Thrombozyten

Thrombozyten, commonly known as platelets in English (also called thrombocytes), are small, anucleate cell fragments in blood that originate from megakaryocytes in the bone marrow. They measure about 2-3 μm in diameter and contain granules but no nucleus. Platelets circulate for about 7-10 days and are essential to hemostasis and wound repair.

Upon vascular injury, platelets adhere to exposed subendothelial matrix primarily through von Willebrand factor binding to the platelet receptor GPIb-IX-V. Activation leads to shape change and release of granule contents, including ADP, serotonin, calcium, and thromboxane A2, amplifying recruitment of additional platelets. Surface glycoprotein IIb/IIIa becomes activated and binds fibrinogen, producing a platelet plug (primary hemostasis).

Platelets also contribute to coagulation by providing phospholipid surfaces and releasing growth factors; they interact with coagulation factors in secondary hemostasis to stabilize the platelet plug into a fibrin clot.

Thrombopoietin is the main regulator of platelet production; megakaryocytes extend proplatelets into the bloodstream. Normal platelet counts range roughly 150-450 × 10^9/L. The spleen stores a fraction of platelets and can sequester them during certain conditions.

Clinical relevance: Thrombocytopenia (low platelet count) increases bleeding risk; thrombocytosis (high count) has thrombotic risk; platelet function disorders can cause bleeding even with normal counts. Drugs such as aspirin or P2Y12 inhibitors impair platelet function; antiplatelet therapy is used in cardiovascular disease. Platelet transfusions are used to treat significant bleeding or severe thrombocytopenia.