Use of the BMP inhibitor protein, Noggin, confirms that BMP signaling is required for erythroid specification and inhibits EC differentiation. (A-B) White light and red filter images, respectively, of the same embryo showing targeting of Noggin expression to the VBI region. The reporter for protein distribution is H2B-RFP. (C-T) All embryos were assayed by in situ hybridization at st32 using probes indicated at lower left of each panel. Number of embryos showing illustrated effect is at lower right. (C-J′) Embryos treated with Noggin. An enlarged ventral view (′) is provided for each embryo. (C-F′) Inhibition of BMP signaling using Noggin results in elimination of blood marker expression (hba1) and appearance of EC marker expression (aplnr) in the ventral region. (G-J′) Inhibition of BMP signaling using Noggin has no effect on expression of myeloid markers (spib and runx1). (K-T) Ventral view of controls and embryos treated with LDN193189. Erythroid marker expression assayed with gata1 is decreased, whereas expression of endothelial differentiation markers is increased. (U) qPCR analysis of VBI explant tissue treated with the BMP inhibitor, LDN193189, from st13 to st32. Compared with controls, erythroid marker expression is decreased, myeloid marker expression is unchanged, and endothelial marker expression is increased.