Mouse yolk sac and intraembryonic tissues produce factors able to elicit differentiation of erythroid burst-forming units and colony-forming units, respectively.

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Abstract
This work was aimed at elucidating the environmental conditions that account for the production of embryonic erythrocytes in the mouse yolk sac (YS), while the adult-type hemoglobin and erythrocytes are generated in the fetal liver. Differentiation of YS hemopoietic stem cells (YS-HSC) of 9.5-day mouse embryos (prior to the colonization of the liver rudiment by HSC) was investigated in vitro. The influence of well-characterized erythroid growth factors, burst-promoting activity (BPA) and erythropoietin (EPO), which trigger the differentiation of early erythroid burst-forming units (BFU-E) and late erythroid colony-forming units (CFU-E), respectively, was tested on the YS-HSC in two different systems of culture: (i) organ culture and (ii) clonal culture in methylcellulose. When studied in organ culture, where the YS microenvironment was maintained, YS-HSC required only additional EPO to attain complete maturation into adult erythrocytes within 7 days. In contrast, YS hemopoietic single cells grown in methylcellulose, and thus released from the influence of the YS, required the presence of both BPA and EPO to generate BFU-E-derived colonies synthesizing high concentrations of hemoglobin. It is concluded that 9.5-day YS from mouse embryos is by itself able to promote the first differentiation steps of the adult lineage YS-HSC due to an intrinsic production of a BPA-like activity. In contrast, these experiments demonstrate that EPO or an EPO-like activity is not produced by YS tissues. As demonstrated earlier, if embryonic tissue is added to YS organ culture, although separated from it by a filter preventing cell contact, YS-HSC differentiate into adult erythrocytes producing adult-type hemoglobins. This shows that, in contrast to YS tissues, the early embryo produces EPO or a factor that can substitute for EPO.