Inhibition of FGFR causes ectopic blood formation and down-regulation of endothelial markers. (A-F) Control DMSO bead has no effect (A,C) and SU5402 bead has excessive surrounding ρ⁺ clusters (B,D). Panels E and F show additional examples of blood induction by SU5402 beads. Arrows point to grafted beads. (G-N) Whole embryo culture in the presence of DMSO (G,I) or SU5402 (H,J) does not affect general growth (I-J, insets: Paraxis in situ). SU5402 drastically expands medial limit of blood formation (H,L) compared with DMSO (G,K), as outlined by yellow dotted line. Sections of panels L (Ni-v) and K (M) show clear induction of ectopic ρ⁺ clusters (arrowhead). Panels Nii-v show presence of endothelial-like cells (arrows) surrounding induced (Niii-v) and endogenous (Nii) clusters. (O-X) Down-regulation of Ets1 (Oii), Vegfr2 (Pii), and Vecad (Qii) by SU5402 compared with respective controls (Oi, Pi, and Qi). Lmo2 expression is shown for HH4 (Ri), 5 (Rii), and 8 (Riii). At HH10 (S), it is expressed in both endothelial and blood cells laterally (Ui and section in Uii) and only in endothelial cells medially (Wi-ii). SU5402 abolishes endothelial expression of Lmo2 while maintaining blood expression laterally (Vi-ii). Ectopic blood clusters in the medial region also express Lmo2 (Xi-ii) but only in inner cells like ρ globin (Niii-v). Arrows indicate blood expression; arrowheads, endothelial expression.