Practice question

Autopod digit patterning and identity along anterior-posterior axis depend on posterior-derived Sonic Hedgehog gradient from ZPA. Duration and concentration of SHH exposure differentially regulate Gli3 processing from repressor to activator and 5-prime HoxD expression, specifying digit one thumb anteriorly through digit five little finger posteriorly with distinct sizes. High prolonged SHH maintains posterior Hoxd13 and posterior identity, low transient yields anterior. BMP4 sculpts interdigits through apoptosis, Wnt3a governs dorsal-ventral polarity, TBX5 governs forelimb field. Thus SHH determines which morphological digit character forms in autopod and controls number.