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
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
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
- Formation of temporary plug
- 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