The Effect of Anti-i on Early Myeloid Progenitor Cells in Human Bone Marrow

Bone marrow cells incubated for several days in liquid suspension culture with an appropriate stimulator frequently produce more CFU-c than marrow cells assayed without prior incubation. The increase in assayable CFU-c produced by the suspension culture is known as ΔC (2, 11). In the differentiation stage, the cells responsible for ΔC probably fall between CFU-c and the human CFU-s equivalent (2, 8). In support of this hypothesis, it has been shown first that, unlike CFU-c, the ΔC progenitor cells can rarely be detected in the S-phase of the cell cycle (8). Second, two types of stimulatory molecules have been detected in leukocyte-conditioned media. The larger molecules (15,000, 35,000, and 90,000 Daltons) are strong stimulators of human CFU-c but have little or no AC effect (6,9). On the other hand, a class of small molecules (1,200 Daltons) often have a large ΔC effect, but as a class, it is a relatively poor stimulator of CFU-c (6, 12). Third, 4-methyl-hista- mine, which has been shown by Byron (1) to cause murine CFU-s to start DNA synthesis, has been shown to have a similar effect on the human ΔC progenitors, but to have no detectable effect on the cell cycle phase of human CFU-c (8). This evidence suggests that there are significant similarities between the differentiation stage of the human ΔC progenitors and murine CFU-s.