Fig. 4. Convergent extension in wild-type definitive endoderm. Whole-mount in situ hybridization with Ttr probes of wild-type mouse embryos at E7.5 (A,B), E8.0 (C-F) and E8.5 (G-J). Lateral and posterior views illustrate how definitive endoderm (white) grows along the anterior-posterior 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 images are at the same magnification. At E7.5, some VE cells (arrowhead in A,B) still overlay the exterior of the embryonic region; the line in A,B delimits embryonic-extraembryonic parts. Gut closure prevented visualization of all the definitive endoderm in I and J. (b,d,f,h,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 each section is shown (midline at the right edge of 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 dashed lines in 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 s.d. See Fig. 5H for primary data.