Reactive Microglia and Ischemic Injury

This chapter elaborates the role of reactive microglia during ischemic injury. Microglia is a class of mononuclear phagocytes intrinsic to the central nervous system. These cells are the principal immune effector elements of the brain and are associated with diverse clinical problems including cerebrovascular disease. Presently, most investigators agree that microglia arise during early embryonic development from common stem cell populations which enter the neuroectoderm and later proliferate in situ . Microglia during the perinatal period are found in ameboid forms that eventually develop long, thin projections reaching up to several hundred microns in length. Rat neocortex damaged by occlusion of the middle cerebral artery shows well-defined regions of cortical infarction identified both by gross examination and by the inability to reduce tetrazolium. Significant numbers of reactive mononuclear phagocytes (both microglia and invading macrophage) appear within the infarct and along neighboring marginal areas by 2 days postischemia. This inflammatory cell population continues to increase with a peak at about 7 days. A second animal model suggests that neurotoxic microglia actually influence recovery of function after ischemic injury.