It is associated with the thrombocytes (platelets).
The activation of the thrombocyte system is caused by linking glycoproteins to the thrombocyte membrane in a process through the von-Willebrand-Factor (vWF). This leads to a first adhesion of the thrombocytes and to the formation of a thin wound cover.
The vWF is normally in the wall of a blood vessel under the inner endothelium layer. Only after an injury of the endothelium the platelet can link to the vWF. In the following further thrombocytes are activated by the release of platelet factors (ADP, serotonin and others). They change their appearance through actin- and myosin filaments: the formerly round platelets broaden and become thorny (they form pseudo feet [cytoplasmic pseudopods]).
Actin and myosin are proteins in the cell inside. As filaments they are connected with the cell membrane and can – under ernergy consumption - change their position towards each other and thus the cell shape (e.g.: muscular contraction).
Supported by chemotaxis (= attraction by chemical substances) the thrombocytes accumulate (aggregation): the activated thrombocytes release messengers with locally restricted impact as serotonin and ADP.
Induced by ADP also the structure of an integrin, an integral membrane protein in the plasma membrane of thrombocytes is modified, which enables adhesive contact to the net with the clotting factor I. This integrin consists of the glyco proteins IIb and IIIa.
Fibrinogen leads to a firm net of thrombocytes of the primary formed white separation thrombus.