Fig. 1. Expression of Sox9 mRNA and SOX9 protein in 5-day-old (E5) chick GI tract. (A) Whole-mount in situ hybridization of E5 dissected gut using antisense Sox9 riboprobe. (B1-F2) Paraffin cross-sections along AP axis as indicated in A, stained with anti-SOX9 (B1,C1,D1,E1,F1) or anti-HNK1 (B2,C2,D2,E2,F2) antibodies. Cross-sections of the gut were taken at the following levels: esophagus and lung bud (B1,B2), posterior region of the stomach (C1,C2), midgut anterior to the umbilicus (D1,D2), caeca (E1,E2) and hindgut (F1,F2). Note SOX9 expression in the mesoderm of the posterior region of the stomach (C1). Mesodermal SOX9 expression is also detected in the lung and caeca (red arrows, B1,E1). SOX9 is strongly expressed in the endoderm of the esophagus, lung, pancreas and hindgut (A,B1,E1). Some SOX9-positive cells are present in the midgut epithelium (insert, D1). SOX9 expression is observed in the mesentery (black arrow, E1), but not in the Remark's nerve, which is essentially composed of neural crest-derived cells as shown by anti-HNK1 antibodies (E2). cae, caeca; cl, cloaca; eso, esophagus; gizz, gizzard; hg, hindgut; mg, midgut; panc, pancreas; pyl, pyloric sphincter; RN, Remark's nerve; sto, stomach.