Platelet-derived microparticles on synthetic surfaces observed by atomic force microscopy and fluorescence microscopy

Platelet activation on a thrombogenic surface includes the release of membrane-derived microparticles that provide catalytic sites for blood coagulation factors. Here, we describe a quantitative investigation on the production and dimensions of platelet-derived microparticles observed on glass and polyethylene under aqueous conditions, using atomic force microscopy (AFM) and complementary fluorescence microscopy. The results show that contact-activated platelet microparticles are not evenly distributed over a thrombogenic surface, but in clusters in close proximity to adherent platelets. The microparticles are localized near the platelet periphery, and in some cases appear to emanate from platelet pseudopodia, suggesting that formation may result from vesiculation of the pseudopodia. The microparticles measured 125±21 nm (n=73) in the x–y dimensions and 5.2±3.6 nm in height. The results compared closely with 125±22 nm width and 4.1±1.6 nm height obtained for control preparations of thrombin activated microparticles, that were filtered and deposited on glass. Large differences between the measured widths and heights of adsorbed microparticles suggest that platelet microparticles may undergo spreading after attachment to a surface. The adsorbed microparticles expressed platelet membrane receptor GPIIb/IIIa, and many expressed the platelet activation marker P-selectin as determined by fluorescence microscopy. The high number distribution of procoagulant microparticles per unit area of surface compared with platelets suggests that platelet-derived microparticles provide a mechanistic route for amplifying thrombus formation on a thrombogenic surface.