It is a well known fact that the injured PNS can successfully regenerate, on the other hand, the CNS such as retinal ganglion cell (RGC) axons of adult mammals is incapable of regeneration. After injury, RGC axons rapidly degenerate and most cell bodies go through the process of fell death, while dial cells at the site of injury undergo a series of responses which underlie the so-called dial scar formation. However, it has become apparent that RGCs do have an intrinsic capacity to regenerate which can be elicited by experimental replacement of the inhibitory dial environment with a permissive peripheral nerve milieu. Schwann cells are a major component of the PNS and play a role in regeneration, by producing various kinds of functional substances such as diffusible neurotrophic factors, extracellular matrix and cell adhesion molecules. RGC regeneration can be induced by cooperation of these substance;. The contact of RGC axons to Schwann cells based upon the structural and molecular linkages seems to be indispensable for the stable and successful regeneration. In addition to cell adhesion molecules such as NCAM and L1, data from our laboratory show that Schwann cells utilize short focal tight junctions to provide morphological stabilization of the contact with the elongating axon, as well as a small scale of gap junctions to facilitate traffic of substances between them. Moreover, our results show that modifications of functional properties in neighboring dial cells of optic nerve are induced by transplantation of Schwann cells. Astrocytes usually considered to form a dial scar guide the regenerating axons in cooperation with Schwann cells. A decrease of the oligodendrocyte marker O4 and migration of ED-I positive macrophages is observed within the optic nerve stump. Accordingly, RGC regeneration is not a simple phenomenon of axonal elongation on the Schwann cell membrane. but is based on direct and dynamic communication between the axon and the Schwann cell, and is also accompanied by changes and responses among the glial cell populations, which may be partly associated with the mechanisms of optic nerve regeneration. (C) 2000 Elsevier Science Ltd. All rights reserved.
