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Although platelets are the smallest cells in the blood, they are implied in various processes ranging from immunology and oncology to thrombosis and hemostasis. Many large-scale screening programs, genome-wide association, and "omics" studies have generated lists of genes and loci that are probably involved in the formation or physiology of platelets under normal and pathologic conditions. This creates an increasing demand for new and improved model systems that allow functional assessment of the corresponding gene products in vivo. Such animal models not only render invaluable insight in the platelet biology, but in addition, provide improved test systems for the validation of newly developed antithrombotics. This review summarizes the most important models to generate transgenic platelets and to study their influence on platelet physiology in vivo. Here we focus on the zebrafish morpholino oligonucleotide technology, the (platelet- specific IntroductionBlood platelets play part in a myriad of processes, such as inflammation, tumor growth and metastasis, immunology and, of course, thrombosis and blood clotting where they provide a first and crucial line of defense against vascular injury, thus maintaining normal hemostasis. 1,2 Primary hemostasis starts when platelets recognize a site of vascular injury where the subendothelial matrix is exposed, bind to collagen, and become activated. 3 The subsequent rise in intracellular calcium triggers conformational changes in integrin receptors, degranulation, exposition of a procoagulant surface, and generation and release of secondary agonists resulting in a thrombus that will cover the site of injury and prevent further blood loss. 4 Platelets are furthermore an important factor in thrombotic events, such as stroke and myocardial infarction. 5 To identify more proteins regulating platelet function that may serve as new targets for the development of anti-thrombotics or in the prevention of bleeding, the platelet research community has seen the completion of several large-scale screening programs and the spectacular rise in the "platelet-omics" field. Several genome-wide association studies and subsequent meta-analysis in patients with coronary artery disease and healthy volunteers identified numerous genetic loci that are possibly involved in regulating platelet formation, count, volume, and function and might confer a risk for coronary artery disease. [6][7][8][9][10][11] On the other hand, gene expression profiling of healthy volunteer platelets, in combination with comparative microarray analysis between in vitro differentiated megakaryocytes (MKs) and closely related cell types, established a comprehensive platelet transcriptome. 6,[12][13][14][15][16][17][18][19] Finally, advanced proteomics studies identified proteins of the platelet sheddome, secretome, interactome, kinome, and phosphoproteome potentially involved in platelet function. 20 The overall result is a large number of newly identified gene products for which we are only beginning to understand their ...

Xenotransplantation systems have been used with increasing success to better understand human hematopoiesis and thrombopoiesis. In this study, we demonstrate that production of human platelets in nonobese diabetic/severe combined immunodeficient mice after transplantation of unexpanded cord-blood CD34 ؉ cells was detected within 10 days after transplantation, with the number of circulating human platelets peaking at 2 weeks (up to 87 ؋ 10 3 /L). This rapid human platelet production was followed by a second wave of platelet formation 5 weeks after transplantation, with a population of 5% still detected after 8 weeks, attesting for long-term engraftment. Platelets issued from human hematopoietic stem cell progenitors are functional, as assessed by increased CD62P expression and PAC1 binding in response to collagenrelated peptide and thrombin receptoractivating peptide activation and their ability to incorporate into thrombi formed on a collagen-coated surface in an ex vivo flow model of thrombosis. This interaction was abrogated by addition of inhibitory monoclonal antibodies against human glycoprotein Ib␣ (GPIb␣) and GPIIb/IIIa. Thus, our mouse model with production of human platelets may be further explored to study the function of genetically modified platelets, but also to investigate the effect of stimulators or inhibitors of human thrombopoiesis in vivo. (Blood. 2009;114:5044-5051) IntroductionHuman platelets are anucleated cells that not only play a crucial role in primary hemostasis and wound repair, but are also particularly important in pathologic conditions such as thrombosis, vascular remodeling, and inflammation. Platelets originate from megakaryocytes (MK) in the bone marrow (BM) by fragmentation of pseudopodial elongations called proplatelets in a process that consumes the entire cytoplasmic content and is tightly regulated by thrombopoietin. 1,2 Human megakaryopoiesis has been studied ex vivo by measuring colony-forming units (CFUs; eg, CFU-MK, CFU-granulocyte, erythrocyte, macrophage, megakaryocyte), 3,4 MK polyploidy state, 5,6 expression of MK markers, 7 and novel genes expressed during MK differentiation. [8][9][10] Unraveling molecular mechanisms involved in megakaryopoiesis and thrombopoiesis is particularly relevant in the light of thrombocytopenia and pancytopenia associated with widespread use of high-dose chemotherapy for treatment of most cancers, also occurring after stem cell transplantation. Therefore, there has been an increasing interest in generating human platelets from MK in culture as well as in developing animal models of human hematopoiesis.Human platelet production has been described from differentiation of CD34 ϩ progenitor cells, isolated from mobilized peripheral blood (PB) or umbilical cord blood (CB), cultured in medium with a cytokine mixture containing thrombopoietin. 5,[11][12][13][14][15] Such produced platelets are functional, as demonstrated in aggregation assays and by expression of P-selectin on the platelet surface or by activation of glycosylphosphatidylinosito...