Practice question

AER persistence depends on balanced BMP antagonism protecting ridge from involution. BMP2 BMP4 BMP7 from mesenchyme and AER itself promote AER regression and induce apoptosis through upregulation of Dkk and activation of Smad signaling antagonizing FGF. Gremlin1 secreted by distal mesenchyme under combined SHH and FGF control binds and sequesters BMP ligands, protecting AER from premature involution. Loss of Gremlin causes early BMP activity elevation, early AER flattening, reduced FGF8 expression, and truncated distal skeleton. Wnt7a dorsalizes, Lmx1b dorsalizes mesenchyme, BMP4 itself would accelerate regression.