Unusual aspects of inflammation in the nervous system: Wallerian degeneration

Wallerian degeneration in the PNS is accompanied by the rapid recruitment of monocytes, but monocytes do not invade CNS fibre tracts undergoing Wallerian degeneration. In recent years it has become apparent that the acute inflammatory response to cell degeneration in the CNS is unlike that in other tissues. We have been interested to learn why Wallerian degeneration does not provoke a typical inflammatory response. We investigated whether the vascular endothelial cells express adhesion molecules during Wallerian degeneration in PNS and CNS. We found that in the degenerating sciatic nerve there was upregulation of ICAM-1 and VCAM-1 expression on endothelial cells in the distal stump of the injured nerve as well as at the site of the lesion. However, in the degenerating optic nerve, the endothelium failed to upregulated these molecules in the distal stump of the nerve and ICAM-1 expression was only increased in the crush site. The lack of adhesion molecule expression on CNS endothelium molecules may be an explanation for the poor leukocyte recruitment during Wallerian degeneration in CNS when compared with PNS.