Fig. 1. Expression of Sox17 and Sox7 during gastrulation and early organogenesis. (A-C) Pre-streak (A,B; 6 and 6.25 dpc, respectively) and early-streak (C; 6.5 dpc) embryos showing the progressive expansion of Sox17 expression (arrow) to the whole extra-embryonic region of the visceral endoderm. (D) Transverse section of the extra-embryonic region of an early-streak embryo showing expression of Sox17 in the visceral endoderm (arrows). (E) Mid-streak (7.0 dpc) stage embryo showing strong expression of Sox17 in the extra-embryonic visceral endoderm (unbroken arrow) and the endoderm at the anterior end of the primitive streak (broken arrow). (F,G) Lateral (F) and anterior (G) views of early-bud stage (7.5 dpc) embryo revealing Sox17 expression in the extra-embryonic visceral endoderm and the anterior-most endoderm (broken arrows). (H,H') Early- to late-bud stage (7.5 dpc) embryos demonstrate the increasing number of Sox17-expressing cells in the anterior definitive endoderm (H; broken arrow), but no expression of Sox7 in the definitive endoderm (H'). (I-L) Embryos at early-bud stage (7.5 dpc): whole embryo (I) and histological section (J) showing strong Sox17 expression in the visceral endoderm (arrows). Section in situ hybridisation by radioactive riboprobes display strong expression of Sox17 (K) and Sox7 (K') in the parietal (arrowheads) and extra-embryonic visceral endoderm (arrows). Strong Sox17 expression in the definitive endoderm (L; broken arrows). Planes of sections are indicated by arrowheads in the embryo (I) of a similar developmental stage. (M,M') Early neural plate stage (7.75 dpc) embryo showing the expansion of Sox17-expressing cells (M; broken arrow) and the absence of Sox7-expressing cells in the prospective foregut and midgut endoderm (M'). (N-Q) Early headfold stage (8.0 dpc) embryo (arrowheads indicating the plane of the histological sections) showing the descending gradient of Sox17 expression from the ectoplacental cone (O) to the allantoic region of the visceral endoderm (P), and enhanced expression in the definitive endoderm of the prospective midgut and hindgut (Q). (R,S) Section in situ hybridisation by a DIG-labelled riboprobe showing the regionalised Sox17 expression (broken arrows) in the posterior definitive endoderm of the 8.25 dpc embryo (four to five somite stage). The broken rectangle in R demarcates the region magnified in S. (T-W) Sox17 expression in the posterior endoderm of the mid- and hindgut (V,W), but not in the foregut (U) of seven- to eight-somite stage (8.5 dpc) embryo (arrowheads indicating the plane of the histological sections). al, allantois; epc, ectoplacental cone; dc, decidua; fg, foregut; hg, hindgut; mg, midgut. Scale bars: 100 µm.
