Practice question

Dose-response of Activin illustrates morphogen interpretation: cells measure extracellular ligand concentration translating into intracellular Smad2 phosphorylation levels. Intermediate phosphorylation activates medium-threshold genes like Xbra, vertebrate Brachyury T-box factor required for general mesoderm maintenance and convergent extension via regulation of Wnt11 and FGF. Xbra maintains its own expression through FGF feedback loop. High doses switch to goosecoid repressing Xbra, low doses induce ventral genes. Therefore intermediate Activin specifically activates Xbra defining trunk mesoderm and chordamesoderm progenitors. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.