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Thrombocytes, commonly known as platelets, are small, disc-shaped cell fragments in blood that lack a nucleus in humans. They are produced from megakaryocytes in the bone marrow and enter circulation as anucleate particles about 2–3 micrometers in diameter. In healthy adults, platelet counts typically range from about 150 to 450 x 10^9 per liter, and platelets have a lifespan of roughly 7 to 10 days, with a portion stored in the spleen.

Platelets play a central role in hemostasis, the process that stops bleeding. Upon vascular injury, they adhere to exposed subendothelial matrix via the von Willebrand factor and collagen, undergo activation, and release granule contents such as ADP, serotonin, calcium, and growth factors. This promotes further platelet recruitment and aggregation, primarily through fibrinogen binding to the GPIIb/IIIa receptor complex, forming a hemostatic plug. Platelets also provide a surface that supports the coagulation cascade and help localize clotting factors at the injury site. Beyond clot formation, platelets contribute to wound healing, inflammation, and angiogenesis.

Development and variation are notable: platelets are anucleate in mammals, whereas many other vertebrates have nucleated thrombocytes. Disorders affecting platelets include thrombocytopenia (low count), which raises bleeding risk, and thrombocytosis (high count), which can increase thrombosis risk. Platelet function disorders, such as Glanzmann thrombasthenia and Bernard-Soulier syndrome, affect aggregation or adhesion, respectively. Diagnostic tools include complete blood count and platelet function tests; treatments range from platelet transfusion in major bleeds to antiplatelet drugs for cardiovascular disease management.