A light and electron microscopic study of the normal and the degenerating corticospinal tract in the rat

Motor and adjacent cortex of the right hemisphere was removed from 30 adult male rats and the animals were sacrificed at intervals between 1 and 360 days thereafter. Primary fixation was by perfusion with buffered 4% paraformaldehyde followed by preparation of blocks from the medulla and the mid‐cervical region for light and electron microscopy by conventional methods. The normal pyramidal tract contained approximately 100,000 myelinated fibers at the level of the medulla, with 89–99% of these fibers under 3.7 μ in diameter and 29–43% less than 1.0 μ. Shrinkage in the degenerating tract occurred gradually and progressively from the third day on and amounted to 70% in the medulla and 72% in the mid‐cervical region by 260 days. In the degenerating tract, throughout the entire period of degeneration, no mitotic figures or positively phagocytic cells were noted and there was no numerical increase or decrease in neuroglia or blood vessels. The rows of interfascicular oligodendrocytes were not altered in any manner except that adjacent rows became close together due to shrinkage. The degenerating fibers were replaced by astrocytic processes oriented in a longitudinal direction. The neuroglia displayed no observable role in the breakdown or removal of the degenerating fibers. Numerous alterations in the fine structure of axoplasm and myelin previously reported by others were confirmed.