Any isolated fragment of a chick embryo, right up until the time of primitive streak formation, is spontaneously able to form a complete embryonic axis. Why then does only a single axis form in intact embryos? New results by Bertocchini et al. () reveal that the normal site of axis formation inhibits other regions from initiating this process. Vg1, synthesised in the posterior marginal zone, induces a fast-moving inhibitor that travels across the 3-mm embryo in under 6 hours, and which acts either in parallel with or upstream of Nodal and Chordin. This inhibitor is distinct from Cerberus, a Nodal antagonist produced by the extraembryonic hypoblast. Instead, the authors suggest that it belongs to the Lefty family of long-range Nodal antagonists. Bertocchini and colleagues also find a new role for FGFs in embryonic axis initiation, and suggest that FGF8 acts synergistically with Nodal to initiate primitive streak formation.
