Electron microscopy of the platelets in artificial thrombi

Abstract

The white head of the artificial thrombus which forms when rat blood is made to flow continuously round a loop of plastic tubing consists mostly of aggregates of platelets. At the centres of the aggregates the platelets retain their characteristic organelles; they are bounded by a well-defined, intact membrane, but they are closely packed together and for the most part there is no visible material between them. At the periphery of the aggregates a fringe of similarly packed bodies appears to consist of platelets which have lost their granules. Leucocytes can be seen adherent to these non-granular bodies. The tight packing of the platelets and the loss of granules occurs at an early stage in the formation of the artificial thrombus and precedes widespread fibrin formation. Appearances indicate that the contents of granules are extruded into the space between platelets when the limiting membrane of a granule joins with the platelet boundary membrane. The morphology of the platelet aggregates is unaffected by anticoagulant doses of heparin.