Practice question

Digit identity within autopod depends on anterior-posterior gradient of Sonic hedgehog duration and concentration from ZPA. High prolonged SHH posteriorly prevents Gli3 repressor formation, induces Hand2, posterior HoxD13 targets specifying posterior digits 5 and 4 with ulnar characteristics. Low SHH anterior allows Gli3R to promote digit 1 thumb identity. Manipulating SHH bead exposure time converts anterior digits to posterior fate. Gli3 mutant mice show polydactyly with loss of identity gradient. Hence SHH provides instructional positional information defining digit morphological individuality through Gli transcription factor balance and BMP interactions.