FGFR2 is the key mediator. (A) FGFR1 at HH4 (Ai), 5 (Aii), 6 (Aiii), 8 (Aiv), and 10 (Av). (B) FGFR2 at HH4 (Bi), 6 (Bii), 7 (Biii), 9 (Biv), and 10 (Bv). (C) FGFR3 at HH4 (Ci), 6 (Cii), and 10 (Ciii). (D) FGFR4 at HH4 (Di), 7 (Dii), and 10 (Diii). (E-G) Sections of FGFR1 (E), FGFR2 (F), and FGFR3 (G). Inset in panel F shows FGFR2 in endothelial but not blood cells. (H-M) Brown indicates electroporated cells; blue, blood cells stained with ρ; arrowheads, endothelial distribution; arrows, blood distribution. CA-FGFR2 cells contribute mainly to endothelium (J,K,M), whereas control cells are seen among different lineages (H-I,L). See Table 2. (N-S) Blue indicates morpholino electroporated cells; brown, blood cells stained with ρ. Knockdown of FGFR2 with a splice block morpholino leads to blood localization (O,Q), and control morpholino cells are distributed among different lineages (N,P). Arrowheads indicate endothelial distribution; arrows, blood distribution. FGFR2 splice morpholino blocks FGFR2 mature transcript accumulation in neural ectoderm at HH4 (compare R with Bi). (S) Brown indicates morpholino-positive cells and blue FGFR2 expression (R shows embryo before morpholino staining).