The failure of double-half forelimbs to undergo distal transformation following amputation in the axolotl, Ambystoma mexicanum.

Although capable of initiating early regenerative responses, axolotl forelimb stumps which are composed of double-half limb tissues fail to undergo the events that normally lead to the replacement of missing parts. In the present study, the posterior halves of right forelimbs were exchanged with the anterior halves of left forelimbs, or the dorsal halves of right forelimbs were exchanged with the ventral halves of left forelimbs. Forelimbs were amputated through the graft region 30 days after grafting. Limb stumps bearing double-dorsal, double-ventral or double-posterior tissues either produced hypomorphic regenerates or failed to form any externally visible outgrowth. When the limb stump bore double-anterior tissues, no externally visible structures were formed. Normal and multiple regenerates were never formed by double-half limbs. These results are discussed in terms of the polar coordinate model and suggest that the regeneration blastema requires a complete circumference of positional values in order to complete distal transformation.