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

Platelet activation on a thrombogenic surface includes the release of membr ane-derived microparticles that provide catalytic sites for blood coagulati on factors. Here, we describe a quantitative investigation on the productio n and dimensions of platelet-derived microparticles observed on glass and p olyethylene under aqueous conditions, using atomic force microscopy (AFM) a nd complementary fluorescence microscopy. The results show that contact-act ivated platelet microparticles are not evenly distributed over a thrombogen ic surface, but in clusters in close proximity to adherent platelets. The m icroparticles 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 12 5 +/- 21 nm (n = 73) in the x-y dimensions and 5.2 +/- 3.6 nm in height. Th e results compared closely with 125 +/- 22 nm width and 4.1 +/- 1.6 nm heig ht obtained for control preparations of thrombin activated microparticles, that were filtered and deposited on glass. Large differences between the me asured widths and heights of adsorbed microparticles suggest that platelet microparticles may undergo spreading after attachment to a surface. The ads orbed microparticles expressed platelet membrane receptor GPIIb/IIIa, and m any expressed the platelet activation marker P-selectin as determined by fl uorescence microscopy. The high number distribution of procoagulant micropa rticles per unit area of surface compared with platelets suggests that plat elet-derived microparticles provide a mechanistic route for amplifying thro mbus formation on a thrombogenic surface. (C) 1999 Elsevier Science Ltd. Al l rights reserved.