Practice question

As downstream effector of VegT, Nodal-related TGF-beta signaling links endoderm specification to mesoderm induction critically. VegT induces transcription of Xnrs in vegetal cells, secreted ligands that bind activin type I and II receptors, phosphorylate Smad2, complex with Smad4, and activate mesodermal genes like eomesodermin, Xbra, and goosecoid in overlying marginal zone. Nodal dose determines mesoderm subtype, while BMP specifies ventral fates and FGF maintains competence. Wnt and Sonic hedgehog are not primary Nodal targets here. Thus VegT-dependent Nodal accumulation creates equatorial mesoderm ring essential for gastrulation and axis formation during embryogenesis.