Size of the fusion pore formed during a lingering kiss. In each case (panels A and B) the image montages show the fluorescence of a Proregion-EGFP (top panels) containing WPBs prior to (i and ii) and during (iii and iv) a lingering kiss evoked by histamine (100 μM), in the presence of extracellular alexa-647 (middle panels) and 3 kDa dextran-TRITC (A; bottom panel) or 70 kDa dextran-TRITC (B; bottom panel). The time-courses for the changes in WPB-EGFP (top traces), alexa-647 (middle traces), and dextran-TRITC (bottom traces) fluorescence are shown below each montage. The time points corresponding to the images in the montages are indicated (i-iv), and scale bars are 1 μm. Images were acquired simultaneously on a Leica SP2 confocal microscope at 7.4 frames per second. Panel C summaries the proportion of WPBs that accumulated dextran-TRITC, over the size range 3 kDa to 2000 kDa, in collapsed WPBs. The numbers of WPBs analyzed at each dextran size is indicated in parenthesis by each point. The size range for the dextrans used (where known) are shown in parenthesis below the ordinate for each point, and scale drawings of the mean diameters of the dextrans used are show as black circles. The diameter of Proregion (Pro) estimated by gel filtration²⁸  is indicated as a gray circle.