The power of regeneration and the stem‐cell kingdom: freshwater planarians (Platyhelminthes)

Abstract

The great powers of regeneration shown by freshwater planarians, capable of regenerating a complete organism from any tiny body fragment, have attracted the interest of scientists throughout history. In 1814, Dalyell concluded that planarians could "almost be called immortal under the edge of the knife". Equally impressive is the developmental plasticity of these platyhelminthes, including continuous growth and fission (asexual reproduction) in well-fed organisms, and shrinkage (degrowth) during prolonged starvation. The source of their morphological plasticity and regenerative capability is a stable population of totipotent stem cells--"neoblasts"; this is the only cell type in the adult that has mitotic activity and differentiates into all cell types. This cellular feature is unique to planarians in the Bilateria clade. Over the last fifteen years, molecular studies have begun to reveal the role of developmental genes in regeneration, although it would be premature to propose a molecular model for planarian regeneration. Genomic and proteomic data are essential in answering some of the fundamental questions concerning this remarkable morphological plasticity. Such information should also pave the way to understanding the genetic pathways associated with metazoan somatic stem-cell regulation and pattern formation.

Brain regeneration from pluripotent stem cells in planarian

“…How can planarians regenerate the brain from a small fragment of almost any portion of the body? It is believed that this is accomplished by pluripotent stem cells that are distributed throughout the body, which can give rise to all types of cells, including brain neurons Saló & Baguñ à 2002;Reddien & Sánchez Alvarado 2004;Agata et al 2006;Saló 2006). However, we do not know the process by which the brain is regenerated or the genes involved in brain regeneration.…”

Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms

“…The agametic asexual strain reproduces by fission behind the pharynx and has no functional gonads (12). Thus, we hypothesize that it has developed somatically active mechanisms for the maintenance of chromosome ends without sexual reproduction per se.…”

“…We set out to test this idea in potentially immortal planarian flatworms. Planarians have been described as "immortal under the edge of the knife" (11), and may have an indefinite capacity to renew their differentiated tissues from a pool of potentially immortal planarian adult stem cells (pASCs) (12,13). For long-term survival over evolutionary timescales, these cells need to overcome the end-replication problem (6).…”

“…The model planarian Schmidtea mediterranea has both asexual and sexual strains, both with apparently indefinite regenerative capacities (3,12). The agametic asexual strain reproduces by fission behind the pharynx and has no functional gonads (12).…”

Stem Cells and the Temporal Boundaries of Development: Toward a Species-Dependent View

“…By contrast, restorative-facultative regeneration requires a high potential for differentiation and a high recruitment of stem cells (Sánchez Alvarado 2000). Thus, organisms that perform facultative-restorative regeneration during their life possess stem cells described in the previous TDP as specific to the embryonic period (see Salo 2006). Consequently, those organisms present a different TDP compared to those described earlier [TDP (A) and (B)] and the persistence of pluripotent or totipotent stem cells do not allow for making the distinction between phases.…”