Lecture 8: Platelets in Health and Disease Flashcards
What are some symptoms of Platelet disorders
1) Mucocutaneous bleeding
2) Pertechiae (non blanching rash)
* Non-blanching rash (NBR) is a medical term used to describe a skin rash that does not fade when pressed with, and viewed through, a glass. It is a characteristic of both purpuric and petechial rashes.
3) Purpura
4) Hematoma
Describe the features of Megakaryoblasts and megakaryocytes
1) Megakaryoblast
- Large
- Doesn’t have much cytoplasm
- Have nucleioli.
2) Megakaryocyte
- A lot more cytoplasm
- Granular appearance
- Platelets are just thousands of cytoplasm that buds off (which is why platelets haev the granular appearance)
- After DNA replication granules and membrane formed
- Multinucleated
- Endomitotic synchornous nuclear replication
- 18-32 nuclei
- Located next to BM sinusoidal endothelial cells
- Filopodia extend into capillaries
- Each megakaryocyte -> ~4,000 platelets
Where are Megakaryocytes found?
Mature megakaryocyte is located adjacent to bone marrow sinusoidal endothelial cells and develops long filopodia (proplatelets) that extend into marrow capillaries.
These projections fragment to produce mature platelets (each megakaryocyte produces ~4,000 platelets).
Describe the process of megakaryocyte differentiation/maturation
Platelets are produced in bone marrow by fragmentation of cytoplasm of megakaryocytes.
- 1) Megakaryocyte matures by a process of endomitotic synchronous nuclear replication with enlargement of cytoplasm and i_ncrease in number of nuclei_ in multiples of 2.
- most commonly 18 or 32 nuclei.
- 2) After DNA replication has ceased, megakaryocytes begin to show evidence of cytoplasmic differentiation with production of components that constitute the mature platelet (e.g. fibrinogen, coagulation factor V, platelet factor 4, von Willebrand factor, platelet-derived growth factor and some platelet glycoproteins).
- 3) With continued maturation, the cytoplasm develops an extensive membrane system termed demarcation membranes, formed by invagination of the plasma membrane.
- Mature megakaryocyte is located adjacent to bone marrow sinusoidal endothelial cells and develops long filopodia (proplatelets) that extend into _marrow capillarie_s. These projections fragment to produce mature platelets (each megakaryocyte produces ~4,000 platelets).
- There is recent evidence that platelets may undergo limited replication in blood, utilizing pre-RNA for protein synthesis.
Platelets are produced in ______________ by ____________________________ of megakaryocytes.
Platelets are produced in bone marrow by fragmentation of cytoplasm of megakaryocytes.
What regulates platelet development/Thrombocytopoiesis?
Thrombocytopoiesis (Figure Above Right)
Major regulator of megakaryocyte and platelet development is thrombopoietin TPO.
Cytokines steel (stem cell) factor, leukaemia inhibitory factor (LIF), interleukin 6 and 11 (IL-6 and IL-11) also modulate megakaryocyte development.
Describe Platelet homeostasis
Platelet numbers in circulation are maintained at a constant level, normally ~1/3 platelets do not circulate but remain in spleen.
- Body requires ~40,000 new platelets / μl blood / day.
- Normal platelet life-span to be 7-10 days.
- Platelet consumption is by a combination of senescence and utilization in haemostatic reactions.
Describe the Platelet Ultrastructure
Platelet cells are discoid and contain an intricate system of channels continuous with plasma membrane.
-
Phospholipid membrane (canalicular) system
- __together with the plasma membrane present a large reactive surface for selective absorption of plasma coagulation proteins.
- Glycoproteins of the surface coat are important for the platelet reactions of adhesion and aggregation which form the initial events that lead to platelet plug formation during haemostasis.
- Submembranous area contains contractile filaments, and a circumferential skeleton of microtubules is responsible for the maintenance of the discoid shape.
Cytoplasm contains at least three distinct types of secretory granules.
- Electron-dense granules contain calcium and magnesium ions, ATP, ADP, and smaller amounts of other nucleotides plus several vasoactive amines, particularly serotonin.
-
Alpha granules contain coagulation factors (fibrinogen, factor V, von Willebrand factor), platelet-derived growth factor, TGF-__b__, heparin antagonist (platelet factor 4), thrombospondin, b-thromboglobulin, fibronectin and albumin.
- PDGF, TGF-b, PF-4, TS bind heparin, protect surface coagulation reactions.
- Lysosomes contain hydrolytic enzymes.
Describe the process of Primary haemostasis
Primary haemostasis is the process of forming a platelet plug at the site of vessel injury
1) The initial phase of the process is vascular constriction.
2) Next, platelets become activated and aggregate at the site of injury, forming a temporary, l_oose platelet plug. (unstable on its own)_
* Central to this function are reactions of platelet adhesion, aggregation, secretion and contraction, in addition to procoagulant activity of platelets.
Platelets in the flowing blood are not activated (roundish). They get pulled into the site of vascular injury, as they do this, they become activated which does several things
- Shape change where we get long filopodia
-
Release of granules
- Released as part of the platelet activation pathway (contain lots of components which lure in more platelets and activate more platelets)
-
Change in membrane surface to allow the blood clotting pathway
- This is triggered firstly by Von Willerbrand Factor- binds to exposed collagen
- __Key factor that is involved in anchoring down the palelets to form the loose plug.
- Binds to GP 1B-V-IX receptor on the platelets
- Membrane change involves exposure of another platelet receptor - Integrin-a-2-B3.
- There are a number of drugs that target this.
- This is triggered firstly by Von Willerbrand Factor- binds to exposed collagen
4) Granules
* (amplification process)
Describe the process of platelet activation and adherance to the membrane
- In circulation, platelets do not adhere to each other or to endothelium.
