Platelets (Thrombocytes): Clot Formation, Vessel Repair, and Lifecycle

Platelet Biology and Hemostasis

Platelets are anucleate cell fragments (2–3 µm) derived from megakaryocytes in bone marrow. Lifespan: 7–10 days. Normal count: 150–450 × 10⁹/L.

Primary Hemostasis (Platelet Plug)

1. Adhesion: vWF bridges exposed collagen (subendothelium) ↔ platelet GPIb-IX-V.
2. Activation: Thrombin, ADP, TxA₂, collagen → shape change, granule release, GPIIb/IIIa activation.
3. Aggregation: Fibrinogen bridges activated GPIIb/IIIa on adjacent platelets → plug.

Secondary Hemostasis & Repair

• Platelets provide phospholipid surface (PF3) for coagulation factor assembly → thrombin burst → fibrin mesh stabilizes plug.
• Release PDGF, TGF-β, VEGF, FGF → stimulate endothelial proliferation, smooth muscle migration, collagen synthesis → vessel wall repair.

Clinical Notes

• Thrombocytopenia (<100k) → bleeding risk (petechiae, mucosal).
• Thrombocytosis (>450k) → thrombotic risk (essential thrombocythemia, reactive).
• Antiplatelet drugs (aspirin, P2Y12 inhibitors) target activation pathways.