Practice question

Apical ectodermal ridge functions as distal organizer secreting FGF4 FGF8 to keep underlying mesoderm proliferative, maintain expression of distal programs including Meis repression and FGF10, and induce progressive distal specification under RA antagonism via Cyp26b1. It also influences anterior-posterior pattern indirectly via maintaining SHH-Gremlin loop and duration of SHH. However dorsal-ventral specification depends on non-AER ectodermal cues: Wnt7a from dorsal ectoderm inducing Lmx1b, and Engrailed-1 ventrally repressing Wnt7a and promoting BMP. Therefore specifying DV axis, nail versus pad fate, is not an AER function.