Several times in posts and comments in this blog we have discussed the necessity of more comparisons between not only different un-related (phylogenetically) regenerative models but also between closely related species with different regenerative potentials. In a paper published last year, Maroko Myohara tells us about the role that neoblasts may have during regeneration in annelids (http://www.ncbi.nlm.nih.gov/pubmed/22615975). Although these days the term “neoblast” is mainly associated to the somatic pluripotent stem cells found in planarians (see some recent posts and comments), the truth is that, as the authors remind to us, the term “neoblast” was coined by Randolph in the late 19th century to refer to some cells that participate during regeneration in the oligochaete annelid Lumbricus. In fact, planarian and annelid neoblasts are not so similar, neither at the morphological level nor in terms of stem cell properties. In planarians, neoblasts are small cells, widely distributed in the animal, representing about 20-25% of total cells and the only cells that can divide. Moreover, at least a proportion of them, behave as real pluripotent stem cells in terms of self-renewal and pluripotency (all this, despite some recent proposed needs to re-define planarian neoblasts). On the other hand, neoblasts in annelids are large cells, relatively few in numbers, appear to occupy rather specific locations in the body (at the intersegmental septa along the nerve cords) and are more prominent in those species that reproduce asexually. In terms of their differentiation potential, annelid neoblasts appear to have a role in the regeneration of mesodermal tissues, whereas ectodermal and endodermal tissues regenerate from the dedifferentiation and proliferation of cells from the same layers. Thus, in annelids, neoblasts are not the only cells that divide. More importantly, self-renewal and pluripotency have not yet determined experimentally in annelid neoblasts.
In this paper the author tells us about two species of the same genus of oligochaetes that show very different regenerative capabilities, Enchytraeus japonensis and Enchytraeus buchlolzi. Whereas E. japonensis worms have neoblasts in each of the body segments (except the 7 head segments and the first trunk segment), E. buchlolzi lack neoblasts throughout their body. Another important difference is that E. japonensis reproduces asexually (also sexually under some circumstances) and E. buchlolzi undergoes exclusively sexual reproduction. In terms of regeneration, E. japonensis worms amputated at any level along the AP axis of the trunk (neoblast-bearing segments) regenerate anteriorly normal heads and posteriorly normal tails. One remarkable feature of anterior regeneration from trunk pieces is that regardless of the level of amputation E. japonensis regenerates only the seven head-specific segments (no matter how many anterior segments were missing). That suggests that after amputation the most anterior facing trunk piece must adopt a positional identity value equivalent to the original 8th segment, regardless of its position along the AP axis before amputation. On the other hand, when worms are amputated through the head region, only the segments missed are regenerated. But respect to posterior regeneration from head fragments, these typically regenerate head segments instead of tails, producing dicephalic worms (that is, there is a reversal of polarity in the regenerated part). So, head fragments that lack neoblasts are also able to regenerate, as in fact it has been also shown for other annelids that lack neoblasts.
What happens in E. buchlolzi? In anterior regeneration, after amputation through the head or anterior trunk (8th-14th segment) regions, worms regenerate few head segments (never the seven head-specific segments), which result in hypomeric heads. Also, some examples of reversal of polarity are found when the amputation is made in the posterior trunk region as sometimes tails instead of heads are regenerated. During posterior regeneration, trunk pieces normally regenerate proper tails but head pieces are incapable of regenerating any tissue and in fact die in few days.
In summary this paper shows how annelids display different regenerative strategies and capabilities. Some of them can regenerate despite lacking neoblasts; others, have neoblasts and use them to regenerate although regeneration is also possible from regions that lack neoblasts (for example from head pieces in E. japonensis); even in the same animals different tissues regenerate either from neoblasts or dedifferentiation of mature tissues (mesodermal derivatives from neoblasts and ectodermal and endodermal from the same layers). In addition, this paper strengths the known relationship between regenerative potential and sexual vs asexual reproduction. In the future, it will be interesting to characterize better, among others, how annelid neoblasts behave during regeneration, determine their potentiality and self-renewal capacity and also, what are the factors responsible for the different regenerative responses observed depending on the level of amputation along the AP axis.