Practice question

Sonic Hedgehog from ZPA establishes posterior identity and expands limb field. Shh null mouse limbs still initiate outgrowth because early FGF10-FGF8 loop activates independently of SHH, so stunted stylopod and zeugopod form, but anterior-posterior digit patterning collapses. Without graded SHH and prolonged Gremlin-mediated maintenance, Gli activator diminishes, digits lose individual identity, fuse partially, and often appear as disorganized polydactylous condensations without clear anterior-posterior distinction. SHH thus dispensable for budding yet essential for organizing digit number and morphology, producing dysmorphic extra structures when absent.