Practice question

Proximal-distal outgrowth relies on positive feedback between mesenchymal FGF10 and ectodermal FGF8 forming self-sustaining loop. Mesenchymal FGF10 activates Wnt3a beta-catenin cascade in ectoderm to induce AER formation and FGF8 expression in ridge cells. FGF8 then signals back to mesenchyme through FGFR1-2 activating Pea3 and maintaining FGF10 transcription, Gremlin expression and progress zone proliferation. This FGF10-FGF8 loop keeps limb elongating while balancing RA proximally via Cyp26b1. Interruption leads to truncation. SHH-Gremlin loop modulates BMP but does not directly drive PD outgrowth, Wnt7a loops pattern DV.