Positional identity highest along proximal-distal axis is at:
Proximal-distal positional identity is graded with highest positional value at proximal base decreasing toward distal tip. Classical experiments measuring Prod1 concentration, Meis1/2 expression, and cell sorting affinities show proximal blastema cells possess highest adhesion and displace distal cells in mixed aggregates, reflecting maximal positional information. Retinoic acid concentration highest proximally reinforces this gradient. Consequently proximal cells can generate entire distal pattern via intercalation, while distal cells cannot generate proximal beyond origin unless proximalized by RA. Hence proximal base represents reference point of greatest positional value establishing polarity for regeneration and development.
Ref: Gilbert, Developmental Biology, 12th ed., Chapter 21: Positional identity highest at proximal base of limb.