Practice question

Activin, related to Nodal and Vg1, serves as model mesoderm inducer. In animal cap experiments, threshold concentrations differentially induce gene expression: very low triggers ventral mesoderm, intermediate concentration activates Xbra and MyoD driving muscle and notochord lateral, while higher doses specify organizer and endoderm. Crucial mesoderm specification events such as somite formation depend on intermediate Activin levels balancing Smad2/3 signaling with FGF. FGF cooperates maintaining Brachyury. This dose dependency underlies morphogen theory where concentration provides positional information for mesoderm patterning, illustrating French Flag principle in Xenopus marginal zone.