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Platelet Plug Formation


Damage to endothelial cells and exposure of collagen

Von Willebrand Factor (vWF) is released by damaged endothelial cells

vWF binds to exposed collagen.

Platelet Adhesion

  • Through following receptors, the platelet attaches to the exposed collagen:

    • Gp Ib ↔ vWF ↔ Collagen

    • Gp Ia/IIa ↔ Collagen

    • Gp VI ↔ Collagen

Platelet Activation & Release Reaction

  • Activation of the receptors → intracellular signaling events → release of alpha and dense granule.

  • Alpha (α) granules:

    • Growth factors → Wound healing

    • Von Willebrand factor → Platelet aggregation

    • Fibrinogen → Platelet aggregation and coagulation

    • Factor V → Coagulation

  • Dense granules:

    • ADP → Platelet aggregation

    • Serotonin → Vasoconstriction

    • Calcium → Coagulation

  • Enxyme COX-1 and thromboxane synthase synthesize TxA2 from membrane phospholipids TxA2 is released into the blood.

  • Parallel coagulation cascade on platelet surface → formation of thrombin → released into the blood.

  • Platelet changes shape (extend finger-like projection) → Increase surface area.

  • Conformational changes in Gp IIb/IIIa → binds with fibrinogen.

Platelet Aggregation

  • More platelets are activated by:

    • ADP ↔ P2Y12

    • TxA2 ↔ TP

    • Thrombin ↔ PAR-1 (Protease-activated receptor-1)

  • Activated platelets undergo similar changes as described above including:

    • Conformational changes in Gp IIb/IIIa → bind with fibrinogen which acts as bridge between two platelets. vWF can also serve as bridge between platelets.

    • More release reaction→ More platelet activation → So on… → in this way, platelets are recruited till the wound is plugged → this plug is called primary hemostatic plug or primary platelet plug → fibrin synthesized by coagulation strengthens it → called secondary plug.


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