Practice question

Dorsal mesoderm from Spemann organizer involutes as chordamesoderm: anterior prechordal plate induces forebrain, while posterior notochordal and paraxial dorsal mesoderm secretes Wnt antagonists Frzb plus FGF and retinoic acid gradients that posteriorize overlying neural ectoderm, converting default anterior neural fate to hindbrain rhombomeres expressing Krox20 and spinal cord trunk expressing HoxB9. Experiments grafting dorsal mesoderm beneath animal caps induced Krox20-positive hindbrain and HoxB9 trunk markers. Ventral mesoderm and epidermis lack this caudalizing ability, confirming dorsal mesoderm induces hindbrain and trunk, essential for AP neural patterning.