Axonal regeneration from injured dorsal roots into the spinal cord of adult rats

Injury to the central processes of primary sensory neurons produces less pr ofound changes in the expression of growth-related molecules and less vigor ous axonal regeneration than does injury to their peripheral processes. The left L4, L5, and L6 dorsal roots of deeply anaesthetized adult Sprague-Daw ley rats were severed and reanastomosed, and in some animals, the ipsilater al sciatic nerve was crushed to increase the expression of growth-related m olecules. After between 28 days and three months, the sciatic nerve of most animals was injected with transganglionic tracers and the animals were kil led 2-3 days later. Other animals were perfused for electron microscopy. Ve ry few regenerating axone entered the spinal cord of the rats without sciat ic nerve injuries. Labelled axons, however, were always found in the spinal cord of rats with sciatic nerve injuries. They often entered the cord arou nd blood vessels, ran rostrally within the superficial dorsal horn, and avo ided the degenerating white matter. The animals with a conditioning sciatic nerve crush had many more myelinated axons around the dorsal root entry zo ne (DREZ) and on the surface of the cord. Thus, a conditioning lesion of th eir peripheral processes increased the ability of the central processes of myelinated A fibres to regenerate, including to sites (such as lamina II) t hey do not normally occupy. Astrocytes, oligodendrocytes, and meningeal fib roblasts in and around the DREZ may have inhibited regeneration in that reg ion, but growth of the axons into the deep grey matter and degenerated dors al column was also blocked. (C) 1999 Wiley-Liss, Inc.