Defective stress-fiber formation during spreading of MYH9Δ platelets. Platelets were pretreated with thrombin (1 U/mL) and allowed to adhere to fibrinogen-coated coverslips for 45 minutes. Actin filaments were visualized (A) by confocal fluorescence microscopy after phalloidin-AF 488 labeling (bars 25 μm, inset 7.5 μm) and (B) by SEM after Triton X100 permeabilization (bars 2 μm). Representative of 3 experiments.