Practice question

Proximal-distal positional values determining which segments regenerates are encoded by counter-gradients of retinoic acid proximally and FGFs distally. High RA proximal limb activates Meis1/2 specifying proximal identities, distal FGF8/10 from apical cap promotes HoxA13 distal fate. Blastema cells interpret RA-to-FGF ratio through intracellular receptors regulating Prod1 adhesion molecule gradient and Hox codes. Experimental manipulation: RA treatment proximalizes, excess FGF distalizes. Affinophoresis experiments sorting proximal and distal blastema cells based on differential adhesion corroborate gradient encoding positional memory and intercalary regeneration behavior following positional disparity grafting.