The clotting time or gel point is commonly used in a variety of clinical assays, but little has been known about what it represents structurally because of the limitations of imaging technologies. The necessity of fixation and dehydration for electron microscopy means that only final clot structures, often distorted by preparation artifacts, can be observed, and there are severe limitations of image quality for conventional light microscopy. For similar reasons, we know almost nothing about the branching and lateral aggregation of fibrin that are essential for clot or thrombus stability. Turbidity, light scattering, or clot stiffness are useful to follow the time course of polymerization but reflect only overall clot properties. We visualized in real time fibrin network formation in the hydrated state, using deconvolution microscopy, which allows optical sectioning without the bleaching that accompanies confocal microscopy. Thus, the events during polymerization could be followed quantitatively over long periods of time. Videos will demonstrate the major observations. Individual mobile fibers were observed before the gel point. After gelation, an initial fibrin network, or scaffold, was seen, which evolved over time by addition of new fibers and elongation and branching of others. Furthermore, some fibers in the network moved chaotically for some time. A detailed, quantitative morphological analysis of network formation was carried out by superposition of images from different time points colorized to distinguish changes. We quantified network formation by the number of branch points, and longitudinal and lateral growth of fibers as a function of time. The distributions of fibers that reached a maximum of longitudinal growth and branch point formation both had maxima at the gel point but, surprisingly, some longitudinal growth continued and new branch points appeared after the gel point, requiring modification of existing models of fibrin polymerization. The cumulative percentage of fibers reaching their final length and the number of branch points attained maximum values at the time corresponding to that at which the turbidity reached approximately 60% of its maximum. Lateral growth reached a plateau at the same time as turbidity. Measurements of clot mechanical properties revealed that the clots achieved maximum stiffness and minimum plasticity well after branch point, as well as length changes and lateral growth of fibers, were completed. These results provide new information on the time sequence of events during fibrin network formation, which is important to understand both clotting and thrombosis and to allow modulation of clot properties.

Author notes

Disclosure: No relevant conflicts of interest to declare.