Axonal and glial responses to a mid-thoracic spinal cord hemisection in the Macaca fascicularis monkey.

A comprehensive understanding of the pathology of spinal cord injury (SCI) in non-human primates may facilitate greatly the development of new strategies to promote recovery in humans with SCI. Relatively few studies, however, have been conducted to systemically examine pathological changes in the monkey, a non-human primate, after SCI. We report axonal, glial, and fibrotic responses in the spinal cord of monkey Macaca fascicularis after a thoracic (T) 8-9 lateral hemisection. We examined these changes at three regions--i.e., the lesion epicenter, the peri-lesion area, and the lateral white matter of the intact, contralateral hemicord at 7 (subacute) and 30 (early chronic) days post-injury. The lateral hemisection resulted in a marked axon and myelin loss, along with tissue loss, at the lesion epicenter at both time points. Unexpectedly, axonal loss and myelin degeneration, along with reactive gliosis and microglia/macrophages activation, were also observed in the contralateral spared hemicord, indicating a spread of the initial damage to the contralateral side. In addition, activated microglia/macrophages were found both within the injury epicenter and the peri-lesion area, indicating that they participate in injury-induced immune responses that may exacerbate the secondary damage. In contrast to rodents, substantial reactive astrocytic responses at the lesion border were not observed in the monkey. Conversely, a deposit of robust fibrotic scar was observed at the injury epicenter, which filled the space originally created by the hemisection. Thus, understanding the pathology of monkey SCI may provide clinically relevant information in designing repair strategies targeting specific problems associated with human SCIs.

Glial response and myelin clearance in areas of wallerian degeneration after spinal cord hemisection in the monkey Macaca fascicularis.

Spinal cord injury (SCI) in mammals not only damages the focal area, but also leads to wallerian degeneration (WD) of axons and myelin distal to the injury. In the present study, we investigated cellular responses within areas of WD of a sensory pathway, the fasciculus gracilis, after a T8-9 lateral spinal hemisection in the adult monkey Macaca fascicularis. Spinal cord segments rostral and caudal to the injury at two clinically-relevant time points, 1 week and 4 weeks post-SCI, representing subacute and chronic stages, respectively, were examined. We observed marked axon degeneration in the areas of WD at the subacute stage, and minimal axonal neurofilament staining at the chronic stage. At the ultrastructural level, however, many degenerating axonal profiles remained at the chronic stage. Myelin breakdown was a much-delayed process. A large number of residual myelin sheaths was observed at the chronic stage. In contrast to rodents, a substantial astrogliotic response was not found in the WD regions up to 4 weeks post-injury. Microglia activation was evident in the WD areas at the subacute stage, and was enhanced at the chronic stage. However, the lack of round reactive microglia/macrophages in these regions suggests that microglial activation was either delayed or incomplete. Thus it appears that many pathological characteristics of WD in monkeys are much delayed compared to those in rodents, but are similar to those in humans. Our results suggest that non-human primate SCI models are useful for evaluating repair strategies before they are translated to clinical trials of human SCI.