Selective loss of GPIbα and GPVI in high-Ca²⁺PS-exposing platelets in thrombi. Platelet thrombi were formed in flow chambers on collagen type I using whole-blood perfusion at a wall shear rate of 1000 s⁻¹. The thrombi were postactivated with thrombin, incubated with vehicle or GW280264X (GW) for up to 180 minutes at 37°C, and stained for GPIbα and GPVI expression and PS exposure. (A) Representative confocal fluorescence images of GPIbα and GPVI expression and PS exposure. PS-exposing ballooned platelets are indicated by arrows and arrowheads. (B) Regions of interest corresponding to aggregated platelets and ballooned platelets were analyzed for fluorescence staining. Pixels representing GPIbα⁻ or GPVI⁺ platelets were determined and expressed as percentages of surface area coverage of ballooned or aggregated platelets. Data are mean ± SD, n = 3. *P < .05.