Practice question

Animal cap sandwich experiments with Activin-soaked beads reveal concentration-dependent gene activation mirroring in vivo Nodal gradient. Cells directly contacting high dose bead experience maximal Smad2 signaling activating high-threshold genes goosecoid and chordin forming head organizer and prechordal mesoderm marked by Cerberus and Frzb. Slightly distant cells receiving moderate dose activate Xbra and form trunk mesoderm, furthest cells become ventral mesoderm. Therefore high concentration near beads induces goosecoid expression defining dorsalmost axial mesoderm, analogous to dorsal vegetal cells receiving highest Nodal. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.