Fig. 8. Incorporation of a new r4 signaling center in promoting hindbrain development. A zebrafish hindbrain showing seven rhombomeres (r1-r7). Many FGFs are expressed at the midbrain-hindbrain boundary, and zebrafish fgf8 is required for cerebellar/r1 development (reviewed by Rhinn and Brand, 2001). Retinoic acid likely signals from more posterior tissues (neural and/or mesodermal) (reviewed by Gavalas and Krumlauf, 2000). fgf3 and fgf8 expression overlap in r4 and promote the development of r5 and r6. FGF3 and FGF8 are required for otic development, but it is not clear how directly the FGF/r4 signals act on the head periphery. FGF3 and FGF8 from r4 may also have an influence on the development of more rostral rhombomeres (such as r3).

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