Microglia: origins, homeostasis, and roles in myelin repair.

Microglia are the resident macrophages of the central nervous system (CNS), implicated in developmental processes, homeostasis, and responses to injury. Derived from the yolk sac during development, microglia self-renew, self-regulate their numbers during homeostatic conditions, and show a robust proliferative capacity even in adulthood. Together with monocyte-derived macrophages (MDM), microglia coordinate the regeneration of CNS myelin around axons, termed remyelination. Gene expression analyses and experimental modelling have identified pro-remyelination roles for microglia/MDM in clearance of myelin debris, secretion of growth factors, and remodelling of the extracellular matrix. Further investigations into the molecular mechanisms controlling these regenerative functions will reveal novel therapeutic strategies to enhance remyelination, by harnessing the beneficial effects of the innate immune response to injury.