Practice question

Vertebrate limb has three orthogonal axes regulated by distinct organizers acting simultaneously. Apical ectodermal ridge with FGFs governs proximal-distal outgrowth and progressive specification via Meis and Hox clock, Wnt7a and Engrailed-1 regulate dorsal-ventral polarity via Lmx1b induction, while zone of polarizing activity at posterior margin secretes SHH establishing anterior-posterior polarity and digit number through Gli3 processing. SHH concentration and duration impart posterior values, highest posteriorly forming digit five. Grafting ZPA anteriorly duplicates digits in mirror image. Thus ZPA specifically controls AP axis exclusively.