MESODERMAL SUBDIVISION ALONG THE MEDIOLATERAL AXIS IN CHICKEN CONTROLLED BY DIFFERENT CONCENTRATIONS OF BMP-4

Molecular mechanisms by which the mesoderm is subdivided along the med iolateral axis in early chicken embryos have been studied, When the pr esomitic mesoderm (medial mesoderm) was transplanted into the lateral plate, the graft was transformed into lateral plate tissue, indicating that the primitive somite was not fully committed and that the latera l plate has a cue for mesodermal lateralization, Since the lateral pla te expresses a high level of BMP-4 mRNA, a member of the TGF-beta fami ly, we hypothesized that it is the molecule responsible for the latera lization of the somite. To test this, we transplanted COS cells produc ing BMP-4 into the presomitic region, Those cells locally prevented th e presomitic cells from differentiating into somites, converting them instead into lateral plate mesoderm, which was revealed by expression of cytokeratin mRNA, a marker for the lateral plate, The effect was de pendent on the level of effective BMP-4: with a high level of BMP-4, t he somite was transformed completely to lateral plate; with a low leve l, the somite formed but was occupied by the lateral semitic component expressing cSim 1, a marker for the lateral somite, These results sug gest that different thresholds of effective BMP-4 determine distinct s ubtypes of the mesoderm as a lateralizer during early development.