Platelets

Outline the physiological production of blood and its constituents

Describe the process and regulation of haemostasis, coagulation and fibrinolysis

Platelets are small cell fragments which are vital in haemostasis via forming a platelet plug. They:

• Have a lifespan of 7-10 days
• Are removed by the reticuloendothelial system in the spleen and liver

Production

Platelets are:

• Anuclear circulating cell bodies, which bud from megakaryocytes
  As the megakaryocyte cell volume increases, the cell membrane invaginates and small platelets bud off.
  • The time from stem cell to platelet is ~10 days, and is stimulated by thrombopoietin
• New platelets are held in the spleen for 36 hours until they mature

Contents

• α-granules
  Contain fibronectin, fibrinogen, vWF, PDGF, and Thrombospondin, platelet factor 4.
• δ-granules
  Contain 5-HT, ATP, ADP, and Ca²⁺.
• Contractile proteins
  Facilitate platelet deformation when activated.

Activation

• Platelets are activated by:
  • Collagen
    Exposed by damaged endothelium.
  • Adrenaline
  • ADP
  • Thrombin
• Activation results in several events:
  • Exocytosis of granules
  • Activation of membrane phospholipase A₂ to form thromboxane A₂
  • Deformation from a disc to a sphere with long projections
  • Promotion of the coagulation cascade
  • Change in glycoprotein (GP) expression by the action of ADP:
    ADP antagonists (e.g. clopidogrel) prevent expression of the GPIIb/IIIa complex.
    • GP Ib/IIb/IIIa facilitate platelet attachment to vWF
      vWF also binds to sub-endothelial connective tissue.
    • GP IIb/IIIa are also receptors for fibrinogen, which encourages platelet aggregation

References

1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
2. Krafts K. Clot or Bleed: A Painless Guide for People Who Hate Coag. Pathology Student.