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Platelets (Thrombocytes)

Platelets are small packages of cytoplasm that are nipped off from the cytoplasm of large mother cells in the bone marrow known as megakaryocytes. Platelets are referred to as thrombocytes because of their sealing function i.e. they form a thrombus, or plug, when there is a cut in the circulatory system.

Properties of platelets: 1. Are about 40 times more common than leukocytes in the blood. 2. Platelets do not have nuclei.. Platelet count: the normal concentration of platelet in adult range between250,000–500,000 platelets/c.mm. of blood. 3-Thrombocytopenia: is a condition in which the platelet count is greatly reduced (aplastic anemia). thrombocytosis (platelets count is greatly increased as in( splenectomy). 4.Life span of platelets: is 8 –12 days (10) days is the mean life span.

5. Thrombopoiesis: is the process of platelet production or development: All platelets are derived from cells in bone marrow called megakaryoblasts, these cells differentiate into another cells called megakaryocytes which actually release platelets from their cytoplasm. The maturation of megakaryocytes is regulated by a factor called thrombopoietin..

Structure of platelets: Morphology: 1. platelets are discoid-shaped packages of cytoplasm surround by a membrane. 2. Platelets surface has many openings that extended deeply into the interior of the structure which give the platelets a sponglike appearance. 2. Platelets surface has many openings that extended deeply into the interior of the structure which give the platelets a sponglike appearance. The surface of platelets has glycoprotein That attached to collagen in the connective tissue of blood vessels. Also the membrane contain large amount of phospholipids, (platelet factor 3) that plays an important role in clotting process. 3.The platelet cytoplasm contains large quantities of ADP, ATP, serotonin, Ca++, thromboxanes A2, factor XIII. (Actin & myosin) that cause contraction of the platelet (these contractile proteins are important in clot retraction).

Function of platelets : Platelets form the second major component of the haemostatic system. Platelets have several functions, all of which are related to haemeostasis (prevention of blood loss). Platelets play an important role in preventing blood loss in the following ways: 1.Blood loss from intact blood vessel is prevented by the structure of the endothelial cells that form the vessel walls. Endothelial cells lined the inner surface of blood vessels & are attached to the collagen fibers in the connective tissues of blood vessel.Endothelial cells form tight junctions with each other and prevent loss of blood cells & plasma proteins from the circulation.

Platelets are required to maintain the structural integrity of the vessels walls. When endothelial cells separate in the intact blood vessels, platelets fill small gaps that may have occurred ; thus prevent the loss of blood cells & plasma from the circulation. When the number of platelets is reduced (i.e. thrombocytopenia), the gaps appear between the cells & blood leaks out of blood vessels into the tissues(purpura) so Platelets play an important role in keeping endothelial cells in good shape. Platelets are required to maintain the structural integrity of the vessels walls. When endothelial cells separate in the intact blood vessels, platelets fill small gaps that may have occurred ; thus prevent the loss of blood cells & plasma from the circulation. When the number of platelets is reduced (i.e. thrombocytopenia), the gaps appear between the cells & blood leaks out of blood vessels into the tissues(purpura) so Platelets play an important role in keeping endothelial cells in good shape.

2.The formation of platelet plugs, that seal holes in damaged blood vessel walls as a result of vessel injury, is known as the primary haemostatic plug formation & it greatly limits the loss of blood from the circulation. 3.The formation of clots, which help seal off larger wounds in the vessels. This process of haemostasis is called secondary haemostasis. Haemostasis: means arrest of bleeding which is very important to the maintenance of homeostasis.

Haemostasis can be divided into two types: 1. Primary haemostasis. 2. Secondary haemostasis Primary haemostasis: INCLUDES:. 1. (vasoconstriction).2. platelet adhesion 3.Release of ATP,ADP,Ca+++,Thromboxane A2. 4.aggregation of the platelets & formation of temporary platelet plug. Vascular spasm: Local vasoconstriction is a immediatebut (temporary closure of a blood vessel )resulting from contraction of smooth muscle within the wall of the blood vessel, which reduces the flow of blood from the rupture vessel such as endothelin released from damaged blood vessels, thromboxane A2 is the most potent vasoconstrictor released by the platelets at the site of injury, and serotonin from aggregating platelets. Vascular spasm (constriction) is a transient process, usually lasting less than a minute. However this time is sufficient for plug formation by platelet.

Platelets plug formation: a platelet plug: is an accumulation of platelets that can seal up small breaks in blood vessel. The steps of the primary haemostatic plug formation are: Step I: platelet adhesion occurs when platelets bind to exposed collagen fibers of endothelial damaged blood vessel. This adhesion requires certain essential coagulation factor such as von willebrand factor which is secreted by endothelial cells of blood vessels. This factor is a part of complex clotting factor number VIII. Step II: activation of platelets by collagen cause release of ADP, thromboxane A2 (derivative of prostaglandins) which are in turn activate additional platelets.

Step III: a plasma protein fibrinogen will bind platelets together resulting in the formation of a platelet plug which seal the wound and prevent bleeding but this plug is temporary stops within few 2 – 6 minutes which is called bleeding time.