Delayed glial cell death following Wallerian degeneration in white matter tracts after spinal cord dorsal column cordotomy in adult rats

The devastating consequences of spinal cord injury (SCI) result primarily f rom damage to long tracts in the spinal white matter. To elucidate the seco ndary injury processes occurring after SCI, we investigated the relationshi p between apoptosis and Wallerian degeneration in spinal white matter tract s. In the rat spinal cord, the corticospinal tract (CST) and the dorsal asc ending tract (DAT) are separated from each other in the dorsal column and r elay information in opposite directions. A dorsal column cordotomy at the e ighth thoracic (T8) level simultaneously induces Wallerian degeneration in the CST caudal to and in the DAT rostral to the injury. Using the terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) method , we demonstrate that apoptosis occurred in areas of Wallerian degeneration in both tracts throughout the length of the cord segments studied (from T3 to T12), This delayed cell death, more apparent in the DAT, began at 7 day s after injury and peaked at 14 days for the DAT and 28 days for the CST, A lthough a few TUNEL+ cells, slightly above the noninjury control level, wer e found in intact areas of both tracts, statistically significant differenc es in the number of TUNEL+ cells were found between the intact and the lesi oned tract segments (CST, F < 0.01; DAT, F < 0,001). Within a particular sp inal segment, a mean number of 64 and 939 TUNEL+ cells in the degenerating CST and DAT, respectively, were estimated stereologically at 14 days postin jury, TUNEL+ cells in degenerating tracts outnumber their intact counterpar ts by 3.8:1 in the CST and 4.1:1 in the DAT, although a statistically signi ficant difference between the two was only found in the DAT at this time po int (P < 0,05), Finally, we demonstrated that oligodendrocytes, the myelin- forming cells in the central nervous system, constitute at least a portion of the cells undergoing apoptosis within areas of Wallerian degeneration. < (c)> 2001 Academic Press.