Platelet spreading and thrombin-mediated clot retraction. (A) Platelets (6 × 106/mL) were allowed to spread on collagen or fibrinogen for 20 minutes and fixed, followed by observation of filopodia and lamellipodia formation under a differential interference contrast microscope (Zeiss LSM 880) (×63 oil objective). (B) Platelets (12 × 106/mL) were placed on glass coverslips that were precoated with collagen or fibrinogen at 37°C for 90 minutes and then stained with Alexa Fluor-546–labeled phalloidin followed by analysis of the covered area using ImageJ software (National Institutes of Health). Images (×100) (scale bar = 20 μm) are representative of 3 independent experiments (mean ± standard deviation, n = 3; unpaired t test). (C) Clot retraction was triggered after addition of thrombin (1 U/mL). Representative images at 30, 60, 90, and 120 minutes from 3 independent experiments are presented. Data were quantified as clot volume (%) and are presented as mean (n = 3; two-way analysis of variance). **P < .01, ***P < .001.