Platelets
Platelets or thrombocytes are only about 20% of the diameter of red blood cells and are the most numerous cell of the blood. Their diameter is about 2-3 µm; hence they are much smaller than erythrocytes. Their density in the blood is 200000-300000 /mm3 and a normal platelet count in a healthy individual is between 150,000 and 450,000 per μl (microlitre) of blood (150–450)×109/L) . Platelets are not only the smallest blood cell, they are also the lightest. Therefore they are pushed out from the center of flowing blood to the wall of the blood vessel. There they roll along the surface of the vessel wall, which is lined by cells called endothelium. The endothelium is a very special surface, like Teflon, that prevents anything from sticking to it. However when there is an injury or cut, and the endothelial layer is broken, the tough fibers that surround a blood vessel are exposed to the liquid flowing blood. It is the platelets that react first to injury. The tough fibers surrounding the vessel wall, like an envelope, attract platelets like a magnet, stimulate the shape change that is shown in the pictures above, and platelets then clump onto these fibers, providing the initial seal to prevent bleeding, the leak of red blood cells and plasma through the vessel injury. Their function is to stop the loss of blood from wounds (hematostasis). They do so by releasing factors like serotonin which reduce the diameter of lesioned vessels and slow down the blood flux, the fibrin which trap cells and forms the clotting. Even if platelets appear roundish in shape, they are not real cells and with Giemsa stain they have an intense purple color. Platelets are produced in the bone marrow from cells known as megakaryocytes by the process of fragmentation which results in the release of over 1,000 platelets per megakaryocyte. The dominant hormone controlling megakaryocyte development is thrombopoietin.
Functions of Platelet:
When there is a wound, platelets aggregate using the protein fibrinogen and von Willebrand factor (vWF) as a connecting agent. The most abundant platelet aggregation receptor is glycoprotein IIb/IIIa which is a calcium-dependent receptor for fibrinogen, fibronectin, vitronectin, thrombospondin vWF. Activated platelets adhere with the help of glycoprotein Ia, to the collagen that is exposed by endothelial damage. Aggregation and adhesion act together to form the platelet plug. Platelet aggregation is stimulated by ADP, thromboxane, and α2 receptor-activation, but inhibited by other inflammatory products like PGI2 and PGD2. Other role of platelets includes modulation of inflammatory processes by interacting with leukocytes and by secreting cytokines, chemokines, and other inflammatory mediators. They Platelets also secrete platelet-derived growth factor (PDGF). Diseases like thrombocytopenia and thrombocytosis may present with coagulation problems due to low platelet counts which increase bleeding risks.