Practice question

During salamander limb regeneration blastema itself carries and maintains positional information acquired from stump level. Cells retain memory through persistent expression of Meis1/2, HoxA9, HoxA13, and cell surface protein Prod1 gradient correlating with proximal-distal origin. Transplantation of proximal blastema to distal location results in intercalation, distal to proximal does not unless proximalized by RA. Blastema sorting assays demonstrate differential adhesion based on Prod1 levels preserving positional disparity. Therefore blastema constitutes repository of positional identity integrating retinoid and FGF cues ensuring regenerate restores correct pattern with faithful proximal-distal polarity.