Onset of neuronal differentiation is regulated by paraxial mesoderm and requires attenuation of FGF signalling.

While many neuronal differentiation genes have been identified, we know little about what determines when and where neurons will form and how this process is coordinated with the differentiation of neighbouring tissues. In most vertebrates the onset of neuronal differentiation takes place in the spinal cord in a head to tail sequence. Here we demonstrate that the changing signalling properties of the adjacent paraxial mesoderm control the progression of neurogenesis in the chick spinal cord. We find an inverse relationship between the expression of caudal neural genes in the prospective spinal cord, which is maintained by underlying presomitic mesoderm and FGF signalling, and neuronal differentiation, which is repressed by such signals and accelerated by somitic mesoderm. We show that key to this interaction is the ability of somitic mesoderm to repress Fgf8 transcription in the prospective spinal cord. Our findings further indicate that attenuation of FGF signalling in the prospective spinal cord is a prerequisite for the onset of neuronal differentiation and may also help to resolve mesodermal and neural cell fates. However, inhibition of FGF signalling alone does not promote the formation of neurons, which requires still further somite signalling. We propose a model in which signalling from somitic tissue promotes the differentiation of the spinal cord and serves to co-ordinate neural and mesodermal development.