Development of a liquid culture system for megakaryocyte terminal differentiation: fibrinogen promotes megakaryocytopoiesis but not thrombopoiesis.

Megakaryocyte differentiation is composed of three distinct stages: formation of erythromegakaryocytic progenitor cells, maturation of megakaryocytes and production of platelets. We have developed a liquid culture system for megakaryocyte terminal differentiation from haematopoietic stem cells into proplatelets. In this system, CD34+ cells isolated from human cord blood, differentiated to CD41+ cells, were classified either as propidium iodide (PI)+ cells (large) or PI- cells (small) by fluorescence-activated cell sorting analysis on the late-stage CD41+ cells. Transmission electron microscopy showed that the cultured small cells were morphologically identical to platelets isolated from normal peripheral blood. Moreover, the number of differentiated cells that were CD42b-positive attained an approximately 60-fold expansion over that of the primary CD34+ cells in this culture system. Furthermore, gene expression of megakaryocytopoietic transcriptional factors, GATA-1 and NF-E2, and several megakaryocytic markers such as glycoprotein (GP)IIb and thromboxane synthase was observed in the individual differentiation stage. Treatment with fibrinogen, a ligand of GPIIb/IIIa, increased the number of CD41+/PI+ cells, but treatment in the late stage suppressed CD41+/PI- cell formation, suggesting that fibrinogen promotes megakaryocytopoiesis, but not thrombopoiesis. We conclude that this liquid culture system using human CD34+ cells may be used to mimic the physiological development from haematopoietic stem cells into megakaryocytes, as well as promote subsequent thrombopoiesis.