The origin of lipid phagocytes in the central nervous system: I. The intrinsic microglia

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Abstract
The potential for the transformation of the normal microglial cell to a lipid phagocyte was studied by light and electron microscopy in the brains of rodents and by light microscopy only in primates. All were subjected to some form of hypoxia‐ischemia and the microglial response was examined in regions of selective neuronal destruction (SND) so that infarction was deliberately excluded. In vivo perfusion‐fixation was employed in all animals and light microscopic examination was carried out on paraffin‐ and sometimes celloidin‐embedded material. Semithin plastic sections from several regions of the rodent brains were used for light microscopy but ultrastructural studies were confined to the hippocampus. In all animals the microglia were activated and transformed into rod cells exhibiting phagocytic properties but only a minority gave rise to lipid phagocytes. Blood vessels were normal in all animals and no hematogenous elements were identified in the parenchyma. As neuronal ghosts could be identified for up to 3 weeks it was concluded that the capacity of the microglia for phagocytic activity was limited.