Stem cells and regeneration, the case of planarians

Planarians are free-living flatworms, capable of regenerating any missing body part. This remarkable regenerative capacity is due to the presence of an abundant population of stem cells, the neoblasts, dispersed throughout the planarian body. Neoblasts are extremely heterogeneous in terms of X-ray sensitivity, expression profiles and differentiation fates, and include pluripotent clonogenic stem cells, the c-neoblasts. Planarians represent a suitable model system for studying stem cells in their physiological environment, and for the identification of both cell autonomous and extrinsic regulators of stem cell fate. In the last years we identified several neoblast-specific genes, most of which are involved in epigenetic modifications and post-translational regulation. Silencing of some of them by RNA interference induces lethality, inability to regenerate and early differentiation failure. Recently, we also focused our attention on the biosynthetic pathway of polyamines, as putative extrinsic factors involved in stem-cell/differentiated tissue crosstalk. We identified 6 planarian homologues of the ornithine decarboxylase (ODC) gene, the rate-limiting enzyme in polyamine synthesis: four of them are expressed in neoblast late progeny, and functionally involved in stem cell commitment towards the epidermal lineage.