Practice question

Anterior-posterior polarity and digit identity are dictated by graded Sonic Hedgehog morphogen from posterior margin ZPA. SHH diffuses anteriorly, creating high posterior low anterior gradient, regulating Gli3 processing: high SHH preserves full-length Gli activator posteriorly, anterior Gli3 repressor predominates restricting posterior genes. This gradient specifies digit five posterior to digit one anterior and controls number through proliferative expansion via Cyclin D1. Wnt3a induces AER formation, FGF8 drives PD outgrowth, RA proximalizes stylopod. SHH therefore serves as instructive AP morphogen integrating growth and pattern specification.