Practice question

John Saunders and Mary Gasseling established ZPA function by transplanting posterior margin mesenchyme to anterior border of chick wing bud. Recipient buds developed mirror-image digit duplications with pattern 65432 alongside normal 23456, revealing a diffusible morphogen controlling anterior-posterior polarity and concentration-dependent digit specification. Later work identified morphogen as SHH acting through Gli3 processing and Hoxd activation. AER removal truncates proximo-distally, retinoic acid inhibition proximalizes, FGF8 overexpression extends outgrowth but does not duplicate AP pattern. Only anterior ZPA graft reproduces posterior digits.