Practice question

Apical ectodermal ridge at the distal margin secretes fibroblast growth factors maintaining underlying progress zone mesenchyme in proliferative undifferentiated state essential for proximodistal elongation. Induction of ridge requires Wnt3a through beta-catenin activation, while sustained maintenance depends on reciprocal signaling with mesodermal FGF10. Core maintenance molecule produced by ridge itself is FGF8, complemented by FGF4, FGF9, and FGF17. Loss of FGF8 leads to rapid ridge involution, distal apoptosis, and truncation. FGF10 resides in mesenchyme, SHH in posterior ZPA supporting loop via Gremlin antagonism of BMP.