Mononuclear phagocytes were first seen 7 h after ischemia by Schröder (1983; see also Wisniewski 1961) in a perivascular position (Fig. 14.12a, b), diffusely distributed (Fig. 14.12c), focally aggregated or perineuronal (Fig. 14.12d). Mononuclear phagocytes - in the first stage - are seen in a perineuronal location (Fig. 14.12d, 14.13), and later after a neuronal loss -in the second stage - these phagocytes replace the neurons (Figs. 14.14, 14.15). These cell types will be transformed into an ameboid cell type after 14 h, and into a gitter cell after 32-48 h (Escourolle and Poirier 1973; Schröder 1983). They are accumulated peri-neuronally in the central cortex and/or basal ganglia and are obviously of hematogenous origin (Schroe-ter et al. 2001). The earliest microglial response was observed in the rat hippocampal formation after 20 min of reperfusion following global cerebral ischemia in the four-vessel occlusion model (Morioka et al. 1991). In another study, a biphasic microglial reaction was detected in the hippocampus evoked by cerebral ischemia or kainic acid administration (Jorgensen et al. 1993). An early, widespread activation of microglial cells could be seen in all hippocampal regions on the first day after ischemia. This reaction subsided during the next few days in the areas devoid of neuronal degeneration, while it progressed into a protracted, lesion-specific reaction in the areas where neuronal degeneration occurred (Freund and Magloczky 1993; Jorgensen et al. 1993).
Abraham and Lazar (2000) could demonstrate an activation of resting microglia within 20 min after ischemia, but no increase of the number of cells. Beschorner et al. (2002) could show an upregulation of CD14 antigen in mononuclear phagocytes in cases of focal cerebral infarction within 1-2.5 days that remained elevated until late stages. This early CD14 expression is suggested to be an essential part of CD14 in the acute inflammatory response following stroke. As mentioned above, Lin et al. (1998) induced
global ischemia in rats and observed reactive microglia in the striatum which peaked at 1 week, as in the CA1 hippocampus.
Single hemosiderin- containing macrophages may be demonstrated after 3 days of survival time (Hammes 1944; Oehmichen and Raff 1980). Lipid-con-taining foam cells are demonstrable after 3-8 days (Baggenstoss et al. 1943; Strassmann 1945), while myelin-containing macrophages are described after a survival period of 44 h (Schröder 1983).
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