Figure 5(A) Below the schematics of the experiment are representative confocal images of donor neuron-like cells (CAD cells labelled with Cell Tracker Green (CTG)) and acceptor primary neurons after 24 hours in co-culture. The upper panels show control conditions (non-challenged CAD cells, CTL), and the bottom panels show donor CTG-CADs that were loaded with K18-ATTO 594 fibrils prior to co-culture with primary cortical neurons. The images are representative Z-stack projections covering the whole cell body of donor and acceptor cells. In the merged images, white show neurons labelled with MAP-2, green are CTG-labelled donor CAD cells, red corresponds to K18-ATTO 594 puncta and nuclei are stained in blue. The single channel pictures are gray scale images. Yellow arrowheads point to K18 puncta inside acceptor cells. MAP-2 and K18-ATTO 594 panels are threefold enlargements of the boxed regions in the merged pictures. On the right are the orthogonal views of the same regions covering 14 slices. Scale bars are 20 μm in the merge panels, and 5 μm in the insets and orthogonal views.(B) Below the schematics of the experiment are representative confocal images of acceptor primary neurons (labelled by MAP-2) cultured with donor cells, consisting in RD-YFP SH cells, first K18-challenged for 2 days, therefore expressing RD-YFP aggregates (upper panels) or with supernatant (SN) of donor cells (lower panels) after 24 hours. The single channel pictures are gray scale images. Yellow arrowheads point to Tau RD-YFP puncta inside acceptor cells, red arrowheads point to aggregates inside donor cells. Scale bars are 20 μm.(C) Below the schematics of the co-culture experiment are representative confocal images of neuron-like cells (CTG-CADs growing on top of organotypic slices (n=6), in which the astrocytes were labelled with GFAP. The upper panels show control CTG-CADs and the middle panels show donor CTG-CADs loaded with K18-ATTO 594 fibrils, co-cultured with an acceptor hippocampal slice. The images are representative Z-stack projections covering the whole cell body of donor cells. In the merged images, white are acceptor astrocytes (GFAP positive), green are donor CTG-CADs, red are K18-ATTO 594 puncta and nuclei are stained in blue. Insets are threefold enlargements of the boxed region in the merged picture. The orthogonal views of the bottom panels show 25 slices of K18-ATTO 594 fibrils in donor cells (1), or transfer of K18-ATTO 594 fibrils from donor CTG-CADs to hippocampal astrocytes of the organotypic slice (2 to 5). Yellow arrowheads point to Tau puncta inside acceptor astrocytes. Orthogonal views show each of the five K18-ATTO 594 positive-puncta contained inside acceptor cells as indicated in the inset of the middle panel. Scale bars are 100 μm in the merged panel and 10 μm in the insets.(C) Below the schematics of the co-culture experiment are representative confocal images of neuron-like cells (CTG-CADs as in A) growing on top of organotypic slices (n=6), in which the astrocytes were labelled with The upper panels show control CTG-CADs and the middle panels show donor CTG-CADs loaded with K18-ATTO 594 fibrils, co-cultured with an acceptor hippocampal slice. The images are representative Z-stack projections covering the whole cell body of donor cells. In the merged images, white are acceptor astrocytes , green are donor CTG-CADs, red are K18-ATTO 594 puncta and nuclei are stained in blue. Insets are threefold enlargements of the boxed region in the merged picture. of K18-ATTO 594 fibrils in donor cells (1), or transfer of K18-ATTO 594 fibrils from donor CTG-CADs to hippocampal astrocytes of the organotypic slice (2 to 5). Yellow arrowheads point to Tau puncta inside acceptor astrocytes. Orthogonal views show each of the five K18-ATTO 594 positive-puncta contained inside acceptor cells as indicated in the inset of the middle panel. Scale bars are 100 μm in the merged panel and 10 μm in the insets. the neurons in the slices were labelled with the dendrite marker MAP-2 and the axonal marker β-III-tubulin (white), and the orthogonal views show 26 slices.