Thrombocytes 2-Life span =8-12 days 1-Minute round or oval discs (2-4 ) non-nucleated 2-Life span =8-12 days 3- number =150.000 – 400.000/mm3 removed by tissue macrophage as spleen so,the number increases after spleenectomy. increases during exercise, pregnancy, high altitude, surgical trauma spleenectomy It decreases during menstruation 4-They are formed in the bone marrow as buds on the surface of megakaryocytes and released into the blood under stimulation of thrombopoietin from the kidney

Platelet structure 1- mucopolysaccharide surface coat important in adhesion and aggregation of platelets Glycoproteins A- bind with the Von Willebrand factor which are necessary for platelet adhesion to the damaged endothelial cells. B-act as fibrinogen receptors. 2- Plasma membrane: is formed of 3 layers and invaginates inside to form open membrane system and contain phospholipids. This membrane is the basis structure of platelet factor 3 (PF3)

3- Beneath the membrane there are: a- Microfilaments of actin, myosin and thrombasthenin b- Microtubules maintains the disc shape of the platelets. 4- The platelets contain 3 types of granules: a- Dense granules ADP,serotonin and calcium. b- Specific (Alpha) granules contain platelet growth factor, beta thromboglobulin, fibrinogen ,clotting factors as V, XIII platelet factor 4 (PF4) which is heparin antagonist factor.

c- Dense tubular system contains calcium and may be site of synthesis of prostaglandin and thromboxane A2 5- The platelets also contain mitochondria, glycogen and lysosomes which contain hydrolytic enzymes

Von Willebrand factor (VWF): an adhesive protein formed by :- A-the vascular endothelial cells and secreted in the plasma.to be bound to clotting factor VIII B- megakaryocytes and stored in the platelets.