Thrombopoiesis: new concepts.

Although platelets were already discovered light-microscopically in the 19th century, it became only clear in 1906 that they originate from megakaryocytes. The discovery of thrombopoietin in 1994 facilitated research on the origin of platelets, because this growth factor enabled expansion of megakaryocytes in culture. Thus, cell biological and molecular studies could be undertaken. Knowledge accumulated further by studying mouse models and by unravelling the defect in patients with hereditary thrombocytopenia. How a megakaryocyte originates from a hematopoietic stem cell and how this cell undergoes further maturation and differentiation is a complex process, controlled at different stages by several transcription factor. These transcription factors influence the expression of their own target genes, leading to megakaryocyte maturation and platelet release. One of the transcription factors that is most studied in this regard is GATA1, that forms a transcriptional complex with its cofactor FOG1. Families with hereditary thrombocytopenia have been described due to mutations in the GATA1 gene, and target genes were studied. This broadens our insight in normal megakaryocyte differentiation. The final release of platelets from the megakaryocyte is strongly dependent on the formation of so-called proplatelets. The formation of pseudopodia requires the presence of microtubuli. Beta 1-tubulin is a major part of these microtubuli and plays an important role not only in the genesis of platelets but also in the final discoid form of the platelet. Despite a renewed interest and expanding knowledge in this area, there are more questions than answers at this day.