The loss of VAMP8 delays fusion pore formation and dilation. Platelets from the indicated strains were isolated and fixed in resting or thrombin-stimulated states (0.1 U/mL thrombin for 90 or 300 seconds). EM tomographs were analyzed for fusion pore structures, and the diameters were measured in z-series. Representative EM images of WT platelets at 90 seconds (A) and at 300 seconds (B), V8^−/− platelets at 300 seconds (C), and V2^Δ3^Δ8^−/− platelets at 90 seconds (D). The arrows indicate fusion pores. (E) The distribution of fusion pore size in platelets at early (90 seconds) and late (300 seconds) times poststimulation for the indicated strains is graphed as a scatter plot. Mean pore sizes are as follows: (WT: 90 seconds, 82.4 nm; WT: 300 seconds, 166 nm; V8^−/−: 90 seconds, 72 nm; V8^−/−: 300 seconds, 160.8 nm; V2^Δ3^Δ8^−/−: 90 seconds, 21 nm, V2^Δ3^Δ8^−/−: 300 seconds, pores could not be detected). Pores detected per platelet profile are indicated. ND, not detected.