Practice question

Spemann organizer functions not by secreting inducing ligands but by secreting extracellular inhibitors neutralizing ventralizing signals. Embryological and biochemical work identified chordin, noggin and follistatin that bind BMP4 and prevent Smad1/5/8 activation, plus Frzb, Dickkopf1 and Cerberus blocking Wnt8. Combined inhibition creates dorsal territory free of BMP and Wnt allowing expression of chordin-goosecoid and neural genes. Injection of these antagonists ventrally reproduces organizer grafts. Thus organizer activity involves secretion of BMP antagonists establishing permissive environment for dorsal development and neural competence. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.