(C) Immunofluorescence analysis of the old APPPS1 tissue cultured alone or in co-culture with young WT brain slices (14 DIV) and immunostained with CD68 (red), Ki67 (green) and M3.2 (blue) Scale bar: 50 µm.(D) Quantitative analysis of Ki67 and CD68 double positive microglial cells in the old APPPS1 tissue cultured alone or in co-culture with young brain slices (14 DIV) reveals increased microglial proliferation upon co-culturing of old and young brain slices. The values are expressed as percentages of CD68 and Ki67 double positive microglial cells from the total number of CD68 positive cells and represent mean ± SEM from 3 independent experiments, each experiment including 2 independent slice culture dishes (**P < 0.01). 