The generation of blood cells, haematopoiesis, in the mouse embryo begins with the development of primitive nucleated erythroid cells in the yolk sac followed by the appearance of precursors for multiple definitive haematopoietic lineages. The later developing lineages arise from multipotential stem cells, but the relationship of primitive erythroid cells to these other haematopoietic populations is unknown. Using an in vitro embryonic stem (ES) cell differentiation system, we show that primitive erythrocytes and other haematopoietic lineages arise from a common multipotential precursor that develops within embryoid bodies generated from differentiated ES cells. In response to vascular endothelial growth factor and c-kit ligand these precursors give rise to colonies containing immature cells (blasts) expressing marker genes characteristic of haematopoietic precursors. Many blast colonies also expressed betaH1 and beta major globins but not Brachyury, a mesodermal marker. Kinetic analysis demonstrated that the blast colony-forming cells represent a transient population, preceding the establishment of the primitive erythroid and other lineage-restricted precursors. This precursor population may represent the earliest stage of embryonic haematopoietic commitment.

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We describe the construction of a v‐rel estrogen receptor fusion protein (v‐relER) which allows the regulation of v‐rel oncoprotein activity by hormone. In the presence of estrogen, v‐relER readily transformed primary chicken fibroblasts and bone marrow cells in vitro. In both cell types, v‐rel‐specific transformation was critically dependent on the presence of estrogen or the estrogen agonist 4‐hydroxytamoxifen (OHT). Withdrawal of estrogen or application of an estrogen antagonist, ICI164,384 (ICI) caused a reversal of the transformed phenotype. We also demonstrate that the v‐relER protein binds to NF‐kappa B sites in an estrogen‐dependent manner, thereby showing that sequence‐specific DNA binding of v‐relER is critical for the activation of its transforming capacity. In transient transfection experiments, we failed to demonstrate a clear repressor or activator function of the v‐rel moiety in v‐relER. However, in v‐relER‐transformed bone marrow cells, estrogen and OHT induced elevated mRNA levels of two cellular genes whose expression is constitutive and high in v‐rel‐transformed cells. These results suggest that v‐rel might exert part of its activity as an activator of rel‐responsive genes.