Requirements for Schwann cell migration within CNS environments: a viewpoint.

Schwann cells are able to migrate into the CNS and myelinate CNS axons in a number of developmental and pathological situations. Morphological studies based on normal, mutant and experimentally-lesioned tissue have indicated that Schwann cells are only able to enter the CNS when the integrity of the astrocytic glia limitans is disrupted. The significance and subtlety of the interactions between Schwann cells and astrocytes have been further explored by glial cell transplantation studies. These studies support in vitro observations on Schwann cell behaviour in highlighting the importance of extracellular matrix for both migration and myelin sheath formation. The failure of Schwann cells to intermix with astrocytes is an important aspect of glial cell biology which will have a bearing on efforts to remyelinate demyelinated axons by Schwann cell-transplantation.