Practice question

Digit individualization from initially webbed autopod paddle occurs via selective apoptosis of interdigital mesenchyme situated between digit condensations. Interdigital cells express BMP2, BMP4, BMP7 which activate Smad and caspase pathways triggering cell death, protected in digit regions by AER-derived FGFs and Gremlin antagonism. Zone of polarizing activity secretes SHH posteriorly, ridge eventually regresses, lateral plate mesoderm is skeletal origin. Failure of interdigital apoptosis results in soft-tissue syndactyly with retained webbing, underscoring precise spatiotemporal BMP-FGF interplay for free digit formation and separation.