Glial and axonal responses in areas of Wallerian degeneration of the corticospinal and dorsal ascending tracts after spinal cord dorsal funiculotomy

Wallerian degeneration (WD), composed of the breakdown and phagocytosis of damaged axons and their myelin sheaths distal to the injury, is a major sequela of spinal cord injury (SCI). To understand the microenvironment within WD that may affect repair following SCI, we investigated the fate of major glial types and axons in this region following acute (1 h), subacute (10 days), and chronic (30 days) dorsal funiculotomy at the eighth thoracic (T8) level. This lesion induces a confined WD in two distinct functional pathways, that is, the corticospinal tract (CST) and dorsal ascending tract (DAT) in opposite directions. Here we report that astrocytes, reactive microglia and macrophages were all significantly increased in areas of WD in both the CST and DAT at subacute and chronic stages compared to the sham-operated or acute stage. While the level of GFAP(+) astrocytes remained stable after the subacute stage, the number of OX-42(+) microglia and ED-1(+) macrophages markedly decreased at the chronic stage. Interestingly, a mild but significant increase in ED-1(+) macrophages was also found in the intact fiber tracts 3 mm proximal to the injury at the chronic stage, coinciding with axonal dieback observed at that level. Axons distal to the injury experienced a continued and prolonged degeneration in both fiber tracts. Finally, although a significant decrease of Olig2(+) oligodendrocyte lineage (OL) cells was found in areas of WD, the presence of these cells at the chronic stage indicates that they are available for endogenous repair. Taken together, our data have provided spatiotemporal evidence for the dynamic pathogenic changes of major cellular components in areas of WD remote to an SCI. Information obtained in this study should be useful for designing experiments aimed at modifying this region to accommodate endogenous or exogenous repair following SCI.