No VBI-derived cells were detected in the embryonic endothelium. (A) Schematic outline of Etv2 knockdown experiment. (B-C) In situ hybridization detection of endothelial differentiation marker (cdh5) in (B) control and (C) Etv2 knockdown embryo, showing elimination of marker gene expression. (D) Experimental embryo visualized after in situ hybridization for EC marker (cdh5). (E) The same embryo viewed under a red filter and showing abundance of lineage traced cells migrating from the implant. (F-G) White light images of 2 independent cdh5-stained embryos showing near absence of detectable ECs. Rare examples of staining are circled. (F′-G′) Red filter images of the same embryos shown in F and G. None of the red Dextran lineage-traced cells correspond precisely to the position of the in situ stain (circles). (H) Schematic outline of transgenic endothelial marker experiment, using Flkp::GFP donor tissue. (I-K) White light, red filter, and GFP filter images of the same implanted embryo. Although (J) abundant cells have migrated from the VBI implant, (K) none of these cells are expressing GFP marker consistent with ECs. (L) Transgenic embryo that contributed the VBI implant, showing prominent endothelial GFP expression.