Haemostasis — Platelets

Platelet adhesion:

• Vessel wall subendothelium is exposed
• Platelets bind to subendothelium via
  • GpIa directly to collagen
  • GpIIa/IX via vWF attached to subendothelial microfibrils

Platelet activation:

• Collagen binding or presence of thrombin
• Thromboxane A₂ produced by COX-1
• Granules degranulate
• Dense granules release ADP and serotonin
• ADP binds to ADP receptors on external membrane
• Increased expression of GpIIa-IIIb
• Platelet also changes in morphology → long pseudopods

Platelet aggregation:

• ADP and thromboxane A₂ released from the first binding platelets
• Aggregation and activation of further platelets
• ADP → increased expression of GpIIa-IIIb
• Fibrinogen linkage via GpIIaIIIb
• Formation of a platelet plug
• Thrombin (from clotting cascade)  converts the fibrinogen into fibrin → stable plug

Factors increasing aggregation: Platelet activating factor

Factors inhibiting aggregation: Nitric oxide (via eNOS), Prostacycline (PGI₂)

Mooney 2016

Answer by Mooney