Proposed model of FGF-mediated regulation of the Shh pathway in the spinal cord. Neural tube/plate cells adjacent to the presomitic mesoderm are exposed to FGF4/8 (red) and Shh (blue, produced by the underlying notochord). Shh triggers a gene regulatory network that activates downstream genes, including Ptch2, a component of a negative feedback loop of the pathway. In this region, FGF signaling further promotes Ptch2 expression and therefore reinforces the negative feedback loop in the Shh pathway, delaying the onset of ventral gene expression.