Schwann cells are excluded from the CNS during development by
the glial limiting membrane, an area of astrocytic specialisation
present at the nerve root transitional zone, and at blood vessels in
the neuropil. This barrier, however, can be disrupted and, with the
highly migratory nature of Schwann cells, can result in their invasion
and myelination of the CNS in many pathological situations. In this
paper we demonstrate that this occurs in a number of myelin mutants,
including the myelin deficient (md) and taiep rats
and the canine shaking (sh) pup. While it is still relatively
uncommon in the rodent mutants, the sh pup shows extensive
Schwann cell invasion along the neuraxis. This invasion involves the
spinal cord, brain stem, and cerebellum and increases in amount and
distribution with age. In situ hybridisation studies using a
P0 riboprobe suggest that the likely origin of these cells
in the sh pup is the nerve roots, primarily the dorsal roots.
Paradoxically, Schwann cell myelination of the CNS increases with time
in the sh pup despite a marked, progressive gliosis involving
the glia limitans and neuropil. Thus the mechanism by which these cells
migrate into the CNS through the gliosed nerve root transitional zone
or from vasa nervorum remains unknown. Extensive Schwann cell CNS
myelination may have therapeutic significance in human myelin
disease.