Megakaryocytopoiesis in vitro of patients with essential thrombocythaemia: effect of plasma and serum on megakaryocyte colony formation

Summary. TO clarify the mechanism of increased numbers of megakaryocytes in patients with essential thrombocythaemia (ET), we studied in vitro megakaryocytopoiesis in ET and other myeloproliferative disorders, using a megakaryocyte colony assay in methylcellulose containing plasma or serum and medium conditioned by phytohaemagglutinin (PHA) stimulated leucocytes (PHA‐LCM). Megakaryocytic colony formation was supported well by heparinized or citrated plasma and citrated serum which was harvested after clot formation of citrated plasma. Whole serum was inhibitory for megakaryocytic colony growth. The addition of platelet releasates and partially purified platelet derived growth factor (PDGF) resulted in a decrease in the number of megakaryocytic colonies. These findings suggested that platelet‐derived factor(s) in serum was inhibitory to megakaryocytic colony formation. ET plasma supported the megakaryocytic colony formation by normal or ET bone marrow cells better than normal plasma. Moreover, in ET bone marrow cells, spontaneous megakaryocytic colonies were formed in the absence of PHA‐LCM. Increased megakaryocytopoiesis in ET may be ascribed to (i) increased megakaryocyte‐colony stimulating activity (Meg‐CSA) in plasma and (ii) increased sensitivity to Meg‐CSA or autonomous proliferation of megakaryocytic progenitor cells.