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The primary role of the platelet is the formation of the initial platelet plug during the first phase of haemostasis. Later the platelets provide clotting factors and an extensive phospholipid surface for the generation of fibrin.
The platelet is produced by cytoplasmic budding of the bone marrow megakaryocyte. The megakaryocyte is derived from a haemopoietic stem cell. The production of platelets is stimulated by the cytokine thrombopoietin.
Young platelets are sequestered in the spleen for 36 hours before being released into the circulation. The life span of a platelet is 7-10 days and the normal count is 150-450 x10^9 / litre.
The initial haemostatic event is the adherence of platelets to exposed collagen at sites of endotheliam damage. Von Willebrand's factor is important in platelet adherence. The platelet becomes activated. Activated platelets are able to aggregate using receptors for agents such fibrinogen. Aggregation is stimulated by the release of the contents of two classes of platelet granules: