Morphologic activation score of fibrin-forming platelets. (A), Different morphologies of adhered platelets superfused with normal plasma at low shear rate of 250 s⁻¹. Start of fibrin formation was at 580 seconds. Numbers in italic refer to platelets with different structures: 1, platelet with pseudopods and core; 2, platelet with lamellipods and core; 3, translucent platelet without core, forming ruffled edges; 4, bleb-forming platelet with rounded structure (bars, 10 μm). (B), Fluorescence images after postlabeling with OG488–annexin A5. (C), Time-dependent increase in OG488–annexin A5 fluorescence during plasma perfusion (arbitrary fluorescence units per cell, mean ± SE, n = 12). Arrow points to start of fibrin formation. (D), Representative [Ca²⁺]_i rises of single adhered platelets during plasma perfusion, expressed as pseudo-ratio F/F_o values. Where indicated, PPACK (40μM) or 6B4 mAb (20 μg/mL) was present in plasma. Microscopic images and Ca²⁺ traces recorded with a Nikon Diaphot 200 microscope as described in “Methods.”