ROLE OF GROWTH-FACTORS IN SHAPING THE DEVELOPING SOMITE

In the vertebrate embryo, the lateral somite gives rise to limb bud an d body wall muscles whereas the medial somite generates the axial musc ulature. We show that in chick embryos; this polarity along the medio- lateral axis is achieved through the antagonistic influences of the la teral 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 dermomyo tome; Noggin expression in the somite is regulated by the Wnt1 protein which is expressed in the dorsal neural tube and mediates the mediali sing effect of the neural tube. Therefore, somite medio-lateral patter ning results from a signalling cascade in which Wnt1 produced by the n eural tube promotes noggin expression in the medial somite which in tu rn antagonises lateral plate-derived BMP4. This mechanism could lead t o the establishment of a BMP4 activity gradient that would produce app ropriate BMP4 signalling to generate medial and lateral somite pattern ing. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.