Platelet

[Ref: KB:p209; WG21:p533; PK1:p245-249]

Characteristics of platelet

• Lack nuclei
• 2-4 micrometer in diameter
• 150,000 to 300,000 per microliter of blood
  * Spontaneous haemorrhage does not occur until <5,000

Life cycle of platelet

Production

Pluripotent stem cells in bone marrow
--> Megakaryoblast
--> Masophilic megakaryocyte
--> Granular megakaryocytes
--> Mature megakaryocytes
--> Fragmentation of megakaryocytes
--> Platelets

Whole process takes 10 days

New platelets are stored in spleen for 36 hours before release

Elimination

Old platelets are removed by reticuloendothelial system in spleen and liver

Regulation

Main regulator of platelet production
= Thrombopoietin

• Produced by liver and kidney
• Facilitates megakaryocytes maturation

Thrombopoietin receptors

Thrombopoietin receptors are present on platelets

Thus low platelet count
--> Less thrombopoietin is bound
--> More thrombopoietin is available to stimulate platelet production

Vessel walls as well as platelets contain von Willebrand factor

vWF forms complex with factor 8
--> Regulation of factor 8 level

Life expectancy

• Halflife = 4 days
• Life span
  = 8-12 days [KB2:p209]
  = 7-10 days [PK1:p246]

Distribution

• 60% to 75% of platelets are in circulation
• Rest are mostly in spleen
  * Splenectomy causes increase in platelet count

Structures

• Lacks nuclei

Membrane

• Receptors for collagen, ADP, vessel wall von Willebrand factor, fibrinogen, thrombopoietin, thrombin
• Receptors for thrombin and ADP
  * Important for platelet activation
• Receptors for factors 8,9,10a,11
  * [Lecture notes]
• ABO and HLA class I antigens
• Glycoprotein

Receptors for thrombin

• Glycoprotein Ib/IX
  * Also a receptor for vWF
• Protease activated receptors (PAR)
• Possibly others

Membrane glycoprotein

[PK1:p248]

Glycoprotein Ia/IIa
= Facilitates adhesion to collagen

Glycoprotein Ib, IIb, and IIIa
= Required for attachment of platelets to vWF (thus the vascular subendothelium)
* especially GpIb/IX

Glycoprotein IIb-IIIa
= Binds to fibrinogen
= Required for platelet-to-platelet aggregation

Cytoplasm

Actin, myosin, glycogen, lysosomes, mitochondria

2 types of granules (dense granules and alpha-granules)

Dense granules

• Serotonin
• ADP

Alpha-granules

• Clotting factors (fibrinogen, factor 5, 8)
• vWF
• Platelet factor 4
• Beta-thromboglobulin
• Fibronectin
• Thombospondin (TSP)
• Platelet-derived growth factor (PDGF)
  * Also produced by macrophages and endothelial cells
  * Stimulates wound healing and potent vascular smooth muscle mitogen

Function of platelet

2 main functions

1. Formation of temporary plug
2. Serves as a surface on which coagulation factors can act (PF3)

See also Hemostasis

Other notes

ATP is mostly produced by anaerobic glycolysis
* But there is some aerobic metabolism