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Fig. 7. A model for the function of mD2LIC in the establishment of the body axes. (A) In wild-type embryos, mD2LIC is required for the formation of cilia in the node and for the correct morphology of ventral node cells (mottled grey circle). It is also necessary for the normal expression of Foxa2, Shh and T, and for the asymmetric expression of Nodal (dark blue), which leads to the induction of Nodal, Ebaf and Pitx2 in the left-hand-side of the embryo. The notochord (mottled rectangle below the node), also expresses Foxa2, Shh and T, while the adjacent ventral neural tube (light blue) expresses Foxa2 and Shh and the nodal signalling antagonist Leftb. These structures constitute the midline, which is thought to act as a barrier to maintain left-right character in the developing embryo. The anterior definitive endoderm (ADE, yellow) receives survival signals from node derivatives, including the axial mesendoderm and ventral neural tube, and the ADE in turn is thought to maintain an Fgf8-expressing signalling centre in the anterior neural ridge (ANR, purple). This is required for maintenance of the forebrain and anterior identity. (B) In mD2LIC–/– mutants ventral node cells do not form cilia and are flatter than their wild-type counterparts (solid grey circle). Expression of Foxa2, Shh and T is severely reduced or absent (depicted as faded text) and expression of Nodal is usually symmetrical. The compromised signalling properties of the organiser result in reduced expression of Shh, T and Foxa2 in the midline and consequently the absence of Foxa2, Shh and Leftb from the ventral neural tube. The bilaterally symmetrical expression of Nodal and the presumed loss of the midline barrier cause the nodal signalling pathway, normally active only in the left-hand side of the embryo, to be active on both sides. The defective axial mesendoderm does not emit survival signals to the ADE, and the Fgf8 signalling centre in the ANR is lost.





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