Atomic force microscopy of fibrin networks and plasma clots during fibrinolysis

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< 0.001 for both comparisons). Fibrinolysis of both thick and thin fibres proceeded predominantly by lateral section of the whole fibre thickness at a given site, regardless of whether it was initiated by plasmin or by rt-PA. The time to complete fibre section by 2.5 U/ml of plasmin did not differ between thin and thick fibrin fibres (7.6 ± 3.7 min vs. 6.4 ± 4.2 min). With a low concentration of plasmin (0.17 U/ml) some fibrin fibres became thinner along their entire observed length before they were cleaved. The rate of fibre thinning was 3-times faster in the thicker fibres than in the thinner ones. We conclude that the ‘cut-through’ pattern is the predominant way of fibrinolysis in purified gels and in plasma clots, and that proteolysis leading towards fibre cleavage proceeds more efficiently in thick than in thin composite fibrin fibres.