Practice question

Retinoic acid acts as morphogen proximalizing regenerating blastema through nuclear RAR-RXR mediated transcription. Exogenous RA applied to distal blastema upregulates proximal transcription factors Meis1, Meis2, HoxA9, Prod1 surface gradient marker, shifting positional value toward proximal base. Consequently wrist-level blastema normally restricted to hand regeneration regenerates entire limb including humerus, radius. Dose-dependent response shows increasing RA yields more proximal duplications. This illustrates RA sets proximal positional identity antagonizing distal FGF signals and illuminates how chromatin remodeling underlies positional reprogramming during regeneration and limb development.