Practice question

Function of zone of polarizing activity as posterior organizer absolutely requires Sonic hedgehog signaling. SHH-producing cells induce Gremlin1 in adjacent mesenchyme, Gremlin antagonizes BMP that would otherwise repress fibroblast growth factors in apical ridge, thereby maintaining SHH-Gremlin-FGF positive feedback loop essential for continued proliferative outgrowth and posterior patterning. Inhibition of SHH collapses loop leading to posterior truncations and oligodactyly with loss of digits 3-5. BMP4 acts downstream for apoptosis, Wnt8c initiates hindlimb field, Pax6 regulates eye, establishing SHH as central ZPA mediator and morphogen.