Practice question

Digit identity along anterior-posterior axis is specified by Sonic hedgehog secreted from zone of polarizing activity at posterior border. SHH forms diffusible gradient anteriorly interpreted by Gli3 processing: high SHH posteriorly blocks Gli3 cleavage into repressor, elevating Gli activator specifying digits 5 and 4, while anterior low SHH allows Gli3 repressor specifying digit 1. FGF8 drives distal proliferation, BMP4 mediates interdigital apoptosis, Wnt3a initiates ridge. SHH simultaneously sustains apical ridge via induction of Gremlin1 that antagonizes BMP, coupling patterning with outgrowth maintenance and survival.