AbstractErythroid colony formation in agar cultures of CBA cells was stimulated by the addition of pokeweed mitogen‐stimulated C57BL spleen conditioned medium. Both 48‐hour colonies (“48‐hour benzidine‐positive aggregates”) and day 7 large burst or unicentric erythroid colonies (“erythroid colonies”) developed, together with many neutrophil and/or macrophage colonies.In CBA mice, the cells forming erythroid colonies occurred with maximum frequency (650/105cells) in 10‐ to 11‐day‐old yolk sac and fetal liver but were present also in fetal blood, spleen and bone marrow. The frequency of these cells fell sharply with increasing age and only occasional cells (2/105cells) were present in adult marrow. A marked strain variation was noted, CBA mice having the highest levels of erythroid colony‐forming cells.The erythroid colony‐forming cells in 12‐day CBA fetal liver were radiosensitive (Do110–125 rads), mainly in cycle and were non‐adherent, light density, cells sedimenting with a peak velocity of 6–9 mm/hr. These properties are similar to those of other hemopoietic progenitor cells in fetal tissues. The relationship of these apparently erythropoietin‐independent erythroid colony‐forming cells to those forming similar colonies after stimulation by er
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