Expression of tbx5.1 in wild type eye (A-F) and heart (G-L) development. (A-D) tbx5.1 expression is restricted to the posterior halves of the eyes, (A, D) 15-somite stage, (B) 20-somite stage, and becomes restricted to a dorsal sector of the neural retina from 30 hpf (C) to 36 hpf (E, F), before it is lost prior to the organisation of the retinal epithelium into distinct layers. Expression in the lateral plate mesoderm starts around the 7-somite stage, labelling the heart progenitors in 2 lateral stripes (G, H), as well as the precursor cells of the pectoral fin buds (arrowheads in J, K; 15 and 20-somite stage, respectively). At the 15-somite stage, heart and fin expression domains start to separate from each other (I, J). At 30 hpf, tbx5.1 is strongly expressed in the prospective atrium (arrow) and sinus venosus (arrowhead). Arrows in (I-K) point to cardiac expression. (A-D, F, J) anterior is to the left, dorsal is up, (G, K, L) are dorsal views, (E, H, I) are cross-sections. Scale bars, 100 µm.

tbx5.1 expression in pectoral fin buds in wild type and mutant zebrafish. (A-E) tbx5.1 expression during wild type pectoral fin development. (A) 18-somite stage. Somites 1-4 are indicated. (B) 32 hpf. Arrow points to tbx5.1 expressing cells anterior to and outside of the pectoral fin bud. (C) 48 hpf. Expression is restricted to the distal part of the fin bud, which forms the fin endoskeleton proper. (D) 3 day embryo. The band of darker staining in the proximal mesenchyme is due to melanocytes that cover the epidermis at the ventral base of the fin bud. (E) 5 day embryo. Note the absence of tbx5.1 expression in the ectoderm of the apical fold and in the scapulocoracoid (sc). (F-I) tbx5.1 is expressed in dak mutant fin buds. (F) 36 hpf wild type and (G) dak^tw25 pectoral fin buds. (H, I) At 5 days, dak^tw25 mutants (I) lack differentiated pectoral fins. (J-Q) Maintenance of tbx5.1 in wild type (J-M) and syu^t4 mutant fin buds (N-Q). Expression at the 22-somite stage (J, N), 32 hpf (K, O), 36 hpf (L, M, P, Q). Anterior is to the left, (C-G, M, Q) lateral views with dorsal on top, (A, H-L, N-P) dorsal views. Scale bars, 100 µm.