Platelets Flashcards
What are platelets ?
- disc shaped cells that are anucleate
- complex internal structure
- main function = aggregate at any site of injury to vasculature
- lifespan = 8-14 days
How are platelets produced?
- produced in BM from Megakaryocytes
- primitive cell is called a megakaryoblast
- this undergoes a process of endomitotic replication - increases the DNA content & cytoplasmic volume
What is the ref range for platelets?
150 - 400 x 10^9 /L
Where are 30% of platelets stored?
- spleen of normal subjects
What hormone influences the production of platelets?
- Thrombopoietin
Where is Thrombopoietin produced?
- mainly produced in liver
- small amount synthesised in kidney & BM
How many platelets can a single megakaryocyte produce?
- between 2000 - 7000 platelets
Give some structures of a platelet
- glycocalyx
- specific a- granule
- plasma membrane
- lysosome
- mitochondrion
- dense tubular system
- canalicular system
Describe the platelets plasma membrane
- series of glycoproteins & integrins
-these allow for the stimulation of platelets & interactions with other proteins such as fibrinogen & Von Willebrand’s Factor (vWF)
Give some examples of alpha-granules
- Factor V
- Platelet Factor 4
- Fibrinogen
Give some examples of Dense Bodys
- ADP
- ATP
- Serotonin
- Calcium
Describe the Canalicular System
- when platelets degranulate they release their contents externally through tubular system
- platelets respond to many stimuli; collagen, ADP, adrenaline & thrombin
- induce shape change, degranulation & aggregation
What are some platelet functions?
- adhesion
- shape change
- release of granule contents
- aggregation
Describe Adhesion in relation to platelets
- In vivo damage to vessel wall causes exposure of sub endothelial fibres
- platelets rapidly cover the damaged area
- adhesion to this site of damage is dependent upon; platelet integrins & adhesion molecules
Describe shape change in relation to platelets
- platelet adhesion is accompanied by change in shape
- transform from discoid to one with irregular projection
- early stages are reversible
- as time proceeds along with granule releases it becomes irreversible
Describe aggregation in relation to platelets
- many factors can initiate platelet aggregation; ADP, collagen, thrombin
- platelets clump together to form a primary plug
- primary plug is unstable & if not reinforced breaks down results in bleeding re-occurring
- platelet plug is normally reinforced with fibrin
Describe Thromboxane A2
- important prostaglandin formed & released by platelets
- main function = act as an autocrine & paracrine fashion to stimulate platelet aggregation
- acts by binding to a receptor & decreases intracellular cAMP that encourages platelet aggregation
Describe the impact of aspirin
- aspirin inhibits Cyclooxygenase-1
describe the synthesis of TXA2
phospholipid
- (phospholipase A2)
Arachidonic acid
- (Cyclooxygenase)
Endoperoxides
- (thromboxane synthase)
Thromboxane A2
Describe the role of endothelial cells
- synthesise factors to antagonise/inhibit platelet aggregation
- Prostacyclin I2 = Inhibits platelet aggregation & increases intracellular cAMP & reducing Ca2+ fluxes within the platelet
- nitric oxide = inhibits platelet aggregation
