Platelet biology, hemostasis, testing, and therapy

Platelet definition and mammalian role

Platelets (thrombocytes) are blood components that help stop bleeding by reacting to injury of blood vessels. In mammals, platelets are small, anucleate cytoplasmic fragments derived from megakaryocytes in the bone marrow (and also lung tissue). They circulate in the blood and respond to vessel damage by clumping to form a clot, working together with coagulation factors to achieve hemostasis. A major mammalian role of platelets is primary hemostasis: they adhere to exposed subendothelial structures, become activated (changing shape and releasing chemical messengers), and then aggregate via receptor bridges to form a platelet plug. Platelet activation also supports secondary hemostasis by facilitating the coagulation cascade, leading to fibrin deposition that stabilizes the clot. Beyond clotting, platelets contribute to immune functions—participating in innate and adaptive intravascular responses, including inflammation and immunothrombosis—partly through the release of cytokines and growth factors and through interactions with immune cells.

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Platelet structure zones and granule types

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