A long-standing question within the field of planarian regeneration relates to the nature of the neoblasts, planarian totipotent stem cells. These cells are the only cells with proliferative capabilities in these animals and are absolutely indispensable for regeneration. Based on morphological criteria (size, shape, nucleus/cytoplasm ratio) about 20-30% of the planarian cells have been considered as neoblasts. However, it has not been clear for many years whether all these cells represent a uniform neoblast population with identical o similar proliferative abilities and potentiality or, alternatively, neoblast comprise a quite heterogeneous cell population respect to these criteria. Recent studies from several laboratories have identified several genes and transcription factors that are expressed in different populations of neoblasts and are required for the differentiation of specific cell types. Based on these data Peter Reddien proposed two models for planarian neoblasts: 1) the naïve model in which neoblasts are a rather homogeneous cell population with the same potentiality and in which fate specification occurs only in the non-dividing neoblast progeny; in contrast, 2) the specialized model predicts that neoblasts are an heterogeneous population containing many different lineage-committed dividing cells.
Based on several recent studies planarian neoblasts seem to follow the specialized model as several cell type-specific genes are co-expressed with Smedwi-1, a planarian piwi homologue, considered as a neoblast marker. Now, a recent study from the laboratory of Peter Reddien further supports the specialized neoblast model (http://www.ncbi.nlm.nih.gov/pubmed/25254346). In this work, the authors first purified by FACS neoblast that were in the S or G2/M phases of the cell cycle (dividing neoblasts) and analysed their expression profiles by RNAseq. Then, they focussed on transcription factors as these proteins regulate cell differentiation. Here, they identified a list of transcription factors that were upregulated in dividing neoblasts during regeneration. To validate these results they next carried out in situ hybridizations on purified dividing neoblasts and all of the transcription factors tested were expressed within these cells although, evidently, with different percentages of positive cells.
They extended then these analyses to other previously characterized transcription factors specific for different cell types. Again, here, they identified a percentage of dividing neoblasts in which these factors were also expressed. Next, they focussed on the planarian nervous system as a target to test the specialized neoblast model as many different neuronal subpopulations have been identified. From the analyses of the transcription factors identified in their RNAseq experiments as well as for the search of conserved factors with conserved functions on the development of the nervous system they found 26 neural transcription factors that were expressed in dividing neoblasts. One of these factors, a klf homologue was coexpressed with cintillo in mechanosensory cells located around the head periphery. Remarkably, klf RNAi lead to the absence of these sensory cells during regeneration, despite these treated animals were capable to normally differentiate many other neuronal cell types. Similarly, a pax3/7 homologue was found to be expressed in the medial region of the brain in cells some of which also expressed a dopamine β-hydroxylase (DBH) gene. Animals in which pax3/7 was silenced by RNAi regenerated a significant reduced number of DBH positive cells. Altogether, these results suggest that these transcription factors would be necessary for the differentiation of specific neural lineages from distinct progenitor neoblast subpopulations.
Importantly, the authors also found that transcription factors associated to specific cell lineages (pharynx, central nervous system, eye, protonephridia or muscle) were not co-expressed in the same dividing neoblasts. These strongly suggest that distinct neoblast populations expressed specific combinations of transcription factors associated to different differentiated cell types.
In summary, this study reports 36 transcription factors expressed in dividing neoblasts from regenerating planarians. These factors are expressed in different cell types and tissues in adult planarians, which suggests that they may specify distinct subpopulations of lineage-committed dividing neoblasts, further supporting the specialized model.
regeneration in nature 
Let me first of all welcome the new paper from Reddien’s factory on the heterogeneity and whereabouts of neoblasts in regenerating planarians. Once again, it is a well set, well executed, and nicely presented paper.
Said that, I would like to state two main criticisms. Firstly, the much trumpeted dilemma in planarian regeneration on whether neoblasts are a homogeneous population or a heterogeneous population of cells, is a FALSE dilemma. This has recently been revived and renamed, namely from the Reddien lab, as the naïve neoblast versus the specialized neoblast model. And I am saying it is a false dilemma because it never existed. Nobody in his/her senses could imagine, or just think, of all neoblasts (which accounts from 20 to 30% of the total number of cells) as being in an equivalent state, namely as proliferating undifferentiated pluri or totipotent cells. Therefore, the ‘homogeneous model’, now the naïve model, never was seriously considered. Indeed, all papers dealing with neoblasts, namely theoretically or making sensible inferences from other renewing cell systems, did consider neoblasts as an heterogeneous population made by a small, but undisclosed number of true stem-cells (would 5% make it?), a rather large population of progressively commited (progenitor?) proliferating neoblasts (e.g. 45%?) and a rather large populations of non-dividing and already committed neoblasts (50%) (for comprehensive reviews, see Baguñà et al. “Growth, degrowth and regeneration as developmental phenomena in adult freshwater planarians”. In: Experimental Embryology in aquatic Plant and Animal Organisms” NATO-ASI Series (H.J. Marthy, ed). Plenum Press pp. 129-162; (1990); New York; USA; and Baguñà, J. The planarian neoblast: the rambling history of its origin and some current black boxes. Int. J. Dev. Biol. 56: 19-37 (2012).
The second criticism is the lack of a parallel source of information on neoblast heterogeneity for the intact organism. While no wonder the flashing process of regeneration in planarians has attracted much more interest than the daily, and boring to some, renewal of cells during growth and degrowth, the last processes take close to 100 per cent of the life-cycle time of any planarian species. Moreover, regenerative processes have to build up on the existing cells and processes which operate during daily cell renewal. Therefore, regeneration should be viewed as an accelerated process of cell production (cell proliferation) and changing and shifting cell lineages (cell differentiation + pattern formation) driven by the new coordinates of an injured organism: changing body axial polarities, new cell-cell interactions, changing concentrations of morphogens, growth factors and growth inhibitors, etc,… In summary, I would much appreciate reading in a not too distant future a parallel paper on neoblast heterogeneity in the intact organism, going much further than the tepid partial results published in the past (eyes, gut, and excretory system).
In my view, smart planariologist should change gears, leave the boring regeneration with all its o-mics to a few specialized labs, and turn their attention to the flashing processes of degrowth (apoptosis, remodelling), growth (allometric relationships, species-specific size-limits, finding growth factors and inhibitors), cell lineage (transgenesis, gene reporters,..), and aging (role of neoblasts, telomeres, ‘immortal’ asexual versus mortal sexual species, rejuvenation). There is more life in planarians than regeneration, don’t you think so?
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I agree with Professor Baguñà that probably the naïve model for neoblasts was never seriously considered when taking in account the high percentage of cells that fit with the currently admitted criteria to define the neoblasts. Still, there were two main problems until now: 1) first, independently, of the real % of true undifferentiated and totipotent stem cells it was not clear whether during regeneration the neoblasts that form the blastema are still totipotent or are already committed to their final fates. If they are committed, then, when does this commitment occurs? before they enter the blastema or once they are in it?; 2) if we assume that the naïve model is not true for most of the neoblast population we should then be able to identify the molecular factors (i.e transcription factors) responsible for the differentiation of different progenitor cells. To me, what is more rellevant of this study as well as from other studies from other laboratories is that different progenitor populations for different cell types are being identified. Therefore, and in addition to supoort the specialized model, they are extremely useful to follow in time and space how these progenitor cells originate during regeneration. So, still a long way to go before we fully understand the exciting and not boring at all process of regeneration.
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