In a previous study also done with a liquid culture technique, the monoblast was identified and characterized as the most immature cell of the mononuclear phagocyte cell line recognized so far. The present study concerned the proliferative behavior of the monoblast and promonocyte in colonies. The cell-cycle times of both cell types were determined on the basis of four independent methods. The resulting values all show excellent agreement: for the monoblast 11.0-11.9 h, and for the promonocyte 11.4-12.8 h. The DNA-synthesis time found for the two cell types amounted to 5.7 h for the monoblast and 5.5 h for the promonocyte. The duration of the other phages of the cell cycle of the proliferating mononuclear phagocytes proved to be: G2 phase, 0.6 h; mitosis phage, 1.8 h; and G1 phase, 3.5-3.8 h. The individual colonies showed a biphasic pattern of colony growth, an initial phase of rapid proliferation being followed by a stage wtih a markedly decreased growth rate. In the initial stage only monoblasts are present in the colony; when the growth rate slows down promonocytes and macrophages appear. These observations support the earlier conclusion that the monoblast is without doubt the precursor of the promonycyte. Colony size was found to vary widely. The main factor underlying this variation proved to be the lag time between the start of the culture and the time point at which the colony-forming cells begin to divide. Mathematical analysis showed that the variation in colony size probably does not arise from heterogeneity of the population of colony-forming cells. A mathematical approach was used to determine the proportion of self-replicating and differentiating cells among the dividing monoblasts and promonocytes in the colony. The results indicate that initially in vitro the majority of the cells of both types are self-replicating cells, but later an increasing proportion of the dividing cells give rise to another, more mature type of cell. On the basis of the conclusion that the monoblast initiates the mononuclear phagocyte colony, the number of monoblasts (2.5 X 10(5)) present in vivo was estimated to be half the number of the promonocytes. In view of this ratio the mostly likely pattern for the proliferation of mononuclear phagocytes in the bone marrow is that a monoblast divides once, giving rise to two promonocytes which in their turn divide once and form two nonproliferating monocytes.

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