Fate of disseminated dead neurons in the cortical ischemic penumbra: ultrastructure indicating a novel scavenger mechanism of microglia and astrocytes.

BACKGROUND AND PURPOSE Because the mechanism for scavenging acidophilic electron-dense dead neurons disseminated among the neuritic networks of surviving neurons in the ischemic penumbra of the cerebral cortex is still obscure, we investigated the fate of them up to 24 weeks after the ischemic insult. METHODS Stroke-positive animals were selected according to their stroke index score during the first 10-minute left carotid occlusion done twice with a 5-hour interval. The animals were killed at various times after the second ischemic insult. Ultrathin sections including the second through fourth cortical layers were obtained from the neocortex coronally sectioned at the infundibular level in which the penumbra appeared and was observed by electron microscopy. We determined the percentages of resting, activated, and phagocytic microglia and astrocytes in the specimens obtained at various times postischemia. RESULTS The electron-dense neurons had been fragmented into granular pieces by invading astrocytic processes from the periphery of the dead neurons and only the central portion remained. These granular pieces were dispersed along the extracellular spaces in the neuropil. By 8 to 24 weeks, the central core portion became a tiny vesicular particle (3.5 to 5.5 mum in diameter) with a central dot. Microglia and astrocytes phagocytized these dispersed granular pieces. CONCLUSIONS We found a novel scavenger mechanism in the ischemic penumbra, one by which dead neurons were fragmented by invading small astrocytic processes and only a thinned-out core portion remained, which finally became a tiny vesicular particle. The dispersed fragmented pieces were phagocytized by the microglia and astrocytes late, at 8 to 24 weeks postischemia.