Fig. 9. Transferrin-coated colloidal gold is targeted to MVBs in K562 cells. Stably transfected K562 cells overexpressing GFP-Rab11 wt (A,B) or Rab11 S25N (C) were incubated with 10 nm Tf-coated colloidal gold particles (Tf-gold) for 45 minutes at 37°C and processed for transmission electron microscopy following standard techniques (see the Materials and Methods). (A) A typical multivesicular body (MVB) that contains vesicles labeled with Tf-gold is shown. A well defined membrane can often be seen surrounding these vesicles but since exosomes are small the large degree of curvature of this membrane might not be well cross-sectioned and consequently would not always be discernable. Magnification: 100,000x. (B) Two MVBs are shown, one of them has already fused with the plasma membrane, and Tf-receptor-containing exosomes are shed from the cell. Magnification: 100,000x. (C,D) Sections of a Rab11-S25N-transfected cell showing MVBs containing vesicles labeled with Tf-gold are depicted. Magnification: 110,000x.