- Following blood vessel injury, platelets adhere to exposed subendothelial collagen.
- Von Willebrand factor (VWF) is required to support this adhesion.
-
VWF first adheres to subendothelial collagen fibres, leading to a conformational change in VWF, allowing binding to a specific receptor site (glycoprotein Ib) on platelets.
- Congenital lack of either VWF (von Willebrand’s disease) or GP1b on platelets (Bernard-Soulier syndrome) leads to a haemorrhagic diathesis characterised by abnormal platelet adherence.
*
- Congenital lack of either VWF (von Willebrand’s disease) or GP1b on platelets (Bernard-Soulier syndrome) leads to a haemorrhagic diathesis characterised by abnormal platelet adherence.
WHat are the names of 2 abnormalities where it leads to haemorrhagic diathesis characterised by abnormal platelet adherence?
Congenital lack of either VWF (v_on Willebrand’s disease_) or GP1b on platelets (Bernard-Soulier syndrome) leads to a haemorrhagic diathesis characterised by abnormal platelet adherence.
Describe the process of platelet aggregation after platelet activation
Most platelets that accumulate at sites of injury do not adhere directly to subendothelial structures, but adhere to each other in a process termed aggregation.
Full aggregation requires fibrinogen, ADP and thromboxane A2.
- It is believed that fibrinogen binds to receptors on activated platelets and links platelets to each other.
- Appearance of fibrinogen receptors is dependent upon ADP.
After platelets adhere to collagen, they become more spherical and extrude long pseudopods which enhance i_nteraction between adjacent platelets._
Prostaglandin synthesis is activated to produce thromboxane A2, which initiates platelet aggregation and release reaction.
Secretion of ADP, generation of thromboxane A2 and activation of coagulation reactions leading to production of thrombin, in turn result in recruitment of other platelets (20 aggregation), and ultimately plug formation that obstructs wound surface.
Upon this surface, a fibrin clot forms and contracts to seal the vessel wall or wound.
What are the steps required after vessel injury to produce a platelet plug? (General/Broad)
1) Platelet adhesion
2) Platelet activation
3) Platelet aggregation
What is the role of Thromboxane A2 and Prostacyclin?
What produces/synthesies these?
Platelet Secretion/Release Reaction
Thromboxane A2 (By Platelets)
The secretion of alpha and dense granule contents follows initial platelet aggregation. Collagen and thrombin activate platelet prostaglandin synthesis, forming thromboxane A2.
- Thromboxane A2 lowers platelet cAMP levels and initiates release reaction.
- Thromboxane A2 is not only potentiates platelet aggregation, but also has a powerful vasoconstrictive activity.
Prostacyclin (By Endothelial Cells)
- Vascular endothelial cells produce prostacyclin, which increases platelet cAMP, thus i_nhibit release reaction._
- This potent inhibitor probably prevents platelet aggregation on normal vascular endothelium.
Other Secretions (By Platelets)
Secretion of ADP and fibrinogen result in secondary aggregation and serotonin mediates vasoconstriction. ADP binds to G protein-coupled purinergic receptor P2Y12 on platelet surface, leading to aggregation.
What factors and drugs stimulate and what factors inhibit release of platelets?
Platelet Secretion/Release Reaction
1) Thromboxane A2 (By Platelets)
The secretion of alpha and dense granule contents follows initial platelet aggregation. Collagen and thrombin activate _platelet prostaglandin synthesi_s, forming thromboxane A2.
- Thromboxane A2 l_owers platelet cAMP levels_ and initiates release reaction.
- Thromboxane A2 is not only potentiates platelet aggregation, but also has a powerful vasoconstrictive activity.
2) Prostacyclin (By Endothelial Cells)
Vascular endothelial cells produce prostacyclin, which increases platelet cAMP, thus in_hibit release reaction._
This potent inhibitor probably prevents platelet aggregation on normal vascular endothelium.
3) Other Secretions (By Platelets)
Secretion of ADP and fibrinogen result in secondary aggregation and serotonin mediates vasoconstriction. ADP binds to G protein-coupled purinergic receptor P2Y12 on platelet surface, leading to aggregation.
Drugs Treatment
4) Aspirin inhibits (decreases) thromboxane A2 production by covalent _acetylation of cyclo-oxygenas_e. This enzyme modification is permanent. (Since platelets are incapable of protein synthesis, this lasts for the life of platelet.)
5) Clopidogrel binds P2Y12 receptor and irreversibly inhibits platelet aggregation.
What are the 2 receptors on platelets that are really important?
What do they bind to?
GP Ib-IX-V (binds to von Willebrand factor (VWF)- for activation of platelets)
Integrin A2B3 (A receptor for fibrinogen, vWF, fibronectin, and vitronectin, is absolutely required for platelet aggregation.)
Diagnose
no lymphadenopathy or hepatosplenomegaly suggest that the patient has no underlying malignancy
The 2 week history of epitaxis and bruising suggests that the cause of the illness is acute (not genetic)
Could be decreased production due to viral or drug causes (but unlikely because these should have affected the other blood counts too)
Diagnosis: Immune Thrombocytopenia (ITP)
- Immune response is sensitised to the platelets, so the antibodies stick to it
- Macrophages destroy the platelets (mainly in the spleen)
- Treatment:
- Thrombopoietin (TOP)-mimetics
- Or splenectomy
What is Immune Thrombocytopenia?
Immune Thrombocytopenia (ITP)
- Immune response is sensitised to the platelets, so the antibodies stick to it
- Macrophages destroy the platelets (mainly in the spleen)
- Treatment:
- Thrombopoietin (TOP)-mimetics
- Or splenectomy
