Mesoderm induction in amphibians is initiated by:
Mesoderm induction in amphibians follows Nieuwkoop model where vegetal endoderm signals to equatorial region rather than autonomous specification. Vegetal cells expressing maternal VegT transcribe Nodal-related TGF-beta ligands Xnr1, Xnr2, Xnr4, Xnr5, Xnr6. These ligands diffuse to marginal zone activating Smad2 phosphorylation via activin receptors and FoxH1. Gradient interpretation leads to dose-dependent fates, high Nodal plus beta-catenin dorsally inducing organizer mesoderm, moderate levels inducing ventral mesoderm forming blood. Animal cap cells alone lack Nodal and become ectoderm. Removing vegetal hemisphere eliminates mesoderm, confirming vegetal Nodal initiation essential.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Mesoderm induction by vegetal Nodal signals in amphibians.