Fig. 6. Model for the regulation of neuronal differentiation by paraxial mesoderm and levels of FGF signalling. (A) Tissue level regulation of neuronal differentiation. The neural tube in contact with somites contains NeuroM-expressing cells (purple dots) while the CNP/PNT underlain by the PSM does not express this neuronal marker and expresses caudal neural genes. Somites provide signals that repress caudal neural genes and activate neuronal differentiation, whereas the PSM provides signals that maintain caudal neural gene expression and prevent neuronal differentiation. Removal experiments indicate that signals from other tissues in the embryo must also influence the appearance of neurons. *Induction of neurons by other tissues at the level of the CNP/PNT may also be a consequence of the down-regulation of caudal neural genes. (B) FGF signalling and the regulation of spinal cord differentiation. FGF8 and FGF4 are present in the primitive streak and FGF8 is also present in the PSM and the CNP/PNT where these signals promote bra and caudal neural genes and prevent the appearance of NeuroM-positive cells. Somites repress Fgf8 expression in CNP/PNT. Attenuation of FGF signalling down regulates the early mesodermal gene bra but does not elicit neuronal differentiation (NeuroM); somites must therefore also interact with additional pathways to promote NeuroM either by inducing an activator (dashed line) and/or repressing a repressor (see text). (C) Fgf8 expression in stage 9– embryo, showing transcripts in the CNP and PNT, aligned with B. Scale bar: 100 µm.