Fig. 3. Defective gut development of Sox17–/– embryos. (A-F) Lateral views of phenotypically normal (+/+,+/–) and mutant (–/–) embryos at 7.5 (late-bud stage; A), 8.0 (headfold stage; B), 8.25 (five- to six-somite stage; C), 8.5 (10- to 12-somite stage; D), 9.5 (E) and 10.5 (F) dpc. The homozygous Sox17 embryos show no axis turning and poor posterior development from 8.5 dpc onwards. Arrows indicate the plane of histological sections (G-S); unbroken arrow for transverse section and broken arrow for sagittal section. (G) Sagittal and (H,I) parasagittal sections of 8.0 dpc embryos. In the mutant embryo, the visceral endoderm extends further into the prospective gut on the lateral aspects (H,I) than the medial aspects (G) in the posterior region. Arrowheads and arrows mark the frontier of the visceral endodermal cells at the anterior and posterior aspects of the embryo, respectively. (H,I) Insets (right) also show a magnified view of the posterior region. (J,K) Transverse sections at the midgut level of 8.25 dpc embryos. In the mutant (–/–) embryo, the definitive endoderm in the lateral region of the prospective gut is replaced by the cuboidal and vacuolated cells that resemble the visceral endoderm. (L-O) Transverse sections at the fore- and hindgut (L,M) and midgut (N,O) levels of 8.5 dpc embryos. The sections of the heterozygous (+/–) embryo at a similar developmental stage (before axis turning) are shown. In the foregut and hindgut portal of the 8.5 dpc mutant embryos, cells that resemble the visceral endoderm are found in the lateral and ventral regions of the gut tube (L,M). In the lateral region of the open midgut, the endoderm cells are juxtaposed to the mesothelium of the splanchnopleure. By contrast, the mutant (–/–) embryo lacks the layer of splanchnic mesenchyme (red arrows; O) that separates the endoderm from the mesothelium. (P,Q) Sagittal sections of the 9.5 dpc mutant (–/–) embryo, showing normal development of (P) the head, the heart and (Q) the thyroid bud. (R) Transverse sections at the caudal foregut level of 9.5 dpc mutant (–/–) embryos reveals a small foregut, but proper liver bud formation. (S) Transverse sections of the hindgut of 9.5 dpc embryos. In the mutant (–/–) embryo, the hindgut has regressed to a cord-like structure containing significantly fewer endodermal cells. The broken rectangle (J,L,N,P) marks the area shown in the magnified view of the histological section. Arrowheads (J,K,N,O) indicate the border between the definitive endoderm and the presumptive visceral endoderm. ce, coelomic epithelium; da, dorsal aorta; fg, foregut; hg, hindgut; ht, heart; lb, liver bud; mes, mesothelium; sm, splanchnic mesenchyme; thy, thyroid bud. Scale bars: 100 µm.
