Molecular signatures of EryP progenitors. (A) Hemangioblastic marker expression by early EryPs. FACS plots showing expression of Flk-1 and VE-cadherin surface protein in GFP(+) cells from ϵ-globin::H2B-EGFP embryos at E7.5 and E8.5. (B) Selected genes that are activated during EryP progenitor expansion from E7.5-E8.5. Transcripts listed showed a low adjusted P value and median expression level < 9.5 (log₂ scale) for triplicates at E7.5 and were up-regulated ≥ 3.5-fold by E8.5. Samples were amplified before hybridization to the microarray. Expression cutoff, 7.2. (C) Expression of activated β-catenin (*β-cat) in EryP at E8.0. Cells from dispersed E8.0 ϵ-globin::H2B-EGFP embryos were cytocentrifuged onto slides and then immunostained. (D-E) Expression of a TCF/Lef::H2B-GFP transgenic reporter for the canonical Wnt-signaling pathway¹³  in an ∼ E7.75 embryo. (D) Anterior view of late headfold (LHF) stage TCF/Lef::H2B-GFP embryo counterstained with Hoechst to highlight nuclei. The image is a 3D reconstruction of a z-stack and was acquired using a Zeiss LSM 510 microscope outfitted with a plan-apochromat 20×/0.75 NA lens. White box indicates the blood islands of the YS. Scale bar, 100 μm. (E) High-magnification view of the boxed region in panel D. Scale bar, 50 μm.