In reticulocytes, all plasma membrane-derived, intracellular structures contain transferrin. Cells were visualized by confocal microscopy. (A) Reticulocytes were treated with FM 1-43 and subsequently incubated at 37°C in the presence of AG-Tf. R, reticulocyte; E, erythrocyte; arrows, collocalized FM 1-43 and Tf. Scale bar is 5 μm. (B) Huh7 cells were treated with FM 1-43 and then with AG-Tf, as in panel A. To visualize the internalized dye, it was required to increase the photomultiplier tube gain to levels that elicit saturation of the dense, membrane signal surrounding these thin, adherent cells. Arrows indicate FM 1-43 vesicles that do not contain Tf. Scale bar is 10 μm.