Loss of pleckstrin allows granule-to-granule fusion. Morphologies of platelet granules were analyzed in resting platelets and in platelets exposed to PMA for 2 or 5 minutes. Platelets derived from wild-type or pleckstrin-null platelets appeared identical under basal conditions. Wild-type platelets coalesced their granules more rapidly than platelets lacking pleckstrin (compare images in middle column.) However, wild-type and pleckstrin-null platelets appeared identical after 5 minutes of stimulation, with both genotypes of platelets having numerous merged granules (several examples indicated by arrowheads.) This shows that loss of pleckstrin impairs the efficiency of granule-to-granule fusion but does not prevent it. Shown are 20 000× magnifications captured with an FEI Tecnai T12 electron microscope operated at 80-kV accelerating voltage.