Regulation of the Proliferation of M urine

The extent to which mouse megakaryoseparate megakaryocyte progenitor cells cyte progenitor cells (colony-forming unitmay be biased in their capacity for promegakaryocyte, CFU-M) can proliferate in liferation versus endomitosis. Differences semisolid cultures prior to endomitosis, were observed in the cell cycle characand conditions that may regulate that difteristics of CFU-M as determined in vivo ferentiation step, were investigated. The and in vitro that suggest that maturation proliferative capacity of CFU-M was deterof CFU-M into megakaryocytes may be mined by estimating the number of megaregulated within the marrow by control karyocytes per colony. A bimodal distribuof the cell cycle of the megakaryocyte tion was observed (modal values, 10-15 precursorcell. and 25-30 cells/colony), indicating that C OLONY ASSAYS in vitro for the detection of progenitor cells of all hemopoietic cell lineages from mouse marrow, including the megakaryocyte progenitor cell (colony-forming unit-megakaryocyte, CFU-M), have been previously described.’-7 Megakaryocytes formed in these cultures are capable of full maturation, since colony megakaryocytes have normal ploidy values4 and shed platelets.6’7 Using appropriate culture conditions, the number of megakaryocyte colonies was found to be directly proportional to the number of bone marrow cells cultured, allowing quantitation ofthe CFU-M population.7 The clonable megakaryocyte progenitor cell may be the precursor cells of small acetylcholinesterase-positive cells, since the increase in CFU-M precedes the detection of heightened numbers of small acetylcholinesterase-positive cells in experimentally induced thrombocytopenic animals.8’9 This notion is substantiated by analysis of the physical properties of the two cell types. Megakaryocyte colony-forming cells have physical characteristics typical of diploid cells,4’8 while small, acetylcholinesterase-containing cells are markedly more dense than the bulk of nucleated cells,’#{176} a property that may reflect increased cytoplasmic maturation and/or nuclear condensation. Thus it is thought that CFU-M represent a population of cells between the pluripotent stem cells and the morphologically identifiable megakaryoblasts.8 The role of obligatory stimulators on the proliferation and regulation of megakaryocyte colony-forming cells has also been investigated using these culture systems.7’8 Two activities may be involved in megakaryocyte formation and maturation. Entities present in mitogen-stimulated spleen cell supernatant,4’5 erythropoietin,6 and the conditioned medium from a myelomonocytic leukemic cell line (WEHI-3)7 all promote directly the growth of megakaryocyte colonies, while factors that are nonstimulatory alone but that potentiate colony