Practice question

Zone of polarizing activity at posterior limb margin secretes Sonic Hedgehog morphogen that patterns anterior-posterior axis and imparts digit identities through graded exposure and Gli regulation. SHH concentration and duration activate differential Gli target genes and posterior HoxD cluster specifying digits 1 to 5 from anterior to posterior with distinct phalangeal formulas. Low short SHH supports thumb identity, high prolonged SHH supports posterior digits 5 and 4. FGF8 patterns proximal-distal outgrowth, BMP4 promotes apoptosis and chondrogenesis, Wnt7a patterns dorsal-ventral. Therefore ZPA-derived SHH regulates digit identity establishment fundamentally.