Practice question

Different dorsoventral mesoderm subtypes require distinct Nodal and Activin-like TGF-beta concentrations. Classic animal cap assay showed low Activin doses induce ventral mesoderm expressing Xbra and globin, intermediate doses induce lateral somitic mesoderm expressing MyoD, high doses induce dorsal organizer expressing Goosecoid and anterior endoderm. Gradient read by Smad2 phosphorylation levels regulates differential gene thresholds via Mix and Brachyury. VegT dosage less determinant than extracellular TGF-beta gradient. Uniform BMP cannot create such diversity, explaining morphogen gradient concept for mesoderm patterning and French Flag model applied to amphibians.