Dye transfer in HAS–RBC cell pair fusion. Images of HAS–RBC cell pair fusion were captured. pH triggering initiated dye transfer (CF) from the RBC to the HAS cell lumen, and changes in fluorescence intensity (F) were measured in regions of interest (ROI) with a fixed area (35,904 pixel) near the centers of the HAS cells. Time 0 represents the onset of dye transfer. (A) Changes in F of the CF dye, compared to background, can be detected with good time resolution. (B) Two examples of untreated cell pairs with considerable variability in their F kinetics. In one HAS cell, the F increases monotonically to a plateau (i), but the F increase may also be irregular, with different F rates until a plateau is reached (ii). (C) The same two examples on a longer time scale.