Fig. 3. High levels of Bmp signaling activity, principally mediated by Bmp4, tightly regulate Tbx5 expression in the dorsal retina. (A,B) Whole-mount in situ hybridization for Tbx5. Lateral views of E9.0-9.5 embryos (A) and E10.5 embryos (B) probed for Tbx5 transcripts. (A) Dorsal specific Tbx5 expression is lost in the optic vesicle of an advanced Bmp4^-/- mutant embryo (arrowhead). (B) Tbx5 transcripts in the dorsal retina (arrowheads) are detected at comparable levels in both control (top) and Bmpr1a^-/fx;Bmpr1b^+/-;Cre mutant (bottom) samples at E10.5. Tbx5 transcripts are also strongly expressed in the forelimb buds (fl) in both control and mutant. (C) In situ hybridization on coronal sections of E11.5 embryos with dorsal towards the top. Tbx5 expression is lost by E11.5 in the mutant retina (bottom, arrowhead). (D) Immunohistochemistry for P-Smad1/5/8 detects high levels of P-Smad protein in the dorsal retina of control embryos, defining regions receiving the highest Bmp signal (top). This pattern is lost in mutants (bottom). so, somites; fl, forelimb; le, lens; nr neuroretina; rpe, retinal pigment epithelium. Scale bar: 500 µm in A; 1 mm in B; 100 µm in C,D.