Practice question

Programmed death sculpting free digits from webbed paddle is executed in interdigital mesenchyme regulated by bone morphogenetic protein signaling. BMP4 secreted interdigitally phosphorylates Smad1-5-8 and activates MAPK and caspase cascades resulting in apoptosis, counterbalanced by apical ridge-derived FGF8 and BMP antagonist Gremlin protecting condensing digits. Wnt3a induces ridge, SHH sustains loop via Gremlin induction, retinoic acid modulates field boundaries. Experimental implantation of Noggin beads or reduction of BMP signaling inhibits interdigital death causing syndactyly, establishing BMP4 as principal pro-apoptotic regulator and sculptor.