Practice question

Proximal-distal elongation and maintenance persist through reciprocal signaling between distal mesenchyme and apical ectodermal ridge. FGF10 from distal mesenchyme induces and maintains FGF8 in AER via Wnt3a beta-catenin pathway, and FGF8 sustains FGF10 transcription and proliferation through Pea3 Ets factors and ERK activation. This mutually dependent FGF10-FGF8 loop is reinforced by Gremlin blocking BMP to prevent AER involution and SHH enhancing Gremlin. Disruption causes progressive truncation with proximal elements spared. Wnt7a-FGF10 does not maintain PD elongation, SHH-BMP loop governs AP timing, PITX1-Wnt3a is not a maintenance feedback pair.