Glial response to dorsal root lesion in the irradiated spinal cord

Exposure of the rat lumbar spinal cord to X-rays during the early postnatal period results in a marked reduction in the glial populations within the irradiated region. The present study was undertaken to determine what effects this reduction of glia, particularly astrocytes, has on the pattern and characteristics of the scar formation that follows root injury in the normal spinal cord. Morphological assessments 60 days following injury of the right L4 dorsal root revealed a distinct difference in the extent of the astrocyte response between the irradiated and the nonirradiated rats. In the nonirradiated animals, a thick astrocytic scar composed of multiple layers of astrocyte processes formed over the dorsal horn and adjacent portions of the dorsal surface of the cord. This astrocytic response was not confined to the surface of the spinal cord but extended also into the root, i.e., into regions normally considered as PNS. In irradiated rats, the astrocytes did not form a thick scar nor did they extend into the injured root. Instead, they formed a glia limitans and were at most only one or two layers thick over the region of cord comparable to that occupied by the thick astrocytic scar in the nonirradiated rats. Mechanisms involved in the glial response of the irradiated spinal cord to dorsal root injury are discussed, particularly with regard to the possible positive effect that this reduction in scar formation may have on regrowth of injured dorsal root axons into the spinal cord environment.