Figure 4. Convergent extension in the wild-type definitive endoderm.

Whole mount in situ hybridization with Ttr probes to wild type embryos of embryonic stages e7.5 (A, B), e8.0 (C-F) and e8.5 (G-J). Lateral and posterior views respectively illustrate how definitive endoderm (white) grows along the AP axis (length) and narrows laterally (width). Ttr highlights the extraembryonic visceral endoderm (VE). The white tissue covering the embryonic region corresponds to definitive endoderm (DE). Arrows point to white extraembryonic tissue (ext). All pictures were taken at the same magnification. At e7.5, some visceral endoderm cells (arrowhead in A-B) still overlay the exterior of the embryonic region (continuous line in A-B delimits embryonic-extraembryonic parts). Gut closure prevented visualization of all the definitive endoderm in panels I and J. (b’-j’) Representative transverse sections of the embryos in the columns above counterstained with Fast Red. Sections correspond to intermediate levels along the anterior-posterior axis. Only half of the section is shown, with the midline located at the right edge of the panel. (K) Plot of length (blue) and width (green) definitive endoderm measurements in wild type embryos of different stages; data in μm. Dimensions of the endoderm were taken as exemplified by dotted lines in panels G-H. Note that measurements were taken in non-Ttr stained embryos, in which transparency of the tissue allowed for accurate measurements of the whole definitive endoderm. Error bars indicate standard deviation. See Table on Fig. 5H for primary data.