Distinct regulators control the expression of the mid-hindbrain organizer signal FGF8

Local expression of FGF8 at the mid/hindbrain boundary (MHB) governs the de velopment of multiple neurons and support cells. Here we show that the pair ed-domain protein Pax2 is necessary and sufficient for the induction of FGF 8 in part by regulating the expression of Pax5&8. A network of transcriptio n and secreted factors, including En1, Otx2, Gbx2, Grg4 and Wnt1&4, that is established independently of Pax2, further refines the expression domain a nd level of FGF8 at the MHB through opposing effects on Pax2 activity. Our results indicate that the expression of local organizing factors is control led by combinatorial interaction between inductive and modulatory factors.