Hemostasis - Platelet Formation and Function Flashcards
1. The student will be able to accurately summarize the steps involved in normal platelet biogenesis. (MKS 1a) 2. The student will be able to locate the sites of normal platelet production and discuss the major determinants of platelet life-span and fate. (MKS 1a) 3. The student will be able to explain the major features of platelet function and how platelets contribute to hemostasis. (MKS 1a) 4. The student will be able to evaluate altered platelet functions in congenital & acquired platelet
1
Q
What are platelets? Describe their life cycle. What are some diseases that affect platelet function?
A
- Platelets are small anucleate cells that serve as a first defense to prevent hemorrhage following blood vessel injury
- Platelets normally survive in the circulation for approximately 10 days and are primarily cleared in the liver following activation of platelet apoptosis
- A small proportion of platelets are also consumed randomly in normal hemostatic processes
- Disease states may increase the proportion of platelets cleared by non-apoptotic mechanisms, such as in immune thrombocytopenia (ITP)
- In acute leukemia and severe liver disease, thrombocytopenia occurs as a major complication
- Treatment with chemotherapy and hematopoietic stem cell transplantation also can lead to severe thrombocytopenia
2
Q
What are megakaryocytes? What is their role in platelet generation?
A
- Platelets are released into the circulation from highly specialized precursor megakarocytes (MKs)
- Approximately 1011 platelets are normally produced in the body each day
- MKs develop from CFU-GEMM committed progenitor cells in an orderly process of differentiation that yields the mature MK
- MKs undergo a process of endomitosis which leads to polyploidization, gene amplification and increased protein synthesis which is essential for efficient platelet production
- Polyploidization results from a modified mitotic process which involves a block in anaphase and the cell does not enter telophase and cytokinesis
- The MK then undergoes cytoplasmic maturation and takes on a very distinct morphologic ultrastructural appearance with the development of the demarcation membrane system (DMS)
- The DMS is an extensive cytoplasmic network of membrane channels composed of tubules and flattened cisternae
- The exact function of the DMS is still debated, but evidence indicates that it most likely provides reserve membrane for proplatelet formation and extension
- MKs also exhibit specialized alpha and dense granules (also found in platelets) and a dense tubular system where prostaglandin synthesis occurs
3
Q
What are proplatelets and what is their role in platelet generation?
A
- Platelets are formed from specialized proplatelet extensions of the MK cytoplasm
- Proplatelets are long, thin pseudopod-like cytoplasmic processes that extend from the MK through bone marrow endothelial junctions into blood sinuses
- Proplatelets fragment to generate individual platelets and platelet precursor forms
- The slender proplatelet tubules bend and branch and the cytoplasmic bridges between the platelets rupture and release individual platelets into the circulation
4
Q
What is the role of the platelet cytoskeleton in platelet function?
A
- Cytoskeletal actin and tubulin play a crucial role in the morphologic changes of the MK leading to proplatelet formation
- Actin-myosin motors provide the propulsive forces for proplatelet branching needed for generating thousands of platelets from each maturing megakaryocyte
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Microtubules formed in the cytoskeleton reorganize into bundles and provide the propulsive force for proplatelet elongation
- The microtubule bundles also form loops that form the microtubule coils in blood platelets
- Proplatelet elongation is controlled by microtubule sliding regulated by the dynein motor protein
- Microtubules in the proplatelets also serve as tracks for the transport of membrane components, organelles and granules into proplatelets and assembling platelets
- The microtubule motor kinesin provides the force to transport these components along the microtubules
- MK membrane cytoskeletal spectrin tetramers also have an essential role to generate the DMS, proplatelet production and in late stages of platelet formation
5
Q
What are some other places outside of the bone marrow that platelet formation can take place?
A
- Platelet formation can also occur outside the bone marrow, most notably in the lung and in the blood
- Approximately five to 20% of platelet mass in the plasma may come from large MK fragments
- Larger MKs that gain access to the circulation may arrest in the pulmonary circulation and form 10% or more of the circulating platelets
6
Q
What are some determinants of the platelet lifespan?
A
- Platelet life-span is controlled by intrinsic factors involving limited inner reserves and the extrinsic demands of the body
- A “multi-hit” model postulates that platelets encounter aggregation events or “hits” during their normal circulation and function, leading to their clearance near the end of their functional life-span
- Platelets are then cleared when they have accumulated a certain number of “hits”
- Recent data suggests that cell-intrinsic mechanisms involving apoptosis also regulate platelet survival and life-span
- Platelet survival depends upon the pro-survival factor bcl-xL which has a short half-life and may function as a molecular clock
- Bax and especially bak are essential mediators of platelet apoptosis
- Bak activation may occur by a loss of bcl-xL or by activation of BH3-only proteins
7
Q
What are some markers of platelet apoptosis?
