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Fig. 5 RAR function is required in endoderm but not mesoderm or ectoderm for pancreas development. (A,B) RAR function is required in endoderm for pancreas specification. (A) Schematic illustrating transplantation approach, which is essentially the same as that shown in Fig. 3D, except that donor embryos are additionally injected with RKD reagents at the one-cell stage. (B) Dorsal view of 24 hpf embryo generated as in A; no insulin expression is present (compare with Fig. 3E). (C,D) RAR function is not required in mesoderm or ectoderm for pancreas specification. (C) Schematic illustrating transplantation approach. (D) Dorsal view of 24 hpf embryo generated as in C; insulin expression is present (blue; arrowhead). (E-H) A constitutively active RAR is able to induce ectopic insulin-expressing cells in anterior endoderm. (E) Schematic illustrating transplantation approach; DA is dominant active RAR (xVP16-RARα1). (F,H) Dorsal views of 24 hpf embryos generated as in E. Ectopic donor-derived insulin-expressing cells (blue; arrowheads) are found anterior to the first somite. Photographs are bright-field/fluorescent composite images. (G) Summary diagram showing locations of all insulin-expressing cells in 21 embryos generated as in E; red cells represent embryo in F, blue cells represent embryo in H, yellow cells are from 19 additional specimens.