Intracellular actin distribution in platelets adhering to fibrinogen under flow. Platelets adherent to fibrinogen were perfusion-fixed after 1.5 minutes of flow, permeabilized and stained for F-actin (red) and ADAP (blue). (A) Actin-rich structures form in ADAP^+/+ platelets adherent to fibrinogen, in which ADAP is also found (top). These structures were greatly reduced in ADAP^−/− platelets (bottom), in which ADAP staining was negative. (B) Line profile of actin and ADAP fluorescence distribution in a representative ADAP^+/+ platelet; note the almost identical localization. (C) Quantification of actin-rich structures in platelets perfused either over immobilized fibrinogen or a dmA1A2 VWF fragment, the latter lacking binding sites for αIIbβ3. A cluster index, used to express quantitatively the distribution of actin-rich structures, was calculated as follows: (average cluster area) × (average number of clusters per platelet). The results shown are the means ± SEM of 3 separate experiments on fibrinogen (*P < .01) and 2 experiments on dmA1A2 VWF fragment.