Induction of Tbx2 and Tbx5 expression by exogenous BMP4. (A, B) Dorsal views of post-culture embryos showing ectopic expression of Tbx2 (A) and Tbx5 (B) in the proximal optic vesicle (arrowheads) after implantation of a BMP4-soaked bead in the peri-ocular mesenchyme (asterisks). Expression is extended into the presumptive RPE on the treated side but restricted to the presumptive neural retina on the contralateral non-treated side shown by transverse sections through the optic vesicle (arrowheads,C-F). (G, H) Lateral views of a post-culture embryo showing normal Tbx5 expression in the non-treated eye (G) and increased Tbx5 expression after implantation of a BMP4-soaked bead in the periocular mesenchyme (H). Arrows demarcate the Tbx5 expression domain. (I-J) Transverse sections of the non-treated and BMP4-treated contralateral eyes showing the extended Tbx5 expression into the presumptive RPE next to the BMP4-soaked bead (arrow). (K-N) Serial transverse sections showing normal Tbx2 expression in a control eye (K, M) and the extension of Tbx2 expression ventrally after implantation of a BMP4-soaked bead in the treated eye (L, N). Sections M and N are immediately ventral to sections K and L. Scale bars: A, B, 0.1 mm; C-F, 0.05 mm; G, H, 0.5 mm; I-N, 0.1 mm. Abbreviations: L, lens vesicle; MN, mandibular region of the first branchial arch; MX, maxillary region of the first branchial arch; N, nasal; NR, neural retina; P-NR, presumptive neural retina; OS, optic stalk; P-RPE, presumptive retinal pigmented epithelium; T, temporal.