Thrombocytes

Thrombocytes, commonly referred to as platelets in mammals, are small, anucleate cell fragments that circulate in the blood and play a central role in hemostasis. In humans and other mammals they arise from megakaryocytes in the bone marrow through thrombopoiesis. Platelets are normally present at about 150 to 450 x 10^9 per liter, and they have a typical lifespan of 5 to 9 days, after which they are removed by the spleen and liver. They are cytoplasmic fragments rather than complete cells, reflecting their lack of a nucleus, and their production is regulated primarily by thrombopoietin.

Platelets contribute to hemostasis by rapidly adhering to sites of vascular injury, primarily through interactions involving von Willebrand factor and platelet surface receptors such as GPIb-IX-V. Upon activation they change shape and release granule contents, including ADP, calcium, serotonin, and thromboxane A2, which amplify platelet recruitment and aggregation. Platelets aggregate by linking via the fibrinogen receptor GPIIb/IIIa, forming a primary hemostatic plug. In addition to forming a physical barrier, platelets provide a phospholipid surface that supports the assembly of coagulation factor complexes, contributing to the secondary phase of coagulation and fibrin clot stabilization. Platelets also release mediators that promote vasoconstriction, wound healing, and inflammatory responses.

Thrombopoietin (TPO) is the principal regulator of platelet production. Abnormal platelet numbers or function can lead to bleeding or thrombotic disorders. Diagnostic approaches include complete blood counts and platelet function tests. In non-mammalian vertebrates, thrombocytes are nucleated and function similarly in hemostasis, though their biology differs from mammalian platelets.