In a recent paper Passamaneck and Martindale show that cell proliferation is necessary for regeneration in Nematostella (http://www.ncbi.nlm.nih.gov/pubmed/23206430). By blocking cell proliferation the authors are able to block also regeneration, suggesting that contrary to what it has been described in Hydra, Nematostella does not appear to have any other compensatory mechanism to allow regeneration in a context of no proliferation. This finding is relevant because it shows how different cnidarian species may use very different modes of regeneration based on the classically used terms of “epimorphosis” and “morphallaxis”.
Originally, in 1901 Thomas H. Morgan wrote that “… there are known two general ways in which regeneration may take place, although the two processes are not sharply separated, and may even appear combined in the same form. In order to distinguish broadly these two modes I propose to call those cases of regeneration in which proliferation of material precedes the development of the new part, epimorphosis. The other mode, in which a part is transformed directly into a new organism, or part of an organism without proliferation at the cut surfaces, morphallaxis”. Based on this definition it appears that when regeneration requires proliferation then it would be epimorhic regeneration, whereas regeneration in the absence of proliferation would be morphallactic. A more updated use of the term “epimorphic” includes also the definition by Richard J. Goss (The natural history (and mistery) of regeneration, 1991. In A History of Regeneration Research. Milestones in the evolution of a science, Ed. C.E. Dinsmore) and that states that “Epimorphic regeneration refers to the regrowth of amputated structures from an anatomically complex stump”, and that “The first event in epimorphic regeneration is the development of a blastema, or regeneration bud, derived from dedifferentiated cells, out of which the new structure will take shape”. So, the consensus is that epimorphic regeneration requires proliferation and the formation of a blastema. But not all blastemas are derived from dedifferentiated cells as stated in Goss definition. That is valid, for instance, for amphibian limb regeneration. But in planarians, regeneration occurs mainly by cell proliferation and the formation of a blastema that is derived from adult pluripotent stem cells and not after dedifferentiation.
But whereas the idea of epimorphic regeneration is quite well-established it cannot be said probably the same for the term of “morphallaxis”. Thus, Hydra has been usually used as an example of “morphallactic” regeneration because it has been know for many years that they can regenerate in the absence of proliferation or without a significant contribution of this proliferation. However, in 2009 the laboratory of Brigitte Galliot showed that in Hydra, and after midgastric bisection, head regeneration depends on an initial apoptotic response below the wound that triggers a proliferative zone with “blastema-like” features that significantly contributes to oral regeneration (http://www.ncbi.nlm.nih.gov/pubmed/19686688). So, in that particular context regeneration seems to be “epimorphic”. As Morgan already said in his definitions, both processes, epimorphosis and morphallaxis, are not mutually exclusive. A good example of that are freshwater planarians that have been considered to follow a mixed epimorphic/morphallactic mode of regeneration. The basis of that is that in addition to the fundamental role of pluripotent stem cells in giving rise a regenerative blastema where the missing structures are formed, there is also a remodeling of the pre-existing tissues far away from the wound that help to attain the proper body proportions during regeneration. This remodeling, considered as morphallactic, is more evident in for example in head and tail pieces regenerating a new whole planarian. Thus, if you start with a big head piece containing a big brain it regenerates not only the whole body posterior to this head (through proliferation, blastema formation and growth of the regenerated part) but at the same time the original head with its brain go through an extensive remodeling so they decrease significantly in size in order to form a smaller planarian with perfect body (head/trunk/tail) proportions. So, following Morgan’s definition of morphallaxis at the beginning of this post literally, this remodeling would not require proliferation. However, a paper from the laboratory of Alejandro Sánchez-Alvarado on the temporal and spatial dynamics of Wnt genes expression during planarian regeneration shows that “… although pre-existing cells can assess their new A/P position in the absence of stem cells, anatomical tissue remodeling in planarians depends on the presence of stem cells” (http://www.ncbi.nlm.nih.gov/pubmed/20707997). Therefore, this data would point out that “morphallaxis” could depend somehow also on proliferation.
To solve this apparent conflict one possibility could be to restrict the term of “morphallaxis” to the first definition given by the same Morgan in 1898 in which he wrote: “Thus, the relative proportions of the planarian are attained by a remodeling of the old tissue. I would suggest that this process of transformation be called a process of morpholaxis”. Then, “morphallaxis” can be clearly associated to “remodeling of the pre-existing tissues” and this would be “proliferation dependent” or “proliferation-independent” depending on the organism (planarian/Hydra) or the specific context of regeneration.