A
- Platelets undergoing apoptosis express phosphatidylserine on their surface which may mark the platelets for engulfment by macrophages
- Also, platelet carbohydrate antigens are normally capped by sialic acid which is later removed during platelet aging
- Loss of sialic acid from platelet carbohydrates leads to terminal galactose residue exposure which causes platelets to bind to hepatocytes and macrophages through Ashwell-Morell receptors and then be removed from the circulation
- Activation of Bak and bax may trigger these platelet surface changes and platelet clearance from the circulation
8
Q
How is thrombocytopenia managed?
A
- Platelet transfusions are used to correct thrombocytopenia and prevent life-threatening bleeding
- The supply of platelets for transfusion is limited and is further complicated by alloimmunization which can limit the effectiveness of platelet transfusions in many patients
- To overcome shortages in platelet supply, some investigators are working to cultivate platelets in vitro
- Much progress has been made in understanding normal platelet biogenesis and generation of megakaryocytes from hematopoietic stem cells in culture
- A major hurdle is to find conditions that will yield therapeutic quantities of platelets from in vitro cultures
9
Q
Describe the morphology of a platelet.
A
- Resting platelets are discoid anucleate cells 2-3 mM in diameter
- A microtubule coil helps maintain the platelet discoid shape
- Prominent features include mitochondria and glycogen granules for energy metabolism
- Alpha granules and dense granules which are released upon activation
- The surface connected canalicular system provides easy release of granule contents and excess platelet membrane during shape change
- Upon activation, the platelet changes shape from discoid to round and then with many projections including lamellipodia (leaf-like) and filopodia (foot-like) extensions
- The activated platelet expels its granule contents
10
Q
Describe the phases of platelet activation.
A
- Adhesion – binding to the subendothelial matrix
- Activation – change from quiescent state to promote hemostasis
- Secretion – release of granule contents
- Aggregation – binding to other platelets through active GPIIbIIIa
11
Q
What is the responding cascade to blood vessel injury?
A
- Collagen exposure
- Exposure of von Willebrand factor that binds platelet receptors
- Thrombin generation
- Decreased endothelial production of CD39 ecto-ADPase
- Endothelin release (vasoconstriction)
12
Q
What happens to platelets when the endothelial lining of a vessel wall is disrupted?
A
- Platelet adhesion to the subendothelial matrix components collagen and von Willebrand factor occur upon loss of the endothelial lining of the vessel wall
- Primary platelet adhesion (via GP Ib-IX-V Complex) to the subendothelium is promoted by binding to von Willebrand factor under high shear stress, high flow conditions.
- Platelet binding to collagen and von Willebrand factor stimulates platelet activation
13
Q
What are the platelet membrane glycoproteins and receptors? What are their functions?
A
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GP IIb/IIIa (aIIb/b3)
- Fibrinogen/von Willebrand Factor (Aggregation)
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GP Ib-V-IX
- von Willebrand Factor (Adhesion)
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GP Ia-IIa
- Collagen (adhesion)
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GP VI
- Collagen (activation)
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P-Selectin
- Platelet-leukocyte interactions
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PECAM-1
- Cell-Cell Interaction
14
Q
What are some agonists that promote platelet activation?
A
- Platelet activation is caused by several agonists including ADP, thromboxane A2, collagen, thrombin, epinephrine and others.
- Endothelium-dependent platelet inhibitors include Prostacyclin (PGI2) which stimulates platelet cyclic AMP and Nitric Oxide (Endothelium-Derived Relaxing Factor (EDRF)), which stimulates cyclic GMP
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Platelet activation leads to:
- Platelet shape change
- Thromboxane A2 generation and release
- Release of granule contents – recruits and activates additional platelets
- Inside-out signaling presenting the active fibrinogen binding form of GPIIbIIIa receptors on the platelet surface
- Exposure of an anionic phospholipid surface on the outer leaflet of the plasma membrane which promotes the assembly of factor tenase and prothrombinase complexes
15
Q
Describe the process of thromboxane A2 generation.
A
- Thromboxane A2 produced by activated platelets is a potent agonist for platelet activation and aggregation
- Aspirin irreversibly acetylates and inhibits cyclooxygenase, an enzyme necessary for the generation of Thromboxane A2
