Practice question

AER signaling persistence is reinforced by positive feedback incorporating both mesenchymal and ectodermal transcription factors. FGF10 from distal mesenchyme maintains Wnt3a beta-catenin activity in ectoderm, sustaining FGF8 transcription in AER ridge cells marked by Sp6 Sp8 factors. Additionally, FGF8 maintains its own expression via autocrine loop and mesenchymal feedback through Pea3 induction and Spry inhibition modulation. BMP ligands antagonize this, leading to regression unless blocked by Gremlin1 induced by SHH from posterior. Wnt3a initiates early, BMP2 promotes regression, PITX1 patterns hindlimb, not AER maintenance. Hence FGF8 together with FGF10 sustains AER signaling loop.