(A) Structure of thrombi formed at different shear rates in the absence (Control) or presence of monoclonal antibodies blocking GP Ib (LJ-Ib1, 100 μg/ml) or _IIbβ₃(LJ-CP8, 100 μg/ml), visualized by confocal optical sections at 1-μm intervals in the z-axis. Each plane shows an area of 52,345 μm². For clarity, the z scale is expanded and only sections at selected distances from the surface are presented. Confocal sections were obtained after 840 seconds of flow at either 100 or 300 s⁻¹, or after 420 seconds at 1,500 s⁻¹. The inset in the middle panel displays the partial reproduction with less size reduction of a thrombus formed at wall shear rate of 300 s⁻¹, demonstrating the resolution of single platelets in the confocal sections. The two insets in the bottom panel show with greater magnification part of the surfaces exposed to antibody-containing blood flowing with wall shear rate of 1,500 s⁻¹. No thrombi were formed, but single platelets could easily be detected. Note that the confocal sections shown here are intended to give a global view of thrombus formation and not to visualize single platelets. (B) Total volume of thrombi calculated from confocal sections. Representative experiment of two that gave comparable results. Experiments at the shear rate of 1,500 s⁻¹ were terminated after collecting z sections at 420 seconds when thrombi had already reached a large volume. Prolonging perfusion at this shear rate resulted in markedly abnormal flow patterns owing to nearly complete occlusion of the chamber.