Practice question

Apical ectodermal ridge secretes FGF8, FGF4, and other FGFs maintaining progress zone mesenchyme survival, proliferation, and undifferentiated state enabling proximodistal extension. Surgical removal of ridge during early chick wing development triggers immediate cessation of outgrowth, massive distal mesenchymal apoptosis, and truncation where proximal stylopod remains but zeugopod and autopod are lost. Later removal deletes only distal digits, indicating progressive fate acquisition. Extra digits originate from ectopic ZPA, hindlimb transformation from Tbx exchange, highlighting ridge as essential growth-promoting signaling center maintaining distal potential.