Proliferating oligodendrocytes are present in both active and chronic inactive multiple sclerosis plaques

The proliferation marker Ki‐67 labels cell nuclei in the G1, S, M, and G2 phases of the cell cycle. We used Ki‐67 immunohistochemistry to quantify proliferating glial cells in brain tissue sections from twenty‐four patients, comprised of multiple sclerosis, normal brains, and other neurological disease controls. Glial proliferation was greatly increased in MS lesions when compared with control brain white matter. Both actively demyelinating/early remyelinating plaques and chronic inactive plaques of long standing often displayed large numbers of glial cells in the proliferative cycle. The bulk of these proliferating cells were of oligodendroglial lineage in the MS plaques. Ki‐67 positive macrophage/microglial lineage cells were largely restricted to acute lesions. The finding of increased numbers of proliferating oligodendroglia in most MS plaques, regardless of disease duration or activity state, indicates that the MS brain is capable of recruiting unexpectedly large numbers of new oligodendrocytes over long periods of time. The factors within the MS plaque microenvironment that provoke new oligodendrocyte generation and their subsequent loss still need to be identified. J. Neurosci. Res. 65:308–317, 2001. © 2001 Wiley‐Liss, Inc.