The fate of proliferating cells in the injured adult spinal cord

Over the last several decades, determining the extent to which endogenous cells within the spinal cord can replace neurons and glia that are lost following spinal cord injury (SCI) has generated increasing interest. While it is known that neurogenesis occurs regularly in certain regions of the adult brain, this process has not been identified within the adult spinal cord. Interestingly, this is likely to be a factor of the spinal cord microenvironment because cells isolated from adult spinal cords can generate neurons, oligodendrocytes, and astrocytes both in vitro and when transplanted into a neurogenic region of the brain [1]. What clearly does occur after SCI is marked gliogenesis [2-5]. SCI leads to significant and protracted proliferation of endogenous cells, which contribute to the replacement of oligodendrocytes and astrocytes. Indeed, the oligodendrocytes formed along the lesion borders significantly outnumber those found in na？ve tissue, revealing that the spontaneous oligogenic potential of the adult spinal cord is quite robust [2].The source of the new glia after SCI has been more difficult to nail down. In the uninjured adult spinal cord, there are two major populations of dividing cells: the slowly dividing ependymal cells surrounding the central canal and the NG2+ glial progenitors distributed through-out the gray and white matter. Much work has been done to track the fate of NG2+ progenitors after SCI, and reports suggest that in vivo they contribute to robust oligodendrocyte replacement and potentially make some new astrocytes [2,3,5]. Studies have also used cell lineage mapping or specific markers to track the fate of dividing ependymal cells after injury; these studies suggest that ependymal cells proliferate after SCI, migrate away from the central canal, and differentiate into new astrocytes [6-9]. A final possible source of new cells after SCI is mature astrocytes, which divide after injury and thereby increase overall astrocyte numbers.Most of