Ultrastructural studies of dorsal root axons regenerating through adult frog optic and sciatic nerves

Optic nerves Of adult fish and amphibia can successfully regenerate, in par t because their glial cells, unlike those of mammals; provide an environmen t permissive to regrowth. We altered the environment of regenerating dorsal root axone in the frog, Rana pipiens, by grafting segments of optic nerve to test the permissiveness of CNS glial cells to other sensory neurons. We compared these preparations to grafts of segments of sciatic nerve. After a llowing various times for survival, light and electron microscopy were used to evaluate the grafts. An agglomeration of astrocytes, tightly joined by desmosomes, initially formed in the center of the optic nerve grafts. Aroun d this grew regenerating dorsal root axons, accompanied by Schwann cells. A t early stages, some axons formed dilated terminal structures, which were n ot seen in peripheral nerve grafts, The appearance-of blood vessels within the graft and the dispersion of cells allowed larger numbers of axons to gr ow through the graft. By eight weeks, 48% of dorsal root sensory axons had grown through optic nerve grafts, compared to 84% for sciatic nerve. These results suggest that astrocytes from optic nerve are not inhibitory to, and provide a suitable substrate for, regrowing sensory neurons. (C) 1999 Wile y-Liss. Inc.