From hematopoietic stem cells to platelets.

Megakaryocytopoiesis is the process that leads to the production of platelets. This process involves the commitment of multipotent hematopoietic stem cells toward megakaryocyte (MK) progenitors, the proliferation and differentiation of MK progenitors, the polyploidization of MK precursors and the maturation of MK. Mature MK produce platelets by cytoplasmic fragmentation occurring through a dynamic and regulated process, called proplatelet formation, and consisting of long pseudopodial elongations that break in the blood flow. Recent insights have demonstrated that the MK and erythroid lineages are tightly associated at both the cellular and molecular levels, especially in the transcription factors that regulate their differentiation programs. Megakaryocytopoiesis is regulated by two types of transcription factors, those regulating the differentiation process, such as GATA-1, and those regulating proplatelet formation, such as NF-E2. The humoral factor thrombopoietin (TPO) is the primary regulator of MK differentiation and platelet production through the stimulation of its receptor MPL. Numerous acquired or congenital pathologies of the MK lineage are now explained by molecular abnormalities in the activity of the transcription factors involved in megakaryocytopoiesis, in the Tpo or c-mpl genes, as well as in signaling molecules associated with MPL. The recent development of MPL agonists may provide efficient agents for the treatment of some thrombocytopenias.