What happens when a wrist blastema from a recently cut Axolotl forelimb is placed on a host hindlimb at the thigh level?
Blastema retains intrinsic positional memory encoded by differential expression of surface proteins Prod1, Meis1/2, and Hox codes resisting environmental reprogramming. When distal wrist blastema from axolotl forelimb is grafted onto proximal thigh of host hindlimb, despite richer nerve supply and growth factors, it does not intercalate to produce intervening arm segments. It regenerates only structures distal to its original level, producing wrist and hand. This demonstrates autonomous positional information stable through regeneration, principle that proximal cells can make distal but distal cannot make proximal without RA proximalization.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Positional memory and blastema transplantation experiments.