Abstract
The capacity of yolk sac hemopoietic cells to produce either primitive or definitive erythrocytes was analyzed in vitro under three different experimental conditions. (i) Before the 28-somite stage (prior to colonization of the liver rudiment by hemopoietic cells), yolk sac explanted alone produced solely primitive erythrocytes and only for a short time. (ii) When allowed to colonize a liver rudiment, hemopoietic cells from the yolk sac gave rise to definitive erythrocytes. (iii) These cells could express the same capacity when stimulated by various intraembryonic organs, even if no direct cell--cell contact was established between stimulating tissue and target hemopoietic cells. These results provide evidence that humoral factors present in embryos past the 28-somite stage act on hemopoietic cells, inducing the onset of definitive erythropoiesis.
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