Shape of the fusion pore. (A–E) Virtual z-slices through tomograms of cell pairs, showing one small pore entirely enclosed in the tomographic volume (A) and four cases of apparent connection between cells, with local loss of density in the CW, some of which may represent fusion intermediates (B). The CW is colored in purple. Zoomed views are shown at the bottom. (C–E) Four virtual z-slices through tomograms and model of the PM outer leaflet. Z-slice positions from left to right are indicated by purple lines on the model, where the z-slice intersects the green PM outline at a perpendicular angle (see Video 3, Video 4, and Video 6). (C) Several connections, similar to the ones shown in B. (D and E) Large pores with an irregular edge. In D, the larger pore at z = 84 nm is prolonged into a connection as in B at z = 18 and 4 nm. In E, a large pore at z = 5 nm splits into two openings separated by a strand of CW at higher z levels. The CW is colored in purple. Uncolored images are shown in Fig. S2, A–C. (F) Two non–mutually exclusive interpretations of fusion pore formation: fusion may initiate at several locations, forming pores that fuse together to form a nonspherical larger pore (i); fusion may initiate at a single location, but pore expansion occurs nonhomogeneously, limited by CW degradation (ii). Scale bars, 100 nm.