Failure of conversion of fibrinogen to fibrin -platelets (thrombocytes) are involved in the conversion of fibrinogen to fibrin through the action of phospholipids. in thrombocytopenia, fibrinogen will not be converted to fibrin in sufficient quantity to allow normal clotting. the absence of platelet aggregation interferes with normal endothelial maintenance and repair after injury. the endothelium becomes increasingly leaky and eventually may permit thrombocytopenia purpura with seepage of blood from the vessel. platelets (thrombocytes) are fragments of megakaryocytes that function in aggregation, coagulation, clot retraction, and removal. the cytoskeleton of the platelet is extensive and facilitates changes in shape of the platelet as well as contractions, which assist in the release of secretory granules. platelet-derived growth factor (pdgf) is released by platelets and stimulates the proliferation of endothelial cells, vascular smooth muscle cells, and fibroblasts. thrombin is involved in conversion of fibrinogen to fibrin, but it is a plasma protein, not a platelet secretory factor. platelets are not required for the initiation of the blood-clotting cascade, but they are required for the adherence and normal formation of a clot. plasmin is not secreted by platelets but is formed by the conversion of plasma-derived plasminogen under the influence of plasminogen activator secreted by endothelial cells. plasmin is involved in dissolution, not formation, of blood clots.
