Platelets (thrombocytes) are small fragments of cells, about 2 to 3 µm across. They are not complete cells: they have no nucleus, but they are surrounded by a cell surface membrane and contain some cytoplasm with mitochondria and granules.Platelets are formed in the bone marrow when very large cells called megakaryocytes break up into thousands of fragments. There are about 150 000 to 400 000 platelets per mm3 of blood, and each one survives for only about 7 to 10 days before being removed by the spleen and liver.The main role of platelets is to help stop bleeding when a blood vessel is damaged:• normally, platelets pass smoothly over the undamaged lining (endothelium) of blood vessels
• when the lining is damaged, collagen fibres in the vessel wall are exposed
• platelets stick to the exposed collagen and to each other, forming a platelet plug that seals small breaks
• the activated platelets release thromboplastin, which starts the chain of reactions that forms a clot
• they also release serotonin, which makes the smooth muscle of the vessel wall contract, narrowing the vessel and reducing blood lossWithout enough platelets, even small injuries can cause heavy bleeding and bruising. Platelets can also be involved in harmful clots: if an atheroma damages the endothelium of an artery, platelets may stick there and start a clot (thrombosis). The details of the clotting cascade are covered in later lessons.
• when the lining is damaged, collagen fibres in the vessel wall are exposed
• platelets stick to the exposed collagen and to each other, forming a platelet plug that seals small breaks
• the activated platelets release thromboplastin, which starts the chain of reactions that forms a clot
• they also release serotonin, which makes the smooth muscle of the vessel wall contract, narrowing the vessel and reducing blood lossWithout enough platelets, even small injuries can cause heavy bleeding and bruising. Platelets can also be involved in harmful clots: if an atheroma damages the endothelium of an artery, platelets may stick there and start a clot (thrombosis). The details of the clotting cascade are covered in later lessons.