Platelet spreading on immobilized fibrinogen. (A) Platelets were incubated for 45 minutes on fibrinogen-coated coverslips in the absence or presence of 0.5 mM MnCl2. Platelets were fixed and stained with rhodamine-phalloidin (red) to label F-actin or with an antibody to phosphotyrosine (green). Wild-type platelets showed filopodia (arrowheads) and minimal lamellipodial extension, both of which were accentuated by extrinsic activation of αIIbβ3 with MnCl2. Scale bar = 10 μm. (B) Platelets were plated on fibrinogen as in panel A, in the absence or presence of MnCl2, ADP, or PAR4 peptide. After fixation and staining, platelet spreading (mean surface area) was quantified by image analysis (see “Methods” for details). *P < .01. Data represent means plus or minus SEM of 349 to 859 platelets analyzed on 6 to 10 separate coverslips for each experimental condition.
