Practice question

Limb development classic example of epithelial-mesenchymal interaction involves iterative paracrine exchanges forming self-reinforcing loop. Mesenchyme secretes FGF10 inducing Wnt3a and FGF8 in ectoderm forming AER; AER-derived FGFs reciprocally sustain mesenchymal proliferation, FGF10 and Gremlin expression via FGFR signaling. This mutual dependence involves extracellular matrix remodeling, transcription factor activation including Sp6 Sp8 in ectoderm, and maintenance of progress zone undifferentiated and distal. Such cross-talk ensures coordinated outgrowth and patterning across all axes. It is not solely negative feedback or neuronal induction, but dynamic mutual induction.