Platelets & Thrombopoiesis PDF

L 38 Platelets & thrombopoiesis ILOs By the end of this lecture, students will be able to 1. 2. 3. 4. Outline stages of thrombopoiesis. Interpret functional significance of some ultra-structural features of platelets. Identify the hemostatic mechanisms in response to injury Correlate the platelets a...

L 38 Platelets & thrombopoiesis ILOs By the end of this lecture, students will be able to 1. 2. 3. 4. Outline stages of thrombopoiesis. Interpret functional significance of some ultra-structural features of platelets. Identify the hemostatic mechanisms in response to injury Correlate the platelets activation to platelets plug formation Platelets Platelets are small, biconvex disk-shaped, non-nucleated cell fragments derived from megakaryocytes in the bone marrow. There are between 250,000 and 400,000 platelets per mm3 of blood, each with a life span of nearly 10 days. Platelets Production [ THROMBOPOIESIS] (Fig. 1&2) In adults, platelets originate in the red bone marrow by fragmentation of the cytoplasm of mature megakaryocytes that arises by differentiation of a large cell called megakaryoblast. 1. Megakaryoblast A large cell with basophilic cytoplasm. The nucleus becomes highly polyploid (i.e. it contains up to 30 times as much DNA as a normal cell) before platelets begin to be formed. 2. Megakaryocyte It is a large cell with an irregularly lobulated nucleus, coarse chromatin, and no visible nucleoli (Fig 2 A & B). The cytoplasm contains numerous mitochondria, a well-developed rough endoplasmic reticulum, and an extensive Golgi complex. Abundant granules arise from Golgi complexes. With maturation of the megakaryocyte, numerous invaginations arise from the plasma membrane and branch throughout the cytoplasm, forming the demarcation membranes (Fig 2C). The demarcation membranes outline areas of the cytoplasm that will be fragmented forming the platelets that will be extruded into the circulation 1 Platelet Structure: (Fig. 3) From outside-in, platelets are composed of the following: A Cell Coat of 15–20 nm thick, rich in glycosaminoglycan and lies to the outside of Plasmalemma. The latter contains polymorphic glycoprotein molecules [GPs] that interact with vessel wall components and other molecules involved in platelet adhesion. A Cytosol that appears by light micrographs, to display a peripheral clear region, [Hyalomere], and a central darker region, [Granulomere]. A. The Hyalomere; By electron microscopy it displays the following: ▪ Cytoskeleton, comprised of 10 to 15 microtubules arranged parallel to each other and forming a ring under the plasmalemma. The microtubules assist platelets in maintaining their discoid morphology. They are associated to Actin and Myosin filaments that serve in platelet movements then retraction in the context of exerting its haemostatic functions. ▪ A tubular system, comprised of 2 tubular sets, a surface-opening one and a dense one. -The open canalicular system (OCS) connects to invaginations of plasmalemma. It permits entry of external elements to platelets and release of its granule contents to exterior. It is also a site of storage of GPs and facilitates filopodia formation during shape change and activation. - The dense tubular system is a closed dense irregular channel network of residual endoplasmic reticulum, primarily involved in Ca sequestration during resting states and release upon activation. B. The Granulomere; By electron microscopy it displays small number of mitochondria, glycogen deposits, peroxisomes. It houses a system of enzymes that permits platelets to catabolize glycogen, consume oxygen, and generate ATP. It displays three types of granules: ▪ Alpha granules(a-granules) that contain growth Factors (PDGF, VEGF, TGF, PF4), adhesive Proteins (vWF, Fibronectin, Thrombospondin) - Coagulation Factors (Fibrinogen, Factor V, VII, and IX, XIII) - Anticoagulation Factors (PAI-1, TFP). Beyond hemostasis these mediate other functions of platelets as inflammation, repair and angiogenesis. ▪ Delta granules (δ-granules) contain hemostatically active molecules, which are secreted during vasoconstriction such as, serotonin, and during platelet activation such as, ADP, ATP, Ca, Mg and pyrophosphate. 2 ▪ Lambda granules (λ-granules) are actually lysosomes that contain hydrolytic enzymes that helps in clot resorption. Figure 3. Ultra structure of the platelet Platelet Function: In relevance to hemostasis: After injury to the vessel wall, platelets deaccelerate by tethering within second to the site of injury. Its role in haemostatic process involves several steps: (Figs. 4 & 5) 1. Platelet Adhesion: This presents the binding of some plasmalemmal platelet glycoprotein (GP) to the collagen (GP VI) and vWF (GPIb) in the exposed subendothelial surface to form a loosely adherent covering platelet layer. 2. Platelet Shape Changes: The thrombin (formed by the coagulation cascade) begins to act on proteinase receptors (PARs R) on platelet surface. The platelets lose their discoid shape becomes irregular and send out filopodia to permit better adherence to underlying sub-endothelium and overlying recruited platelets. Extracellular Ca also begins to influx triggering more intracellular Ca release which starts activation and platelet degranulation. 3. Platelet Activation (degranulation) 3 Figure 4. Platelets during hemostasis Shape Change Activation TXA2 Activated platelets release TXA2 by the action of COX1 on arachidonic acid of exposed platelet phospholipids. The alpha and dense granules also degranulate, especially releasing their haemostatic contents as serotonin, ADP, fibrinogen, other coagulation factors and Ca. The released TXA2 and ADP act in an autocrine manner on their expressed receptors on platelet surface, and together with other activators (agonists) as thrombin and epinephrine they amplify the common final pathway of the platelet activation, by increasing its GPIIbIIIa receptors affinity. 4. Platelet Aggregation: With more and more platelet recruitment, each two adjacent platelets through the activated GPIIbIIIa receptors bind together by fibrinogen bridges. This amplification step creates a bigger platelet sealing plug in which platelets are irreversibly bound but still the plug is not that stable and can be dislodged. This represents the Primary Haemostatic Plug. 5. Clot Retraction and Stabilization: Before 24 hours have passed, The actin, myosin and ATP of platelets contract. The fibrinogen holding platelets would become activated to fibrin (by thrombin) and then cross-link and polymerize (by activated Factor XIII) to finally stabilize and cement the plug preventing its dislodgment. This represents the Secondary Haemostatic Plug. 4 Figure 5. Steps of Platelet activation CLINICAL RELEVANCE When platelets rich thrombi develop pathologically as in acute myocardial infarction and stroke, the use of drugs that inhibit the platelets’ haemostatic role to halt disease progression has become an essential therapeutic strategy. Such ANTIPLATELET AGENTS target the platelet agonists involved in the activation and aggregation phases of platelet action. They include: 1. Thromboxane (TXA2) inhibitors; inhibiting COX1 2. Purinergic (P2Y12) Inhibitors; targeting ADP receptors 3. Glycoprotein IIb/IIIa Antagonists; targeting GP IIb/IIIa receptors 4. Protease-activated receptor-1 Antagonists; Targeting PAR-1 thrombin receptors.. 5