Penetration of grafted astrocytic scars by regenerating optic nerve axons in Xenopus tadpoles.

The following study was performed in order to determine the effect of a dense glial scar upon the outgrowth of neurites in the regenerating optic nerves of Xenopus tadpoles. Glial scars, primarily comprised of hypertrophic astrocytes, were formed in the optic nerves of postmetamorphic, juvenile Xenopus by unilateral enucleation. After 25--40 days, segments of glial scar tissue were then grafted near the cut retinal stumps of the transected optic nerves in stage 54--56 tadpoles. Within 7--10 days bundles of unmyelinated axons were seen among the cytoplasmic processes of the implanted astrocytes, and many of the fibers had traversed the entire extent of the graft by 7--10 days. The results indicate that in this regenerating system an extremely dense glial scar, formed by mature, hypertrophic astrocytes, does not represent a major obstacle to axonal outgrowth. These observations are discussed in relation to the problem of glial scarring and the general failure of regeneration in the mammalian central nervous system.