Expression of FcγRIIa enhances thrombus formation in arterioles and fibrin formation in veins. (A) Arteriolar laser injury: platelet accumulation. Kinetics of platelet accumulation in laser-injured arterioles. Cremaster muscle arteriole injury was induced using an SRS NL100 Nitrogen laser system. Average platelet accumulation was measured using an Alexa 488–labeled anti-CD41 Fab. Data were collected for 2.5 minutes at 5 frames per second and analyzed using Slidebook software. SE are shown as shaded areas. Note that platelet thrombus formation was markedly enhanced in FcγRIIa^pos, compared with FcγRIIa^neg, mice, consistent with a dominant role for platelets in arterial injury. (B) Femoral vein electrolytic injury: fibrin accumulation. Kinetics of fibrin accumulation in electrolytically injured femoral veins. Electrolytic injury was induced by a positive direct current (1.5 V for 30 seconds) applied to the outer surface of the femoral vein using a steel microsurgical needle. Mice were preinjected with rhodamine 6G to label platelets, and fibrin was detected as described in “Materials and methods”. Fluorescent images were captured every 2 minutes for 60 minutes. Fluorophore intensity was measured within the thrombus zone and normalized for interanimal comparisons. The numbers of animals per group, and the P value calculated by the Student t test are indicated in each panel. Note that fibrin accumulation was markedly enhanced in FcγRIIa^pos, compared with FcγRIIa^neg, mice, consistent with a dominant role for fibrin in venous injury.