Oligodendrocyte progenitors balance growth with self-repulsion to achieve homeostasis in the adult brain

In this study, the authors use time-lapse in vivo two-photon imaging to elucidate the dynamics of NG2 + gial cells in the cortex. They find that proliferation and migration of these cells is intimately linked to the loss of neighboring cells to cell death or differentiation. The adult CNS contains an abundant population of oligodendrocyte precursor cells (NG2 + cells) that generate oligodendrocytes and repair myelin, but how these ubiquitous progenitors maintain their density is unknown. We generated NG2-mEGFP mice and used in vivo two-photon imaging to study their dynamics in the adult brain. Time-lapse imaging revealed that NG2 + cells in the cortex were highly dynamic; they surveyed their local environment with motile filopodia, extended growth cones and continuously migrated. They maintained unique territories though self-avoidance, and NG2 + cell loss though death, differentiation or ablation triggered rapid migration and proliferation of adjacent cells to restore their density. NG2 + cells recruited to sites of focal CNS injury were similarly replaced by a proliferative burst surrounding the injury site. Thus, homeostatic control of NG2 + cell density through a balance of active growth and self-repulsion ensures that these progenitors are available to replace oligodendrocytes and participate in tissue repair.