Atomic force microscopy of fibrin networks and plasma clots during fibrinolysis

Abstract We have used atomic force microscopy (AFM) in order to study the ultrastructure of fibrin fibre dissolution in real time. Thin purified fibrin gels and plasma clots were prepared on glass surfaces and overlaid with isotonic saline or heparinized plasma in an AFM fluid-cell. Fibrinolysis was initiated by introducing plasmin or recombinant tissue-type plasminogen activator (rt-PA) into the solution bathing the clots. Microscopy was performed serially in real time on the Nanoscope III Atomic Force Microscope operating in the tapping or contact mode. The acquisition time for a single image was 2–8 min and the clots were imaged for up to 1 h with fields of view ranging from 128 × 128 μm to 0.7 × 0.7 μm with a resolution of 512 × 512 pixels. In the smallest fields of view fibrin fibres were seen to be composed of globules 40–70 nm in diameter. The diameter of composite fibrin fibres in purified gels depended on the concentration of NaCl in the fibrinogen solution: 250 ± 155 nm in 150 mmol/l NaCl vs. 1.42 ± 0.19 μm in 50 mmol/l NaCl. Plasma clots were composed of thick fibres with interspersed thinner fibres. In clots from platelet-rich plasma both the thick and the thin fibres had significantly smaller diameters than the corresponding fibre types in clots from platelet-depleted plasma (620 ± 195 nm vs. 965 ± 200 nm, and 195 ± 30 nm vs. 260 ± 60 nm, P