Fig. 1. Fusion-pore opening and expansion in the cell-contact zone. (A-F) Time series of syncytia formation for cells that were labeled with FM4-64FX. For each time point, the top view (maximum intensity projection of the whole image stack along the z-axis) is shown on the right panel and the view of the contact zone as seen by looking from one fusing cell into another (maximum intensity projection along the x-axis of the region delimited by two vertical lines) is shown on the left panel. The arrow in B marks the location of the first pore that was observed in this series. (G,H) Pore-size (G) and pore-number per contact zone (H) distribution histograms based on analysis of 498 pores from 175 cell-cell contact zones. Because the average rates of fusion-pore expansion differed from day to day in more than ten experiments investigating the morphology of the fusion pores in fixed cells, these histograms include only the data collected in the same experiment. Cells were fixed 4 minutes after a 1-minute low-pH application. The height of each bar in G represents the number of pores with diameters in the interval between its lower and upper bounds on the x-axis.