Facilitatory and inhibitory effects of glial cells and extracellular matrix in axonal regeneration.

Recent studies have shown that Schwann cells stimulate nerve regeneration by producing nerve growth factor in response to macrophage activation as well as by mediating growth through cell-surface and extracellular matrix adhesion molecules. Neurons sprouting in the central nervous system, however, encounter a hostile environment including mature oligodendrocytes with contact inhibitors of growth cone motility, masses of proliferating astrocytes with surface properties that may block regeneration, and an extracellular environment relatively rich in chondroitin sulfate and tenascin forming a matrix less permissive for regeneration than that found in the peripheral nervous system. In addition, as neurons mature, integrins and cell adhesion molecules are reduced in number (transcriptionally) or in efficacy (post-translationally).