Proliferation of NG2-positive cells and altered oligodendrocyte numbers inthe contused rat spinal cord

Given the numerous reparative roles glia may play after spinal cord injury (SCI), glial proliferation and cell number were examined in a model of trau matic SCI. Emphasis was placed on analysis of oligodendrocytes and NG2-posi tive (NG2+) cells, an endogenous cell population that may be involved in ol igodendrocyte replacement. Overall, proliferation (assessed by bromodeoxyur idine incorporation) was markedly elevated during the first 2 weeks after i njury and declined thereafter; a large portion of these dividing cells like ly consisted of microglia-macrophages. Although the total number of NG2+ ce lls in the epicenter was reduced by half, we noted protracted proliferation in surviving NG2+ cells, with values sevenfold greater than in uninjured c ontrols. Elevated proliferation of NG2+ cells persisted throughout the firs t 4 weeks after injury. However, the absolute number of NG2+ cells was not increased over controls, suggesting that the daughter cells either did not survive or they differentiated into other cell types. As expected, oligoden drocyte numbers were drastically altered after SCI. By 7 d after injury, th e number of oligodendrocytes at the impact site was reduced by 93%. Despite ongoing tissue loss, the number of oligodendrocytes in spared tissue rose threefold at 14 d after injury. Although the function of NG2+ cells within the spinal cord is not completely understood, several studies suggest that they may differentiate into oligodendrocytes. Thus, proliferating NG2+ cell s may contribute to the increased oligodendrocyte number observed at 2 week s after injury. Future studies are required, however, to definitively deter mine the role NG2+ cells play in oligodendrocyte genesis, remyelination, an d other post-injury events.