A conceptual framework for analyzing axial patterning in regenerating urodele limbs.

This review describes what we have learned about mechanisms of patterning in regenerating urodele limbs. Experimental evidence for three concepts is presented. First, the regeneration blastema is a self-organizing system based on positional memories inherited from parent limb cells. Second, the autonomous patterning mechanism involves local cell interactions that determine patterns of Hox gene activation. The effector molecules for positional identity reside in or on the cell surface, and can be altered by retinoic acid. Third, proximodistal patterning of the blastema is linked to blastemal growth, which in turn is dependent on a signal from the apical epidermal cap and on the non-uniformity of positional identity in the transverse axes. Lastly, the question of the degrees of similarity between the mechanisms of urodele limb regeneration, urodele limb embryogenesis, and the embryogenesis of other tetrapod limbs is discussed.