Cell cycle dynamics of NG2 cells in the postnatal and ageing brain

Abstract

Oligodendrocyte precursors (OLPs or ‘NG2 cells’) are abundant in the adult mouse brain, where they continue to proliferate and generate new myelinating oligodendrocytes. By cumulative BrdU labelling, we estimated the cell cycle timeTCand the proportion of NG2 cells that is actively cycling (the growth fraction) at ~ postnatal day 6 (P6), P60, P240 and P540. In the corpus callosum,TCincreased from <2 days at P6 to ~9 days at P60 to ~70 days at P240 and P540. In the cortex,TCincreased from ~2 days to >150 days over the same period. The growth fraction remained relatively invariant at ~50% in both cortex and corpus callosum – that is, similar numbers of mitotically active and inactive NG2 cells co-exist at all ages. Our data imply that a stable population of quiescent NG2 cells appears before the end of the first postnatal week and persists throughout life. The mitotically active population acts as a source of new oligodendrocytes during adulthood, while the biological significance of the quiescent population remains to be determined. We found that the mitotic status of adult NG2 cells is unrelated to their developmental site of origin in the ventral or dorsal telencephalon. We also report that new oligodendrocytes continue to be formed at a slow rate from NG2 cells even after P240 (8 months of age).