The significance of stem cells in free-living flatworms: one common source for all cells in the adult

Abstract

A survey of the current knowledge on stem cells and cell proliferation in turbellarians is presented, with special focus on recent results obtained by the authors when studying cell kinetics and cultivating neoblasts from various species. Differentiated somatic cells do not divide in flatworms. In these animals neoblasts constitute a proliferative compartment. These cells are rather uniform when viewed through a light microscope, but constitute a heterogeneous population comprising actual stem cells, progenitors and early differentiation stages. Cell kinetics were monitored by the incorporation of 5-bromo-2´-deoxyuridine and by immunocytochemical staining for mitoses in the marine microturbellarians Macrostomum, Microstomum and Convolutriloba. Distribution patterns of proliferating cells within the body were documented, and a tentative interpretation of the differences observed is given. Correlations to the nervous system and the mode of reproduction seem to exist. To analyse neoblasts quantitatively and establish primary cultures, neoblasts from the freshwater planarians (triclads) Dugesia and Schmidtea were isolated, purified and subjected to various culture conditions, with the ultimate but not yet achieved goal of establishing a permanent stem cell line. The role of stem cells in flatworms is compared with the situation in mammals, with special focus on embryogenesis, germline and strategies of cell renewal.

The nervous system of Convolutriloba (Acoela) and its patterning during regeneration after asexual reproduction

“…All Wssioning processes depend on regenerative processes, and good regenerative capacity is closely related to asexual reproduction in general. Underlying this high regenerative capacity observed throughout the platyhelminth taxon is a unique neoblast (stem cell) system (Ladurner et al 2000;Peter et al 2004;Reuter and Kreshchenko 2004).…”

The embryonic development of the flatworm Macrostomum sp.

“…we could not resolve the intriguing issue of the origin of the unique post-embryonic stem cell system (neoblast system) from embryonic stem cell precursors (see Peter et al 2001Peter et al , 2004. As summarized in Peter et al (2001), circumstantial evidence even suggests that platyhelminths seem not to have a separate germ line.…”

Pumilio genes from the Platyhelminthes

“…In several free-living Platyhelminthes, somatic cells do not proliferate, and a separate cellular population, called neoblasts, with totipotent capabilities, are responsible for cell proliferation during growth and regeneration (reviewed by Peter et al 2004). Furthermore, evidence suggests that this is a characteristic of the phylum as a whole (Peter et al 2004), including the parasitic clade Neodermata, which includes the well-known classes Cestoda and Trematoda (Baguñà and Riutort 2003).…”

“…Furthermore, evidence suggests that this is a characteristic of the phylum as a whole (Peter et al 2004), including the parasitic clade Neodermata, which includes the well-known classes Cestoda and Trematoda (Baguñà and Riutort 2003).…”