Fig. 5. Duplications and expansions of node and notochord in Tcf3^-/- embryos. (A-B'') Scanning electron microscopy (SEM) images of the ventral surface of wild-type (WT; A,A') and Tcf3^-/- (KO; B-B'') embryos (anterior towards the lower left-hand corner). Arrows: Green, nodes; blue, notochords; red, endodermal strip. White line indicates notochordendoderm boundary in B and B'. White boxes in A and B outline areas magnified in A' and B', respectively. Note the club-like structure of the single node and notochord in the wild-type embryo and the duplicated node and expanded notochord in mutant embryo. (C-F') Whole-mount in situ hybridizations of E8.5 wild-type and knockout embryos, probed with digoxigenin-labeled cRNAs for brachyury or Foxa2 as indicated. Anterior is towards the top of each image. The three embryos in each panel in D and F show different representative aberrant patterns, reflective of the extent of node/notochord multiplication. Opposing arrowheads indicate thickness of notochords (ncd); arrows indicate splitting of the notochord, often seen in mutant embryos. (G,G') Tcf3^-/- embryo with a rostral extension probed for brachyury expression. Anterior is leftwards for both images. The primary primitive streak (1°PS) and a secondary primitive streak (2°PS) are positive for brachyury expression. Emerging from the secondary primitive streak are structures similar to a node (arrows) and notochord (arrowheads).