Practice question

Digit identity along anterior-posterior axis depends on graded Sonic Hedgehog from zone of polarizing activity. SHH induces Gremlin1, which antagonizes BMPs, thereby sustaining AER FGFs and forming SHH-Gremlin-FGF feedback loop that prolongs SHH exposure and expands progenitors. Higher and longer SHH promotes posterior digit fates through sustained Hoxd13 and Gli activator activity and repression of Gli3 repressor. Interruption of Gremlin alters SHH duration and range, changing digit number and morphology. TBX5 specifies forelimb field, BMP4 drives apoptosis, Wnt3a induces AER.