Monensin blocks vacuole formation. (A) Transmission electron microscopic images of mouse fetal liver erythroblasts cultured with 1μM monensin added at 38 hours after start of culture along with control are shown. Scale bar, 1 μm. (B) The effect of MiTMAB or monensin on vesicular trafficking was evaluated by monitoring transferrin AF-594 uptake in mouse primary fetal liver erythroblasts in the absence or presence of 10μM MiTMAB and 1μM monensin. Live cell images were acquired with a Nikon Eclipse C1Si laser scanning confocal microscope with a 100×/ 1.40 oil objective and were visualized with a 32 element multi-anode PMT detector. Images were processes using EZ-C1 Gold Version 3.8.0 and Image J Version 1.44a. Control cells displayed strong transferrin uptake, as evidenced by staining of cytoplasmic vesicles and vacuoles, with a striking alignment in the region between the nucleus and cytoplasm (arrow). In contrast, MiTMAB and monensin potently blocked transferrin uptake and disrupted the distribution of transferrin receptors on the plasma membrane along the nuclear side (arrowhead) of the cells. Scale bar, 2 μm.