Forced expression of the bHLH myogenic factors, Myf5 and MyoD, in various mammalian cell lines induces the full program of myogenic differentiation. However, this property has not been extensively explored in vivo. We have taken advantage of the chick model to investigate the effect of electroporation of the mouse Myf5 and MyoD genes in the embryonic neural tube. We found that misexpression of either mouse Myf5 or MyoD in the chick neural tube leads to ectopic skeletal muscle differentiation,

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... sayed by the expression of the myosin heavy chains in the neural tube and neural crest derivatives. We also showed that the endogenous neuronal differentiation program is inhibited under the influence of either ectopic mouse Myf5 or MyoD. We used this new system to analyse, in vivo, the transcriptional regulation between the myogenic factors. We found that MyoD and Myogenin expression can be activated by ectopic mouse Myf5 or MyoD, while Myf5 expression cannot be activated either by mouse MyoD or by itself. We also analysed the transcriptional regulation between the myogenic factors and the different genes involved in myogenesis, such as Mef2c, Pax3, Paraxis, Six1, Mox1, Mox2 and FgfR4. We established the existence of an unexpected regulatory loop between MyoD and FgfR4. The consequences for myogenesis are discussed.