Practice question

FGF8 from AER maintains distal progenitors and progressive proximal-distal patterning through temporal exposure. Gradual loss reduces time distal cells spend in progress zone under FGF influence, so late-forming structures fail to be specified due to premature Meis activation. Conditional genetic deletion of Fgf8 in mouse AER results in limbs with relatively normal stylopod humerus femur but severely truncated autopod lacking digits, because proximal specification occurred before depletion while distal specification needs prolonged FGF exposure combined with Cyp26b1 mediated RA clearance. Complete loss requires earlier FGF10 elimination, extra digits relate to SHH augmentation.