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Stem cells are undifferentiated cells that have the capability to self-renew and generate differentiated progeny through asymmetric cell division. Planarians possess a population of totipotent stem cells maintained throughout the adult life, the neoblasts, which have no parallel as renewal potentiality in the animal kingdom. Neoblasts are responsible for the replacement of cells lost during the physiological turn-over and also allow regeneration in these organisms. During regeneration, neoblasts activate a proliferation program that results in the formation of a regenerative blastema, the structure from which the missing parts of the body are progressively rebuilt.
The distribution of neoblasts has been defined by the use of molecular markers such as DjMCM2 and DjPCNA capable to detect proliferating cells or through BrdU incorporation. These cells are scattered throughout the parenchyma with the exception of the anterior end of the cephalic region and the pharynx and are
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preferentially accumulated in the dorsal region, along the antero-posterior body axis.
Although all neoblasts share a similar morphology,
heterogeneity in their population has been hypothesized. The recent characterization of two genes (DjPiwi, a PIWI-PAZ family member, and Djnos, a planarian homologue of nanos gene) expressed in neoblast subpopulations, confirms this hypothesis.
A peculiar advantage to study planarian neoblasts is that these cells can be selectively destroyed by high dose (30 Gy) of X-ray irradiation, thus offering the possibility to directly compare worms lacking stem cells with wild-type controls.
With the aim to identify additional neoblast sub-populations we decided to evaluate whether neoblast sub-groups show variability in their radiotolerance level. With this goal we treated animals with low and sub-lethal doses (5Gy) of X-ray and analyzed the stem cell fate by following the expression of different neoblast markers. We identified a radiotolerant population of proliferating cells that at the
4th day after the irradiation starts an intense proliferation activity,
migrates and repopulates the entire planarian body reconstituting the initial neoblast sub-populations.
