Practice question

Retinoic acid influences amphibian limb regeneration by resetting proximal-distal positional value toward proximal identity. Binding RAR-RXR nuclear receptors activates transcription of Meis1, Meis2, HoxA9, and surface protein Prod1 characteristic of proximal cells, while repressing distal genes like HoxA13. Distal blastema exposed to RA behaves as proximal, generating full limb duplication when grafted. Endogenous gradient formed by Raldh2 synthesis proximally and Cyp26b1 degradation distally patterns normal limb. Hence exogenous RA proximalizes positional identity, reprogramming blastema cells to adopt more proximal fate during regeneration and development processes.