Practice question

Digit individualization depends on BMP-driven apoptosis of interdigital tissue executed by conserved cell death pathways and lysosomal activation. BMP4 binds BMPR1B activating phosphorylated Smad1-5 and upregulating Msx2 and proapoptotic Bax caspase-9 cascade in interdigital necrotic zones, while Noggin and Gremlin protect digit rays allowing Sox9 chondrogenesis and collagen II synthesis. Genetic inactivation of BMP4, Bmpr1b or Msx2 causes syndactyly with retained webbing and fused soft tissue. FGF8 from digit tips opposes BMP to maintain digit outgrowth and survival. SHH and PITX1 regulate patterning and hindlimb identity respectively