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Fig. 3. Morphological defects in Tcf3-/- embryos. (A-F') Wild-type (WT; A,A') and Tcf3-/- (KO; B-F') E8.5 embryos. The view displayed is indicated in the lower left-hand corner. (A,A') Wild-type embryos display bilateral symmetry with two neural-folds (nf) and somites along either side of the AP axis. Rhombomere formation in the neural-folds allows identification of forebrain (fb), midbrain (mb) and hinbrain (hb) regions. The broken line in A' refers to the position of the section displayed in G. (B-D) Mildly affected Tcf3-/- embryos display multiple morphological defects. (B,B') Multiple neural-folds are clearly visible in some mutants. (C) Caudal half of a Tcf3-/- embryo with a secondary neural tube (2°nt) and extra row of somites (asterisks) adjacent to the primary neural tube (1°nt) which runs along the primary AP axis. A higher magnification view of the boxed region in C is displayed in C'. (D) Lack of forebrain structures and an enlarged heart appear in the Tcf3-/- mutant shown. Broken line in D refers to the position of the section displayed in H. (E-F') Ventral views of severely affected Tcf3-/- embryos. (E) Multiple neural groove-like structures (ng) are apparent along the ventral surface of the Tcf3-/- embryo but neural-folds appear to be absent. Red arrowheads indicate areas along the ventral surface that appear to lack underlying mesoderm and ectoderm. The boxed region in F is shown in higher magnification in F'. (G,H) Toluidine Blue stained semithin sections of embryos in A' and D, respectively. Notice the single neural-groove in G and the three neural grooves in H. (I) E7.5 wild-type or knockout embryos, as indicated at the top of each embryo. A Toludine Blue stained semithin section from the plane indicated with the broken line is displayed below each embryo. Notice the morphological defects in the primitive streak region of Tcf3-/- embryos manifest either a bulge (arrowheads, middle embryo) or a second primitive streak region (arrowheads, right embryo). ncd, notochord; ne, neurectoderm; fg, foregut, EEX, embryonic/extra-embryonic border.