A: Transverse sections of spinal cords at the contusion epicenter and 1.5 or 3.5 mm rostral (+) or caudal (-) to the epicenter (o) from an unirradiated rat (upper micrographs) and a rat x-irradiated at 4 mm rostral to the epicenter (lower micrographs) at 6 weeks following contusion injury as indicated. The sections were stained with Luxol fast blue for myelin and counterstained with cresyl violet, as described in the Methods section. Typically, the central cavity and peripheral rim of spared tissue appeared histologically similar in x-irradiated and untreated spinal cords, although a greater extent of sparing of spinal cord white matter is apparent at and adjacent to the epicenter following contusion injury of the x-irradiated compared to the unirradiated spinal cord. Areas of white matter and cavitation are indicated, respectively, by striped white and dotted black lines with gray matter area remaining. The calibration bar equals 1 mm. B: The effects of varying the anatomical rostral-caudal position of the x-irradiation beam relative to the contusion site on sparing of spinal cord tissue. Values are means of determinations of the cross-sectional area of spared spinal cord white matter (in myelin-stained transverse sections) as a function of distance from the contusion epicenter. The rats received a contusion injury and radiation treatment, as described for Fig. 1A. Significant increases in spared spinal cord white matter (in a region several millimeters in length, extending rostrally and caudally from the epicenter) due to x-irradiation at the epicenter ( †) and 4 mm, but not 8 mm, rostral (*) and 4 mm caudal ( ‡) to the contusion epicenter were observed at 6 weeks postirradiation compared to rats receiving x-irradiation of the thorax (p < 0.05 [least significant difference]). (continued)→