Figure 4: The bright-field cell image (A) and the simultaneous fluorescence image of QD-EGF (B) in living A431 cell after 15 minutes of 4 nM QD-EGF stimulation at 37ºC. The arrow indicated the the direction of retrograde transport of QD-EGF-EGFR complexes, where the transport speed of wavelike propagation was determined to be 3.5 nm/s.