann cells that have myelinated the CNS can be replaced by myelinating oligodendrocytes. It is unclear, however, why oligodendrocyte remyelination would occur for axons that are already myelinated. The Schwann cells might signal their own replacement by oligodendrocytes, but more probably a third player, the reactive astrocyte, is essential to this phenomenon. We propose that as long as reactive astrocytes do not form fibrous gliosis, they are beneficial to oligodendrocyte remyelination. Unknown is whether reactive astrocytes induce oligodendrocyte progenitor (NG2 immunopositive cells) cells to differentiate, a phenomenon that is absent in multiple sclerosis. So what role do Schwann cells play in CNS remyelination? They appear to serve to protect central axons and might coincidentally prevent reactive astrocytes from laying down scar tissue that impedes oligodendrocyte remyelination.