This study examins the role of leukocytes within a thrombus by demonstrating the morphologic detail of their activities, the chemotactic properties of thrombi and the presence of plasminogen and possible plasminogen activator within eosinophils. A model which produces discrete, reproducible platelet thrombi in arteries and veins of dogs allowed timed studies of their early evolution. In this model, the growth of the thrombus was constantly monitored by a flowmeter and the thrombus could thus be removed at a selected period in its formation. It was then studied histologically for fibrin activity and also ultrastructually. Little fibrinolytic activity was found. In contrast to neutrophils which are concerned particularly with the phagocytosis and disruption of platelet aggregates, we observed that eosinophils participate in the lysis and disruption of the fibrin within these aggregates. The fibrin is rarely phagocytosed but is acted on at the surfaces of the eosinophils, usually in shallow invaginations of the cell membranes. The fibrin shows morphologic changes of lysis. It appears that eosinophils and neutrophils are concerned with the transformation of the early fibrin and platelet thrombus, rather than with the resolution of the formed, mainly fibrin and red cell thrombus.
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