Fig. 7. Model for Gdf3 function. (A) In wild-type embryos, Nodal activity promotes AVE induction and movement as well as primitive streak formation, and maintains its expression through an autoregulatory feedback loop. Gdf3 is also required for AVE formation and movement, and may additionally be necessary for streak formation and maintenance of Nodal expression. (B) In wild-type embryos (top), expression of Nodal antagonists by the AVE provides essential signals for patterning of the anterior epiblast; the distribution of Nodal pathway activity (including Gdf3) in the epiblast is depicted in blue, while that of Nodal antagonists is shown in red. In the absence of Gdf3, the AVE may form but be unable to move (middle), thereby resulting in embryos with downregulated Nodal expression that is restricted to the proximal epiblast. Alternatively, if the AVE fails to form (bottom), the absence of Nodal antagonists will result in upregulation of Nodal expression throughout the epiblast, potentially leading to expansion of the primitive streak and axis duplication.