Convergent inductive signals specify midbrain, hindbrain, and spinal cord identity in gastrula stage chick embryos

In the chick embryo, neural cells acquire midbrain, hindbrain, and spinal c ord character over a similar to 6 hr period during gastrulation. The conver gent actions of four signals appear to specify caudal neural character. Fib roblast growth factors (FGFs) and a paraxial mesoderm-caudalizing (PMC) act ivity are involved, but neither signal is sufficient to induce any single r egion. FGFs act indirectly by inducing mesoderm that expresses PMC and reti noid activity and also directly on prospective neural cells, in combination with PMC activity and a rostralizing signal, to induce midbrain character. Hindbrain character emerges from cells that possess the potential to acqui re midbrain character upon exposure to higher levels of PMC activity. Induc tion of spinal cord character appears to involve PMC and retinoid activitie s.