Platelets (Thrombocytes)- Definition, Structure, Immunity, Functions


Platelets, also known as thrombocytes, are small blood cells that play a vital role in blood clotting and wound healing. They are produced in the bone marrow by a process called megakaryopoiesis, where large cells called megakaryocytes fragment into thousands of platelets. Platelets circulate in the bloodstream for about 7 to 10 days before they are removed by the spleen and liver.

Platelets are essential for maintaining hemostasis, which is the balance between bleeding and clotting. When a blood vessel is injured, platelets adhere to the exposed collagen fibers and form a plug that seals the wound. They also release chemicals that activate other platelets and initiate a cascade of reactions that result in the formation of a fibrin clot. This process is called primary hemostasis.

Platelets are also involved in inflammation and immunity, as they can sense and respond to various stimuli, such as pathogens, cytokines, and shear stress. They can secrete various factors that modulate the activity of immune cells, such as neutrophils, monocytes, macrophages, dendritic cells, and lymphocytes. They can also interact with endothelial cells and regulate vascular permeability and tone. Platelets can also form complexes with leukocytes and bacteria, which enhance their clearance by phagocytosis. This process is called secondary hemostasis.

Platelets are not only important for preventing excessive bleeding but also for preventing excessive clotting. Platelets have mechanisms to limit their activation and aggregation, such as nitric oxide and prostacyclin production, which inhibit platelet adhesion and aggregation. They also have receptors that bind to anticoagulant factors, such as protein C and thrombomodulin, which inhibit the coagulation cascade. Platelets can also undergo apoptosis or programmed cell death, which reduces their number and activity.

Platelets are fascinating cells that have multiple functions in health and disease. They are involved in various physiological and pathological processes, such as wound healing, inflammation, immunity, angiogenesis, cancer metastasis, atherosclerosis, thrombosis, stroke, and myocardial infarction. Understanding the biology and function of platelets can help in developing new strategies for diagnosis and treatment of various disorders related to hemostasis and inflammation.