Haematology

Question 1

Outline the formation, structure, and function of the platelet

Answer 1

Formation
- PHSC –> CFU(meg) –> megakaryocytes –> platelets.
○ Time for production ~10days.
○ Stored in spleen for 36hrs, then released in circulation for ~7-10days (lifespan)
○ Old platelets are removed by the RES
○ Production is controlled by thrombopoeitin (produced in liver or kidneys) - inflammation or decreased platelet count increases thrombopoeitin production

	_Structure_
	-  Platelet structure - disc shaped cells ~ 2-3um diameter. They lack a nucleus but contain the following:
	    ○ External glycocalyx
			○ Platelet membrane with invaginations forming open canalicular system
			    --> conatins important glycoproteins (eg. GPIa facilitates binding to collagen. GPIb, IIb, IIIa are required for attachment to vWF; GP IIb/IIIa important for fibrinogen)
			○ Mitochondria 
			○ Alpha Granules - contain beta-thomboglobulin, fibronectin, fibrinogen, vWF
			○ Dense granules - electron dense core. Contain serotonin, ATP, ADP
			○ Lysosomes, peroxisomes
			
_Function_
- Adhesion - to surface collagen via vWF
- Aggregation - (platelet to platelet) mediated by fibrin + vWF
- Activation - degranulation, shape change, phosphatidylserine exposure on platelet surface (essential for clotting factor binding)
- Amplification - activation of intrinsic pathway by available thrombin
- Propagation - platelet-bound factors VIIIa, Va, X activate thrombin - "thrombin burst". Allows for generation of lge amts fibrin
- Contraction - later stages of clot maturation