Factors involved in megakaryocyte development and platelet production
Sub-pathways within Pathway: Factors involved in megakaryocyte development and platelet production :
Factors involved in megakaryocyte development and platelet production:
Megakaryocytes (MKs) give rise to circulating platelets (thrombocytes) through terminal differentiation of MKs which release cytoplasmic fragments as circulating platelets. As MKs mature they undergo endoreduplication (polyploidisation) and expansion of cytoplasmic mass to cell sizes larger than 50-100 microns, and ploidy ranges up to 128 N. As MKs mature, the polyploid nucleus becomes horseshoe-shaped, the cytoplasm expands, and platelet organelles and the demarcation membrane system are amplified. Proplatelet projections form which give rise to de novo circulating platelets (Deutsch & Tomer 2006).
The processes of megakaryocytopoiesis and platelet production occur within a complex microenvironment where chemokines, cytokines and adhesive interactions play major roles (Avecilla et al. 2004). Megakaryocytopoiesis is regulated at several levels including proliferation, differentiation and platelet release (Kaushansky 2003). Thrombopoietin (TPO/c-Mpl ligand) is the most potent cytokine stimulating proliferation and maturation of MK progenitors (Kaushansky 2005) but many other growth factors are involved. MK development is controlled by the action of multiple transcription factors. Many MK-specific genes are co-regulated by GATA and friend of GATA (FOG), RUNX1 and ETS proteins. Nuclear factor erythroid 2 (NF-E2), which has an MK-erythroid specific 45-kDa subunit, controls terminal MK maturation, proplatelet formation and platelet release (Schulze & Shivdasani 2004). NF-E2 deficient mice have profound thrombocytopenia (Shiraga et al. 1999). MYB (c-myb) functions with EP300 (p300) as a negative regulator of thrombopoiesis (Metcalf et al. 2005). During MK maturation, internal membrane systems, granules and organelles are assembled. Cytoplasmic fragmentation requires changes in the MK cytoskeleton and formation of organelles and channels. Individual organelles migrate from the cell body to the proplatelet ends, with approximately 30 percent of organelles/granules in motion at any given time (Richardson et al. 2005).
The processes of megakaryocytopoiesis and platelet production occur within a complex microenvironment where chemokines, cytokines and adhesive interactions play major roles (Avecilla et al. 2004). Megakaryocytopoiesis is regulated at several levels including proliferation, differentiation and platelet release (Kaushansky 2003). Thrombopoietin (TPO/c-Mpl ligand) is the most potent cytokine stimulating proliferation and maturation of MK progenitors (Kaushansky 2005) but many other growth factors are involved. MK development is controlled by the action of multiple transcription factors. Many MK-specific genes are co-regulated by GATA and friend of GATA (FOG), RUNX1 and ETS proteins. Nuclear factor erythroid 2 (NF-E2), which has an MK-erythroid specific 45-kDa subunit, controls terminal MK maturation, proplatelet formation and platelet release (Schulze & Shivdasani 2004). NF-E2 deficient mice have profound thrombocytopenia (Shiraga et al. 1999). MYB (c-myb) functions with EP300 (p300) as a negative regulator of thrombopoiesis (Metcalf et al. 2005). During MK maturation, internal membrane systems, granules and organelles are assembled. Cytoplasmic fragmentation requires changes in the MK cytoskeleton and formation of organelles and channels. Individual organelles migrate from the cell body to the proplatelet ends, with approximately 30 percent of organelles/granules in motion at any given time (Richardson et al. 2005).
Hemostasis:
Hemostasis is a physiological response that culminates in the arrest of bleeding from an injured vessel. Under normal conditions the vascular endothelium supports vasodilation, inhibits platelet adhesion and activation, suppresses coagulation, enhances fibrin cleavage and is anti-inflammatory in character. Under acute vascular trauma, vasoconstrictor mechanisms predominate and the endothelium becomes prothrombotic, procoagulatory and proinflammatory in nature. This is achieved by a reduction of endothelial dilating agents: adenosine, NO and prostacyclin; and by the direct action of ADP, serotonin and thromboxane on vascular smooth muscle cells to elicit their contraction (Becker et al. 2000).
The chief trigger for the change in endothelial function that leads to the formation of a haemostatic thrombus is the loss of the endothelial cell barrier between blood and extracellular matrix components (Ruggeri 2002). Circulating platelets identify and discriminate areas of endothelial lesions; here, they adhere to the exposed sub endothelium. Their interaction with the various thrombogenic substrates and locally generated or released agonists results in platelet activation. This process is described as possessing two stages, firstly, adhesion - the initial tethering to a surface, and secondly aggregation - the platelet-platelet cohesion (Savage & Cattaneo et al. 2001).
Three mechansism contribute to the loss of blood following vessel injury. The vessel constricts, reducing the loss of blood. Platelets adhere to the site of injury, become activated and aggregate with fibrinogen into a soft plug that limits blood loss, a process termed primary hemostasis. Proteins and small molecules are released from granules by activated platelets, stimulating the plug formation process. Fibrinogen from plasma forms bridges between activated platelets. These events initiate the clotting cascade (secondary hemostasis). Negatively-charged phospholipids exposed at the site of injury and on activated platelets interact with tissue factor, leading to a cascade of reactions that culminates with the formation of an insoluble fibrin clot.