[Megakaryocyte development and platelet production in normal and disease states]

Megakaryocytopoiesis involves the commitment of hematopoietic stem cell, the proliferation and terminal differentiation of the megakaryocytic progenitors and maturation to platelet producing megakaryocytes (MK). MKs align adjacent to bone marrow vascular endothelial cells, and form proplatelets. Platelets are released, or sheared from the MK directly into the circulation from the tips of proplatelets which protrude into the vascular lumen. The regulation of megakaryocytopoiesis is mediated through multiple hematopoietic growth factors, chemokines and cellular interactions via signal transduction pathways and integrated transcription factors. The primary physiological growth factor that regulates megakaryocytopoiesis and platelet production is thrombopoietin. Circulating Levels of thrombopoietin (TPO) induce concentration-dependent proliferation and maturation of MK progenitors by binding to the c-Mpl receptor and signaling induction. Increased concentration of free TPO resulting from decreased platelet turnover rates enables the compensatory response of marrow MKs to drive amplified platelet production. C-MpL signaling is orchestrated by a complex cascade of signaling molecules inducing the action of specific transcription factors to drive MK proliferation and maturation. Newly developed thrombopoietic agents operating via c-Mpl receptor have now been proven useful in supporting platelet production in thrombocytopenic states. In this article, the authors review the regulation of megakaryocytopoiesis and platelet production in normal and disease states, and new approaches of thrombopoietic therapy.