Objective--The platelet glycoprotein (GP) Ib/V/IX complex plays an important role in regulating the morphology of resting platelets and can induce shape change during adhesion to immobilized von Willebrand factor (vWf). In this study we have examined the effects of fluid shear stress on GPIb-dependent changes in platelet morphology during translocation on vWf.

Methods and Results--We demonstrate that translocating platelets undergo a unique series of morphological changes in response to increasing fluid shear stress. Under moderately low shear conditions (600 s^-1), initial shape change involved extension of membrane tethers and/or filopodia from the platelet surface. With increasing shear rate, platelets adopted a spherical morphology with numerous surface projections (1800 to 5000 s^-1). At high wall shear rates (10000 to 20 000 s^-1), translocating platelets retracted filopodia, developing a smooth ball-like appearance. These changes in morphology were dependent on reorganization of the actin and microtubule components of the cytoskeleton and were regulated by intracellular signaling processes linked to Src kinases. Functionally, alterations in platelet shape had a major effect on translocation dynamics in that conversion from discs to spheres resulted in a 3- to 8-fold increase in rolling velocity.

Conclusions--These studies demonstrate that platelets undergo shear-specific morphological changes during surface translocation on vWf that may serve to regulate translocation dynamics under flow.