The effects of thrombopoietin on megakaryocyte-cfc, megakaryocytes, and thrombopoiesis: with studies of ploidy and platelet size.

The effects of partially purified thrombopoietin (TPO), prepared from the plasma of thrombocytopenic rabbits, upon megakaryocyte colony-forming cells (Meg-CFC), megakaryocytes, and platelets were evaluated. The thrombopoiesis-stimulating activity of both types of preparations of TPO used was established by their ability to increase the levels of selenomethionine- 75Se ( 75SeM) in the circulating platelets of mice, as described previously. TPO did not increase the frequency or total numbers of Meg-CFC or granulocyte-macrophage-CFC (GM-CFC) in the bone marrow or spleen of recipient mice. Similarly, the addition of TPO directly to 0.3% agar cultures of bone marrow or splenic cells did not alter the frequency of either Meg-CFC or GM-CFC, although in some experiments, concentrations greater than required to stimulate thrombopoiesis in vivo were used. TPO did not enhance the ability of suboptimal concentrations of spleen conditioned medium to support colony growth. Human erythropoietin (EPO) at concentrations from 0.6 U/ml to 8 U/ml, in vitro, did not increase the frequency of bone marrow or splenic Meg-CFC or GM-CFC. As previously reported, two types of megakaryocyte colonies were present in soft agar cultures. The proportion of 16N cells was significantly increased in the more mature appearing, big cell colonies derived from mice that had previously received thrombopoietin. There was no detectable change in the ploidy of heterogeneous colonies. Addition of TPO directly to cultures produced a slight, but not statistically significant increase in the proportion of 16N, 32N, and 64N megakaryocytes in big cell colonies. Although 16N megakaryocytes remained the most common ploidy group among recognizable megakaryocytes in the bone marrow of mice that had received TPO, there was an increase in the proportion of 32N and 64N megakaryocytes. Both preparations of TPO significantly increased the mean platelet volume of recipient mice, although the platelet counts remained unchanged. Similar protein fractions, prepared from the plasma of normal donors, did not increase levels of 75SeM in circulating platelets or increase mean platelet volume. Our data indicate that thrombopoietin produces a variety of effects, some of them perhaps indirect, on megakaryocyte precursors, megakaryocytes, and platelets.