Two mechanisms are involved in the enhancement of thrombus growth during arterial flow by fibronectin: (1) activated platelets bind to fibronectin cross-linked by factor XIIIa to fibrin (at the bottom, fibronectin molecules [black] cross-linked to the fibrin matrix bind platelets via their receptors); (2) assembly of fibronectin by adherent and aggregating platelets (above, platelet-platelet interactions are mediated by assembled fibronectin). In both cases, platelets are binding to fibronectin via their receptors, including β3 integrins. The arrow indicates the direction of blood flow.