AbstractExperiments were performed to investigate the presence of colony‐forming units (CFU) in the mouse embryonic yolk sac during the developmental period in which the yolk sac is the sole hemopoietic organ. Injection of yolk sac cell suspensions from normal embryos into syngeneic, lethally irradiated adult recipients evoked a very low number of spleen colonies. However, prior cultivation of yolk sacs in vitro caused a dramatic increase in the spleen colony‐forming capacity–as high as 84‐fold–following 48 hours in culture. The yolk sac origin of the spleen colonies was confirmed by: (a) Chromosomal marker analysis; (b) dose‐response analysis; (c) demonstrating that the above colonies were not of endogenous origin induced by the mere injection of grafted cells. We conclude that the yolk sac contains many precursors of colony‐forming cells which though undetectable by immediate grafting apparently become activated in culture by an as yet unknown indu
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