Practice question

Axolotl limb amputation at distal level normally regenerates only distal parts. Retinoic acid treatment proximalizes blastema positional memory activating proximal markers Meis1/2, HoxA9 and Prod1, converting distal blastema to proximal identity. Consequently distal wrist blastema treated with RA regenerates complete limb from stylopod through digits, representing serial duplication beyond amputation plane. Experiments by Maden and colleagues showed dose-dependent proximalization: low doses duplicate forearm, high doses duplicate entire arm. This property demonstrates RA as reprogramming agent altering positional value and Hox gene expression during epimorphic regeneration.