Human megakaryocytes. IV. Growth and characterization of clonable megakaryocyte progenitors in agar

Human megakaryocyte progenitors were cloned in semisolid agar from unfractionated bone marrow cells and recognized by their capability of producing discrete megakaryocyte colonies. Megakaryocyte colonies were identified in situ by immunofluorescence, using antibodies against platelet glycoproteins Ib, IIb, and IIIa, as well as von Willebrand factor (vWf), which are regarded as distinct protein markers for the megakaryocyte-platelet lineage. Megakaryocyte colonies typically contained 20-50 cells arranged in compact configurations, with high nuclear-cytoplasmic ratios, diameters between 10 and 14 micron, and round, oval, or indented nuclei. Colony numbers peaked at days 6 and 7, with a mean of 17.9 megakaryocyte colonies (range, 8-33) per 2 X 10(5) unseparated marrow cells. The in vitro growth characteristics and kinetics of megakaryocytes grown in agar are different from those described for the plasma clot and methylcellulose systems, which suggests selection of distinct progenitor subsets. Consequently, this assay may be a useful complement to other approaches in characterizing the megakaryocyte progenitor population.