Several amphibians are really good in regenerating their limbs after amputation. However, even in these champions of regeneration, this amazing capacity to regrow such a complex structure is often conditioned by the life stage of the animal. Thus, in many cases, especially in frogs, regenerative abilities are very high in tadpoles but decrease significantly after metamorphosis. In fact, this loss of regenerative abilities depending on the life stage is not only observed in amphibians but also in other animal groups, including insects and mammals, in which regeneration power is quite limited after metamorphosis or from post-embryonic stages. Some other amphibians, especially urodeles, retain high regenerative abilities as adults. However, here some variability also exists in terms of the capacity to complete a successful regeneration. In urodeles, another aspect to consider is the influence that metamorphosis may have on their regenerative abilities. Some of them, as some newts, are perfect regenerators as adults but for other species there are unclear results. In any case, it is important to understand how aging, body size and metamorphosis relate to the regenerative capabilities shown not only by urodeles but by other animal groups.
Now a recent paper from the laboratory of James Monaghan and Ashley Seifert has analyzed how inducing metamorphosis in paedomorphic axolotls affect their regenerative abilities (http://onlinelibrary.wiley.com/doi/10.1002/reg2.8/abstract). Axolotls almost never undergo natural metamorphosis but it can be experimentally induced by thyroxine exposure and, therefore, can be a good model to test how body size and metamorphosis affect regeneration. After inducing metamorphosis the authors first checked that the morphology of the limbs of paedomorphic and metamorphic axolotls was quite similar with only slight differences. They used an age-matched cohort of animals with a broad range of body sizes. Contrary to other studies their results suggested that body size (as an independent variable) was not a limited factor for regeneration. Next, they describe how metamorphosis really impairs the regenerative abilities of these axolotls. Although limbs of metamorphic axolotls went through all the normal stages of regeneration, it took to them almost twice as long to regenerate compared to paedomorphs. Moreover, regeneration was not only much slower but also in 100% of the cases the metamorphic axolotls regenerated limbs that showed anatomical defects. These defects included loss of skeletal elements or complete digits or fusion of other skeletal elements. Therefore, metamorphosis reduced regeneration rate and fidelity.
Finally, the authors analyzed cell proliferation dynamics in the blastema of paedomorphic and metamorphic axolotls. By using PCNA, a marker that labels proliferating cells all through the cell cycle, they first showed that there were no differences in the number of proliferating cells. However, when using BrdU to analyze if there were differences in the number of cells in S-phase within those blastemas they found that that metamorphs had a smaller proportion of BrdU-positive cells (relative to total blastema cells), suggesting that there could be a difference in the length of the cell cycle. Remarkably, though, blastemas from metamorphs contained a two-fold higher number of cells.
In summary, this study sows how after inducing metamorphosis in axolotls these animals can still mount a regenerative response after limb amputation. However, regeneration rate and fidelity were severely compromised. Importantly, the defects observed in regeneration after thyroxine treatment were due to metamorphosis and not because of differences in body size or age, which allows to decouple the effects that different parameter may have on the regeneration potential. Moreover, the author suggest that it would be interesting to test whether the putative alterations in the cell cycle timing that they observed after metamorphosis could somehow affect the transcription of some patterning genes required for proper regeneration.