Fig. 7. A model for myogenesis initiation. The expression of Myf5 is autonomously initiated in the presomitic mesoderm. Soon after, Wnt5b, which is autonomously expressed in the presumptive caudal border of somite –I and 0, triggers the activation of MyoD expression. However, this effect is inhibited by the action of Bmp. Noggin, expressed in the lateral region of the rostral presomitic mesoderm and the newly formed somites, is likely to release Bmp inhibition. This leads to the expression of MyoD. Shh (and maybe Wnts), secreted by axial structures, amplifies and/or stabilizes MyoD expression in older somites.

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