Primary hemostasis Flashcards
What are megakaryocytes? What is the platelet normal range? What is the individual variability? How long do platelets survive? What percentage of platelet mass is regenerated daily? What about in times of crisis? What are the 3 stages of megakaryopoiesis?
• Megakaryocytes = bone marrow cells that produce platelets
– Platelet normal range: 150,000-400,000/microliter
• 30K/microliter individual variability
• Platelet survival is 7-10 days
• Regenerate 10-13% of platelet mass daily
– Up to 10-fold increase in production in times of increased demand
What are the 3 stages of megakaryopoiesis?
– Proliferation – Differentiation – Maturation
What are 6 cytokines involved in the proliferative stage of megakaryopoiesis? What is the morphology of the cell at this point?
- Stem cell factor
- Interleukin 3
- Interleukin 6
- Interleukin 11
- Granulocyte-macrophage colony stimulating factor
- Thrombopoietin
• Morphology is not specific for megakaryocyte at this
phase
What are the cytokines involved in the differentiation stage of megakaryopoiesis? What kind of growth begins to occur at this point? Explain. What is the cell like morphologically?
- Interleukin 6
- Interleukin 11
- Thrombopoietin
- Endomitosis = nuclear division without cytoplasmic division – At this point, DNA content is 4-8N without nuclear segmentation.
- Basic morphologic features appear abundant cytoplasm with granules
What cytokine is involved in the maturation stage of megakaryopoiesis? What kind of growth is occuring? To what extent? How many platelets can a mature megakaryote produce before apoptosis?
- Thrombopoietin
- Continued endomitosis with numerous nuclear lobes
- DNA content is 16-128N
- Each mature megakaryocyte can produce thousands
of platelets before undergoing apoptosis.
What is the structure of thrombopoietin like? What is it similar to? What is its function? What happens if it is administered? Where is TPO produced at baseline? In times of need (thrombocytopenia)? What is the relation b/w the TPO levels and platelet counts? What gene encodes the TPO receptor?
- 35kD protein with two domains
- Structurally very similar to erythropoietin
• Stimulates megakaryocyte development, but no effect on terminal release of platelets
– 3-5 day lag in platelet count response after administration of recombinant TPO
• Produced in liver and kidney; spleen and bone marrow
(thrombocytopenia) – TPO levels inversely related to platelet counts
• Receptor is encoded by c-mpl gene
Describe the basic morphology of a platelet. Describe the 3 parts of the membrane skeleton of a platelet. What are their functions? What granules and organelles exist in platelets?
• Granular flat discs, no nucleus (average 3.5 um in diameter)
• Membrane Skeleton
– Cytoskeleton (microtubules)
– Open canalicular system (connected to membrane channels—>activation of platelet)
– Dense tubular system (ER like-activation of platelet)
• Platelet Granules and Organelles
– Alpha granules
– Dense (delta) granules
– “Typical” organelles: mitochondria, lysosomes, peroxisomes
What are the 3 categories things located in alpha granules? What 4 things are inside of delta granules?
Adhesive proteins, growth modulators, and coagulation factors
- Serotonin
- ADP
- ATP
- Calcium
What is the phospholipid bilayer of a platelet like? What happens when it’s activated? Give 3 examples of adhesive membrane glycoproteins? What are they called? What are they like? What is their function?
• Phospholipid bilayer – asymmetrical – Upon platelet activation, phosphatidylserine (negative charge) is flipped onto the outer surface of the membrane thrombogenic
surface
• Adhesive membrane glycoproteins:
– GP IIb/IIIa (A.k.a. alpha IIb beta 3)
• Integrin; most importantly binds fibrinogen
– GP Ib/IX (A.k.a. GP Ib/IX/V)
• Leucine-rich glycoprotein; most importantly binds vWF
– GP Ia/IIa
• Integrin that binds collagen
– Many other surface proteins exist…
Name 4 platelet membrane activation receptors. Describe one of them. Describe one inactivation receptor.
• Protease Activated Receptors (PAR-1)
– Thrombin receptor
– Transmembrane domains linked to G proteins downstream signaling and platelet activation after
thrombin binds
- Epinephrine receptor
- Thromboxane A2 receptor
- ADP/ATP receptors
- Inactivation receptors (main one binds PGI2 [prostacyclin])
Name 3 platelet activation pathways and whether they are stimulatory or inhibitory. Generally, how do they all work?
- Phospholipase C pathway (stimulatory)
- Phospholipase A2 pathway (stimulatory)
- Adenylyl cyclase pathway (inhibitory)
- All three pathways are coupled to G proteins
What is the end product of the phospholipase C pathway? What stimulates it?
– End product: platelet shape change, granule release, and exposure of GP IIb/IIIa – See figure in the syllabus
Stimulated by thromboxane A2, thrombin, and collagen
How is the phospholipase A2 pathway activated? Name the important steps in the pathway. What is the end product? What does the end product do?
- Activated with exposure to agonists (thromboxane A2, thrombin, collagen—same as PLC)
- Stimulates release of arachidonate from phospholipids
- Cyclooxygenase pathway (COX) metabolizes arachidonate with an end product of thromboxane A2 (TxA2)
- TxA2 activates neighboring platelets and stimulates vasoconstriction
Describe how 3 different drugs inhibit cyclooxygenase.
• Aspirin irreversible inhibition of COX no
TxA2 synthesis for remainder of platelet’s life
• NSAIDs (i.e. ibuprofen) reversible inhibition of COX
• COX-2 selective inhibitors no inhibition of
platelet function – COX-2 involved in inflammation (leukotriene response)
Name some steps in the adenylate cyclase pathway. What is the result? Name something that inhibits adenylyl cyclase. name something that increases it.
- ATP is converted to cAMP by adenylyl cyclase
- cAMP –>reduced platelet responsiveness to agonists
- Thrombin inhibits adenylyl cyclase
- Prostaglandin I2 (PGI2) – secreted by endothelial cells – increases adenylyl cyclase activity–> inhibiting platelet activation
What steps of hemostasis is von-willebrand factor involved in? What is it comprised of? Where is it synth. and stored? Where is it secreted from and when?
- Involved in both primary and secondary hemostasis
- Comprised of multimers of varying sizes
• Synthesis and storage in endothelial cells (Weibel-
Palade bodies) and megakaryocytes (alpha granules)
• Secretion: – Platelets – only during activation – Endothelial cells – active release from W-P bodies
in response to thrombin, histamines, and DDAVP
