Reorganization of contractile elements in the platelet during clot retraction.

Electron microscopic studies have been carried out on human platelets in the clot retraction. In the early stage of clot formation, platelets send out filopodia, in which thin filaments run longitudinally. The thin filaments are often observed to attach to the cell membrane where fibrin strands bind from the extracellular surface. In the later stage of clot formation, thick filaments become observable, mainly in the cell body of the platelets. These thick filaments are arranged to form an ordered array, and thin filaments run parallel to them. The thin filaments often attach to the end of the thick filaments. However, thin filaments are not seen between the arrays of thick filaments. Similar structures are also observed in the cytoskeleton of the contracted platelet. These filaments closely resemble the purified myosin aggregates formed under low ionic strength. Thus, during clot retraction, both actin and myosin in platelets are reorganized into thin and thick filaments, respectively.