Practice question

Activin mimics Nodal signaling through Smad2/3 in Xenopus animal cap morphogen assay. Genes respond with different thresholds due to enhancer affinity. High Smad2 signaling near bead induces high-threshold organizer genes like goosecoid, marking dorsalmost mesoderm that will become prechordal plate and head organizer expressing antagonists Frzb and Cerberus. Goosecoid represses Brachyury and Wnt, specifying head mesoderm. At moderate doses pan-mesodermal Xbra appears, so high concentration beads specifically activate goosecoid, reflecting dorsal vegetal signaling gradient in vivo. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.