Fig. 1. FGF8 begins to be expressed in the mouse mid/hindbrain region prior to Fgf18 and can induce Fgf18 expression in mouse midbrain explants. (A-C) Fgf8 is first expressed (arrowheads) in the mid/hindbrain region at the four-somite stage (C). (D-F) Fgf18 is first expressed in the mid/hindbrain region at the five-somite stage (arrowhead in E) and becomes restricted to a narrow transverse band straddling the isthmus by E9.5 (arrowhead in F; asterisk in D indicates the presumptive mid/hindbrain junction region). (G,H) Fgf18 is induced by FGF8b-soaked beads (arrowhead in H) by 48 hours, but not by BSA-soaked beads (G). Inset in H shows that Fgf18 is induced by FGF8b by 16 hours. (I,J) Fgf17 expression is first detectable in the mid/hindbrain region at the six-somite stage and at E9.5 it is in a broad domain on both sides of the mid/hindbrain junction (arrowheads). (K,L) Fgf17 is not induced after 48 hours by either the BSA-soaked or FGF8-soaked beads in rostral midbrain explants. Arrowhead in L indicates the endogenous Fgf17 expression sustained in the explant. Broken lines in F and J indicate the tissues used for the explant assays.