HEMATOPOIESIS AND STEM CELLS BMP-mediated speci fi cation of the erythroid lineage suppresses endothelial development in blood island precursors

The large majority of endothelial cells (ECs) within the embryo arise as individual angioblasts throughout mesodermal tissues. If hemangioblasts, bipotential precursors of blood and ECs (B/ECs), do indeed exist, they are located in those regions where blood and endothelial precursors develop in close proximity; for example, in the primitive streak and early blood island of the avian andmammalian embryos, and presumably within the primitive blood-forming regions of fish and amphibian embryos. Because blood and endothelium are both mesoderm derivatives, it is important to define the putative hemangioblast as an immediate precursor of these 2 lineages and not simply as an early mesodermal cell. The hemangioblast theory was bolstered by molecular evidence that blood and endothelial precursors cells express an overlapping set of marker genes. For example, the cell surface receptor, Kdr, and the transcription factors, Erg, Etv2, Fli1, Gata1/2, Tal1, and Lmo2, are expressed in both blood and endothelial precursors. In vitro studies have demonstrated that cells isolated from mouse embryoid bodies and the primitive streak can generate blood and ECs in clonal culture, and similar results have been obtained using human embryonic stem cells. Although these studies show that cultured single cells can give rise to multiple cell lineages, including blood and endothelium, it has been much more challenging to demonstrate the existence of such cells within the embryo itself. Several studies searching for embryonic hemangioblasts reached different conclusions about the existence, or at least the frequency, of such cells in vivo. Photoactivation of lineage tracers in the zebrafish embryo showed that single cells within the ventral marginal region of the gastrula could generate both blood and endothelial derivatives. It was estimated that as many as 12% of cells in this region had properties consistent with the hemangioblast. In contrast, lineage tracing of cells in the mouse blood islands failed to reveal any cells that generated both blood and endothelial progeny from a single clone. Although it is not possible to demonstrate the nonexistence of a cell type, the authors concluded that if hemangioblasts exist, they must be exceedingly rare. Numerous subsequent studies have failed to generate consensus on the existence of hemangioblasts in vivo, primarily due to questions concerning experimental design or interpretation of results. In this study, we tested for the presence of hemangioblasts in the ventral blood island (VBI) of Xenopus, which provides a uniquely accessible and localized precursor population. Although markers of both endothelial and blood precursor cells are coexpressed in the VBI, our data demonstrate that these cells become blood and do not normally give rise to ECs, effectively precluding the existence of a classic bipotential precursor. However, experimental inhibition of bone morphogenetic protein (BMP) signaling blocks erythroid development and reveals a latent capacity for early VBI cells to differentiate into ECs. Furthermore, blocking the erythroid transcriptional program resulted in endothelial differentiation. These results indicate that precursor cells in the Xenopus VBI possess both endothelial and erythroid potential, but that in normal development, these cells are not hemangioblasts as such and give rise only to