Inhibition of BMP signaling results in reduction in expression of blood precursor markers and persistence of expression of endothelial precursor markers. (A) Single cell PCR analysis of precursor gene expression in st17 VBI. Individual cells were isolated from dissected VBI mesoderm layer and transcripts assayed by PCR. Whole embryo (WE) tissue was used as a positive control. A single cell from the dorsal region of the st17 embryo was used as a negative control for B/EC expression. Transcripts for RNA polymerase II, polr2a, were used as a reference standard. Method details can be found in supplemental Materials. Cell number 3 is probably an endodermal contaminant. Note that the EC precursor markers kdr and etv2 are coexpressed with hematopoietic precursor markers gata2, tal1, and lmo2. (B-S″) Time course of precursor marker expression in VBI. Treatment with BMP inhibitor commenced at st13. All embryos were assayed by in situ hybridization using the probe indicated. All views are ventral. Embryonic stages assayed are indicated at the top of each column. Erythroid precursor markers are gata2, lmo2, and tal1. Myeloid precursor markers are runx1, mpo, and spib. Endothelial markers are etv2, kdr, and fli1. Numbers at the lower right of each panel indicate the proportion of embryos displaying the illustrated phenotype.