Fig. 4. Exclusion of integral membrane proteins from intraendosomal vesicles. Dictyostelium cells were treated with U18 for 2 hours. Cells were then collected and processed for freeze-fracture. Two typical multivesicular endosomes are shown in A and B, with some intra-endosomal vesicles visible on the fracture. C and D are enlarged images of the boxed regions in A and B, respectively. The density of particles is markedly reduced in the membrane of intraendosomal vesicles compared to the limiting endosomal membrane. Bars, 0.5 µm.