Characterization of megakaryocyte spleen colony-forming units by response to 5-fluorouracil and by unit gravity sedimentation.

Properties of megakaryocyte progenitor cells in mouse bone marrow have been examined using an in vivo assay system. Perturbation with 5-fluorouracil (5-FU) and separation by unit gravity sedimentation was used to characterize the cells. Bone marrow was assayed for the presence of megakaryocyte colony-forming cells (MK CFU-S) by transplantation into lethally irradiated mice and examining spleen sections 10 days later. Donor mice were untreated or injected intravenously with 5-FU (150 mg/kg), 1 (FU-1) or 7 (FU-7) days beforehand. There was a lack of correlation between the numbers of MK CFU-S and cells giving rise to macroscopic spleen surface colonies (CFU-S10). The sedimentation profile of MK CFU-S in normal marrow was similar (modal velocity 4.16 +/- 0.05 mm/hr) to that of CFU-S10. In FU-1 marrow, MK CFU-S exhibited a bimodal sedimentation profile, with peaks at 3.26 +/- 0.06 mm/hr and 4.53 +/- 0.07 mm/hr. The marrow content of CFU-S10 was reduced to 5% of normal, while MK CFU-S numbers were only reduced to 60%. In FU-7 marrow, the sedimentation profile of MK CFU-S (modal velocity 4.86 +/- 0.16 mm/hr) differed from that of CFU-S10 (5.5 +/- 0.16 mm/hr). It was concluded MK CFU-S and CFU-S10 are different entities. The MK colonies formed from FU-1 marrow contained on average 3.8-fold more cells than those formed from normal marrow. The enhanced megakaryocyte production may be accounted for on the basis of the generation-age model for cell proliferation. It is proposed that MK CFU-S are a heterogeneous population with regard to proliferation potential and that the FU-1 marrow contains cells that survive 5-FU and have a high proliferative potential. These cells may be equivalent among megakaryocytic progenitors to the high proliferative potential colony-forming cells of the granulocyte/macrophage series. They may be responsible for the enhanced megakaryocytopoiesis seen in the marrow of mice 7 days after the injection of 5-FU.