(B) Immunofluorescence analysis of young WT and old APPPS1 slices cultured alone (young alone, old alone) or in co-culture (young co-culture, old co-culture) at 7 days in vitro (DIV) stained with propidium iodide (PI, red) reveals reduced cell viability in old brain slices compared to young.(C) Quantitative analysis of PI positive cells in young WT and old APPPS1 slices cultured alone (white and black bars, respectively) or in co-culture (grey and white squared bars, respectively) at 7 and 14 DIV. The values are expressed as percentages of PI positive cells from the total number of DAPI positive cells. The values represent mean ± SEM from 3 independent experiments, each experiment including 2 independent slice culture dishes (n.s.= not significant; ***P < 0.001). 