Practice question

Anterior-posterior axis specification depends on coordinated interaction between posterior SHH from ZPA and distal FGFs from AER. FGF signals maintain SHH expression via Gremlin1-mediated BMP antagonism, while SHH maintains FGF expression. Duration and strength of this SHH-FGF-Gremlin feedback loop encodes positional information along AP axis, controlling digit number and identity via Gli3 activator/repressor balance and Hand2 induction. Removal of either center prematurely truncates pattern, while ectopic graft prolongs signaling causing duplications. Hence AP axis emerges from integrated SHH and AER cross-talk rather than independent single signal.