Learning Objective

Understand the structure, function, receptors, and granule contents of platelets (thrombocytes) and their role in primary hemostasis.

Thrombocytes (Platelets)

Platelets are small, anucleate cytoplasmic fragments derived from megakaryocytes in the bone marrow. They play a critical role in primary hemostasis by forming the initial platelet plug at sites of vascular injury. The average lifespan of a platelet is approximately 8–10 days, and about one-third of the total platelet pool is stored in the spleen.

Following endothelial injury, platelets become activated and undergo adhesion, aggregation, and secretion. Activated platelets adhere to exposed subendothelial collagen through interaction with von Willebrand factor (vWF) via the GpIb receptor. Platelet aggregation is mediated by fibrinogen binding to the GpIIb/IIIa receptor, allowing platelets to link together and form a platelet plug.

Platelets contain 2 major types of granules:

• Dense granules (CASH):
  • Calcium (Ca²⁺)
  • ADP
  • Serotonin
  • Histamine
• Alpha (α) granules:
  • von Willebrand factor (vWF)
  • Fibrinogen
  • Fibronectin
  • Platelet factor 4

Production of platelets is regulated by thrombopoietin, which stimulates megakaryocyte proliferation and differentiation in the bone marrow.

A decrease in platelet number (thrombocytopenia) or impaired platelet function commonly presents with petechiae, reflecting defects in primary hemostasis.

Activity