The Role of Platelets in Blood Coagulation

Platelets are generated by the fragmentation of the megakaryocytes in the bone marrow [25-27]. Mega- 

The platelets are mainly composed of protein and lipids. Protein represents 60% of the dry weight of platelet. Many of the platelet proteins are enzymes (catalase, hyaluronidase, esterase, j8-glucuronidase, nucleotidase, tryptase, tyrosinase, and pyrophosphatase). Many of the enzyrries in platelets are hydrolases, which may well reflect platelets physiological role. 

In addition to proteins, platelets contain 15% lipids, of which cephalins constitute 7.5%. Finally, the platelets also contain large amounts of pharmacologically active compounds, namely, serotonin and histamine, and these compounds are released in the blood during platelet disintegration. 

If the platelets are lysed experimentally by osmotic shock, two fractions can be recovered, a water-soluble and a liposoluble one. Each of these fractions contains a variety of compounds involved in blood coagulation. 

if the water-soluble fraction is absorbed on tricalcium phosphate and then centrifuged, the supernatant contains a platelet factor referred to as factor I. The tricalcium phosphate pellet contains another factor called factor II, which can be eluted with 0.2% m sodium citrate. Factor II accelerates the conversion of fibrinogen to fibrin by thrombin. Factor III has thromboplastic activity and reverses the anticoagulant effect of heparin, and platelet thromboplastin, or factor IV, reacts with plasma components on active thromboplastin. At least two other factors have been described in platelets—serotonin and fibrinogen that coagulates under the influence of thrombin. Available evidence suggests that fibrinogens in plasma and pate-lets are identical. 

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