What does the activin gradient experiment in Xenopus embryos demonstrate?
Xenopus animal cap assay provided direct evidence that TGF-beta family ligand Activin acts as a classic morphogen encoding multiple fates. Dissociated ectoderm exposed to increasing doses shows stepwise fate changes: low Activin induces ventral mesoderm markers like Xbra and blood, intermediate doses induce muscle actin and dorsal mesoderm, high doses activate goosecoid and organizer genes plus endoderm markers such as Sox17 and endodermin. Dose-dependent Smad2 phosphorylation and target gene activation occur without cell contact, demonstrating diffusion-based gradient interpretation rather than exclusive high-threshold or juxtacrine inhibition mechanism.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Activin Gradient and Xenopus Mesoderm Induction.