Fig. 4. Bafilomycin A1 induces fragmentation of early endosomes in control cells. To determine the effect of the proton pump inhibitor bafilomycin A1 (Baf) on endocytic organelles, macrophages were incubated in the presence of 16 nm BSA-gold particles for the indicated times in order to form endosomes of various ages. Cells were then treated for 30 minutes with 500 nM of bafilomycin A1 (right panels) or DMSO as a control (left panels), and the resulting morphology of the endocytic structures was observed at the electron microscope. As shown in the top right panel, only early endosomes have been disrupted by the bafilomycin A1 treatment. Bar, 550 nm (bottom-right panel) or 500 nm (all other panels).