Figure 2. SLO-induced endosomes gradually acquire features of early and late endosomes.

Cells were treated as described in Figure 1. A) The average number of internal vesicles per endosome profile increases over time in both control and SLO-permeabilized cells, but is significantly higher after 15 and 30 min of SLO treatment, when compared to control cells. Results are expressed as the mean ± SEM. **p < 0.01, unpaired Student’s t-test. B) The percentage of early endosome-like vesicles (showing flat electron-dense coats and no more than five intraluminal vesicles) is higher between 1 and 15 min after SLO permeabilization, followed by a decrease after 30 min. C) The percentage of MVB-like vesicles (containing six or more intraluminal vesicles) is higher after SLO permeabilization between 1 and 15 min, but is similar to control cells after 30 min. D) Representative TEM images of vesicles containing BSA-gold (arrowheads) and showing morphological features of early endosomes, after 5 min in control and SLO-treated cells. Arrows indicate electron-dense areas on the limiting membrane reminiscent of flat clathrin coats characteristic of early endosomes. E) Representative TEM images of vesicles containing BSA-gold (arrows) showing the characteristic MVB/lysosome morphology, after 30 min in control and SLO-treated cells. The images show the numerous intraluminal vesicles and occasional multilamellar structures inside endosomes. Bar = 100 nm.