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Somites, the segmented building blocks of the vertebrate embryo, arise one by one in a patterning process that passes wavelike along the anteroposterior axis of the presomitic mesoderm (PSM). We have studied this process in Xenopus embryos by analyzing the expression of the bHLH gene, Thylacine1, which is turned on in the PSM as cells mature and segment, in a pattern that marks both segment boundaries and polarity. Here, we show that this segmental gene expression involves a PSM enhancer that is regulated by retinoic acid (RA) signaling at two levels. RA activates Thylacine1 expression in rostral PSM directly. RA also activates Thylacine1 expression in the caudal PSM indirectly by inducing the expression of MKP3, an inhibitor of the FGF signaling pathway. RA signaling is therefore a major contributor to segmental patterning by promoting anterior segmental polarity and by interacting with the FGF signaling pathway to position segmental boundaries.

The vertebrate neural crest arises at the border of the neural plate during early stages of nervous system development; however, little is known about the molecular mechanisms underlying neural crest formation. Here we identify a secreted protein, Noelin-1, which has the ability to prolong neural crest production. Noelin-1 messenger RNA is expressed in a graded pattern in the closing neural tube. It subsequently becomes restricted to the dorsal neural folds and migrating neural crest. Over expression of Noelin-1 using recombinant retroviruses causes an excess of neural crest emigration and extends the time that the neural tube is competent to generate as well as regenerate neural crest cells. These results support an important role for Noelin-1 in regulating the production of neural crest cells by the neural tube.