During regeneration not only missing tissues and structures are re-made but also they need to be precisely patterned and integrated along the preexisting defined body axes. Thus, in the case of axolotl limb regeneration, if you amputate at the level of the wrist, only the hand is regenerated. If you amputate through the lower arm, then a lower arm and a hand are regenerated. Finally, if you amputate at the level of the upper arm, a new upper arm, lower arm and hand regenerate. That is, only the structures distal to the amputation plane are regenerated. This is known as the rule of distal transformation. More amazingly, classical experiments showed that if a hand blastema is grafted onto a stump at the level of the upper arm, a normal limb regenerates with the hand blastema cells contributing only to the new hand. In 2009, the laboratory of Elly Tanaka published a very important paper showing that contrary to the notion of a blastema being composed by a mass of undifferentiated cells, a more realistic view is that of a heterogeneous blastema formed by different populations of lineage-restricted progenitor cells (http://www.ncbi.nlm.nih.gov/pubmed/19571878). In that paper they determined that during regeneration new muscle, Schwann and epidermal cells derive from preexisting muscle, Schwann and epidermal cells, respectively. New dermis and skeletal cells could derive from either pre-existing dermis or skeletal cells. The main message was then that when cells dedifferentiate and proliferate in order to provide the blastema with new cells to regenerate the missing structures they somehow keep a memory of their origin. Then, combining specific GFP labeling and grafts for different cell types and tissues they showed that cartilage cells obey the rule of distal transformation but that Schwann cells do not.
Now, a recent paper by Nacu and collaborators from Tanaka’s lab have expanded the characterization of which cell types and tissues obey this rule (http://www.ncbi.nlm.nih.gov/pubmed/23293283). Thus, when grafting a wrist blastema from an animal in which connective cells express GFP onto an upper arm stump of a non-labeled host, GFP positive connective cells are found only in the regenerated hand. But when the authors transplant an upper arm blastema from an animal in which connective cells express GFP onto an upper arm stump of a non-labeled host, now GFP positive cells are found in the regenerated upper arm, lower arm and hand. This indicates that connective tissue cells obey the rule of distal transformation. However, when using GFP labeled muscle cells they saw that these do not obey that rule. So, if a wrist blastema from an animal in which muscle fibers express GFP is transplanted onto an upper arm stump of a non-labeled host, GFP muscle cells are found in the upper arm, lower arm and hand. In addition to these results, the authors also provide evidences that similarly to what happens during embryonic development the β-catenin pathway appears to have a role during the patterning of the regenerated muscle tissues. This provides a new example of how some processes are regulated by the same molecules and pathways in both development and regeneration.
In summary these results may help to characterize the main actors and processes involved in the establishment of the proximo-distal outcome of axolotl limb regeneration.