Thrombocytotic suppression of megakaryocyte production from stem cells.

Megakaryocytopoiesis in the spleens of lethally irradiated mice transplanted with marrow cells was suppressed by platelet transfusions. In one group of experiments, animals were irradiated and transfused with bone marrow cells on day O. They were then given either no treatment, platelets, platelet-poor plasma, or saline on days 0, 2, 4, 6, and 8, and then were sacrificed on day 10. Megakaryocytes per section in the spleens of mice receiving platelets were 24%-48% of the values in the groups given plasma, saline, or bone marrow only. The number of pure megakaryocyte colonies was also diminished by platelet hypertransfusion. Another experiment examined the effect of platelets or plasma administered on days 1 and 2 or days 6 and 7 after irradiation and bone marrow transfusion. Hypertransfusion on days 6 and 7 was as effective in suppressing megakaryocytopoiesis as hypertransfusion every other day for 10 days. Animals given platelets or plasma only on days 1 and 2 did not have any significant change in their megakaryocyte number. These results implied that committed megakaryocyte precursors were more sensitive to inhibition by increased platelet levels than pluripotential stem cells. Further experiments with plethoric animals indicated that different levels of erythropoietin did not account for the effects of platelet hypertransfusion. The findings could be explained by inhibition of cell proliferation or of differentiation of megakaryocyte precursors by increased platelet levels.