Role of growth factors in shaping the developing somite.

In the vertebrate embryo, the lateral somite gives rise to limb bud and body wall muscles whereas the medial somite generates the axial musculature. We show that in chick embryos, this polarity along the medio-lateral axis is achieved through the antagonistic influences of the lateral plate and the medial neural tube. Bone morphogenetic protein 4 (BMP4) mediates the lateralising signal delivered by the lateral plate and is counteracted locally by Noggin expressed in the medial dermomyotome; Noggin expression in the somite is regulated by the Wntl protein which is expressed in the dorsal neural tube and mediates the medialising effect of the neural tube. Therefore, somite medio-lateral patterning results from a signalling cascade in which Wnt1 produced by the neural tube promotes noggin expression in the medial somite which in turn antagonises lateral plate-derived BMP4. This mechanism could lead to the establishment of a BMP4 activity gradient that would produce appropriate BMP4 signalling to generate medial and lateral somite patterning.