Role of platelets in haemostasis: structure, activation, aggregation and endothelium inhibitor effets

Question 1

Describe the structure of platelets

Answer 1

• they are small cells
• they have no nucleus
• they do have other membrane enclosed organelles
• they contain alpha and dense granules

Question 2

where & how are platelets formed?

Answer 2

they are formed in the bone marrow by budding off from the cytoplasm of large cells called megakaryocytes

Question 3

What do the dense (delta) granules in platelets contain?

Answer 3

• Ca2+, ATP, ADP and seratonin

Question 4

what do alpha granules in platelets contain?

Answer 4

they contain a number of clotting proteins such as factor V, factor VIII and Von Willebrand factor

Question 5

What does a injury to a blood vessel expose?

Answer 5

• an injury to a blood vessel disurpts the endotheium and exposes the collagen fibres

Question 6

How do platelets bind to collagen?

Answer 6

• they** bind via Von Willebrand factor**
• VWF binds to collagen, changes its conformation which allows VWF to bind to collagen
• VWF acts to form a bridge between the damaged blood vessel and platelet
• platelets can then bind to collagen via glycoprotein Vi receptors (collagen receptors)

Question 7

What do the platelets release once they bind to collagen & what effects do these molecules have?

Answer 7

• the platelets release the alpha and dense granules into the extracellular space
• the dense granules release ADP and serotonin which act locally to produce multiple changes in metabolism, shape and surface proteins on the platelets
• platlets express ADP receptors, so ADP stimulates plate activation

Question 8

What molecule is synthesised by the platelets when they adhere to the collagen?

Answer 8

thromboxane A2
this is released into the ECF which also stimulates platelet activation

Question 9

what specific membrane transporters on the platelets are important for the platelet aggregation process? what role do they have?

Answer 9

• integrin transmembrane receptors
• they **bind to fibrinogen **
• fibrinogen acts to form bridges between the aggregating platelets and therefore leads to the formation of a platelet plug

Question 10

What molecules does the endothelium release in order to inhibit platelet activation?

Answer 10

• prostacyclin
• nitric oxide

Question 11

How does prostacyclin reduce platelet activation?

Answer 11

• prostaglycin binds to a g protein coupled receptor on the platelet
• this binding results in an inhibitory cAMP signaling that reduces platelet activation