In vertebrates, the segmented somites, which are the medial-most component in the paraxial mesoderm, are the entity giving rise to the axial bones and skeletal muscles. We previously demonstrated that the mechanism that distinguishes the somite from the more lateral mesoderm (lateral plate) involves different levels of BMP-4 activity which is highest in the lateral plate. We report that Noggin, an antagonist of BMP-4, is expressed in the presumptive somite and appears to control effective levels of BMP-4 to differentiate somitic mesoderm from the lateral plate. When Noggin-producing cells were implanted into the presumptive lateral plate, they produced ectopic somites that were respecified from the lateral plate precursors. These somites exhibited no mediolateral (M-L) polarity, but acquired it when implanted Noggin was eliminated. Thus, in normal embryogenesis no or low BMP-4 activity realized by Noggin specifies the somites in the medial-most portion of the paraxial mesoderm, and then BMP-4 emanating from the lateral plate subsequently establishes the M-L polarity in the somites.
Copyright 1998 Academic Press.