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

Injury to the central processes of primary sensory neurons produces less profound changes in the expression of growth-related molecules and less vigorous axonal regeneration than does injury to their peripheral processes. The left L4, L5, and L6 dorsal roots of deeply anaesthetized adult Sprague-Dawley rats were severed and reanastomosed, and in some animals, the ipsilateral sciatic nerve was crushed to increase the expression of growth-related molecules. After between 28 days and three months, the sciatic nerve of most animals was injected with transganglionic tracers and the animals were killed 2-3 days later. Other animals were perfused for electron microscopy. Very few regenerating axons entered the spinal cord of the rats without sciatic nerve injuries. Labelled axons, however, were always found in the spinal cord of rats with sciatic nerve injuries. They often entered the cord around blood vessels, ran rostrally within the superficial dorsal horn, and avoided the degenerating white matter. The animals with a conditioning sciatic nerve crush had many more myelinated axons around the dorsal root entry zone (DREZ) and on the surface of the cord. Thus, a conditioning lesion of their peripheral processes increased the ability of the central processes of myelinated A fibres to regenerate, including to sites (such as lamina II) they do not normally occupy. Astrocytes, oligodendrocytes, and meningeal fibroblasts in and around the DREZ may have inhibited regeneration in that region, but growth of the axons into the deep grey matter and degenerated dorsal column was also blocked.