(A) Healthy gut fed a control diet showing no sign of tissue damage. Images are maximum intensity projections. See also Figure 3—video 1. Scale bar is 20 µm. (B) Gut from fly fed SDS 0.2% overnight showing progressive tissue repair ex vivo mediated by progenitor cells proliferation and differentiation. As cells differentiate, nlsGFP expression is gradually lost. Area delimited by the yellow rectangle is enlarged in (B’). Images are maximum intensity projections. See also Figure 3—video 2. Scale bar is 20 µm. (B’) Enlargement of panel (B) showing an esg+ cell (white outline) differentiating and replacing dying enterocytes (yellow circles). As the cell differentiates, nlsGFP expression is lost and its nucleus grows larger. Images are maximum intensity projections. See also Figure 3—video 3. Scale bar is 10 µm. (C) Example of stem cell dividing upon tissue damage initiated by SDS feeding. Cell is marked by the expression of nlsGFP (yellow arrowhead). Note that in the 30 hr time-point the cell is in mitosis and so the nlsGFP signal is mostly lost due to nuclear envelope breakdown. After mitosis, the nuclear envelope is reformed and GFP re-accumulates in the nucleus of the daughter cells. Images are single z-slices. Scale bar is 5 µm. (D) Plots of nuclear area (red) and mean nuclear GFP intensity (green) from single progenitor cells from healthy guts. Note that both nuclear size and GFP signal, despite a small gradual dip caused by photobleaching, are stable for the duration of the imaging session. See also Figure 3—video 4, left column. (E) Plots of nuclear area (red) and mean nuclear GFP intensity (green) in single progenitor cells from SDS-damaged guts. Note that nuclear size increases, while GFP signals dims over time, suggesting EB to EC differentiation. See also Figure 3—video 4, right column. (F) Quantification of nuclear area of esg+ cells in control (green) and SDS-damaged (red) intestines cultured ex vivo. Each cell was measured at imaging start (i.e. 0 hr) and 48 hr later. In control guts, esg+ cells remain quiescent and have no change in nuclear size. Due to the SDS treatment, several progenitor cells have a large nucleus already at the time of dissection. Furthermore, most progenitor cells’ nuclear area significantly increases during the course of imaging (Two-way Anova and Šídák’s multiple comparisons test). (G) Ratio of nuclear GFP intensity for individual esg+ cells from control or SDS-damaged intestines cultured ex vivo. While cells in control intestines do not lose GFP expression (except for a minor loss due to photobleaching), several cells in the SDS-treated intestines show a significant loss of GFP intensity, suggesting their differentiation and change of cell identity (T test). (ns, not significant, *, p<0.05; ****, p<0.0001).