Fig. 4. LMX1B and WNT1 slowly induce GDF7 expression. Lmx1b and Wnt1 are competence factors for RP differentiation. At stage HH10-11, Lmx1b (A) and Wnt1 (B) are expressed both on the dorsal midline and widely in the caudal midbrain (between arrowheads). Lateral views (C,E,E') and transverse sections of the midbrain (D-D'',F) of embryos fixed 10 (D-D''), 24 (C) and 34 (E,E',F) hours after coelectroporation of Lmx1b and Egfp expression vectors. Lmx1b (C,E,F) or Wnt1 (D-D'') transcripts are detected in red, Gdf7 transcripts in purple (C,D'',E,F) and EGFP in green (D',E'). Twenty-four hours after electroporation, very few scattered cells express Gdf7 ectopically (C). Gdf7 expression is strongly induced 34 hours after electroporation (E,F). Except in the caudal midbrain (arrowhead in E) and at the midbrain-forebrain junction, the purple staining for GDF7 partly obscures that for Lmx1b (E,F). This fainter induction of GDF7 does not result from a difference in the efficiency of Lmx1b electroporation around the constrictions, as the co-electroporated EGFP reporter is expressed at high levels in the same regions (arrowhead in E').