Platelets

Platelets, also known as thrombocytes, are small, anucleate cell fragments derived from megakaryocytes in the bone marrow. They measure about 2–3 micrometers in diameter and circulate for about 7–10 days before being cleared from the circulation. Platelets lack a nucleus but contain organelles and granules that enable rapid responses to vascular injury.

In hemostasis, platelets rapidly respond to damaged vessels. They adhere to exposed subendothelial matrix mainly through von Willebrand factor binding to the platelet receptor complex GPIb-IX-V. Upon activation, platelets undergo shape change and release contents from alpha and dense granules, including von Willebrand factor, fibrinogen, ADP, calcium, and serotonin. These mediators recruit and activate additional platelets, promoting aggregation via the integrin GPIIb/IIIa, which binds fibrinogen and supports the formation of a platelet plug. Platelets also provide a procoagulant surface that enhances thrombin generation, contributing to clot stabilization.

Thrombopoiesis, the production of platelets, is regulated chiefly by thrombopoietin, produced by the liver and kidneys. Megakaryocytes extend proplatelets into bone marrow sinusoids, from which platelets are released. A normal platelet count ranges roughly from 150 to 450 x 10^9/L, and the spleen contains a reservoir of platelets.

Clinical relevance includes disorders of quantity and function. Thrombocytopenia increases bleeding risk; thrombocytosis can be reactive or due to primary means. Inherited platelet disorders include Bernard-Soulier syndrome (GPIb-IX-V deficiency) and Glanzmann thrombasthenia (GPIIb-IIIa deficiency). von Willebrand disease impairs platelet adhesion. Platelet function can be assessed by aggregometry; management may involve platelet transfusion for significant bleeding or perioperative needs, and antiplatelet drugs (aspirin, P2Y12 inhibitors, GP IIb/IIIa inhibitors) reduce platelet function in various settings. Platelets also participate in inflammation, wound healing, and immunity, releasing growth factors such as PDGF.