Platelet
Platelets or thrombocytes are a blood component whose function is to react to bleeding from blood vessel injury by clumping, thereby initiating a blood clot. Platelets have no cell nucleus; they are fragments of cytoplasm derived from the megakaryocytes of the bone marrow or lung, which then enter the circulation. Platelets are found only in mammals, whereas in other vertebrates, thrombocytes circulate as intact mononuclear cells. One major function of platelets is to contribute to hemostasis: the process of stopping bleeding at the site of interrupted endothelium. They gather at the site and, unless the interruption is physically too large, they plug the hole. First, platelets attach to substances outside the interrupted endothelium: adhesion. Second, they change shape, turn on receptors and secrete chemical messengers: activation. Third, they connect to each other through receptor bridges: aggregation. Formation of this platelet plug is associated with activation of the coagulation cascade, with resultant fibrin deposition and linking. These processes may overlap: the spectrum is from a predominantly platelet plug, or "white clot" to a predominantly fibrin, or "red clot" or the more typical mixture. Berridge adds retraction and platelet inhibition as fourth and fifth steps, while others would add a sixth step, wound repair. Platelets participate in both innate and adaptive intravascular immune responses. In addition to facilitating the clotting process, platelets contain cytokines and growth factors which can promote wound healing and regeneration of damaged tissues.
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