Fig. 4. Lateral expansion of erythroid development at the expense of the pronephric duct. All embryos flat mounted; anterior, top. (A,B) In uninjected embryos, gata1 expression pattern (purple) overlapped with the stronger medial aspect of fli1 expression (red) in the PLM (blue arrow in b). (C,D) In scl/tal1-lmo2-injected embryos (SL), gata1 was excluded from the fli1⁺ cells in the head (green arrow), heart (bracket) and somitic mesoderm (black arrow) but completely overlapped with fli1 expression in the PLM (blue arrows), demonstrating expansion into the weaker, lateral aspect of the fli1 domain. (E,F) In uninjected embryos, the weaker lateral aspect of the fli1 domain overlaps with expression of the PND marker pax2.1. Yellow arrows in A,B,C,E indicate fli1⁺ PND progenitors. (G,H) pax2.1 expression was reduced in the PLM of scl/tal1-lmo2-injected embryos (H). (I) Schematic showing that in scl/tal1-lmo2-injected embryos, gata1⁺ erythroid cells (red) form ectopically from haemangioblasts developing from PLM that normally gives rise to the PND (yellow). Ectopic haemangioblasts induced in the head, heart and somitic mesoderm (purple) do not develop into erythrocytes, showing that erythropoiesis is regionally restricted.