The effects of thrombopoietic activity of rabbit plasma fractions on megakaryocytopoiesis in agar cultures.

: A plasma fraction that stimulates platelet production in vivo also stimulates megakaryocytopoiesis in vitro. The plasma activity is attributed to a humoral regulator, thrombopoietin. Addition of the thrombocytopenic plasma (TP) fraction to an agar system supporting megakaryocyte colonies increased the frequency of colony formation significantly over that stimulated by spleen cell-conditioned medium (SCM). TP had no effect on the size of the colonies and, in the absence of SCM, TP did not stimulate colony formation. In studies of single megakaryocytes, the numbers of small megakaryocytes, specifically those 5-10 microns in diameter, increased significantly after 3 days of incubation with TP alone. SCM supported not an increase in the numbers, but an increase in the proportion of larger (30- to 40-microns) megakaryocytes. A normal plasma fraction contained similar but consistently less activity than fractions containing TP. The findings indicated that TP stimulates differentiation of megakaryocyte precursors from unidentifiable to identifiable cells but does not alone support colony formation. Thus, TP appears to be a potentiator of megakaryocytopoiesis. However, the augmentation of colony frequency by TP further suggests that TP may also play a role in early colony development, either by enhancing progenitor responsiveness to a megakaryocyte colony-stimulating factor or by recruiting additional colony progenitors from a noncycling progenitor population. These studies establish a link between the stimulation of platelet production observed after TP administration in vivo and the effects of TP on early events in megakaryocytopoiesis.