Regeneration: Rewarding, but potentially risky

Abstract

Some bilaterally symmetric animals, such as flatworms, annelids, and nemerteans, are renowned for their outstanding regeneration capacity-even a fraction of the body can give rise to a complete new animal. However, not all species of these taxa can regenerate equally well-some cannot regenerate at all. If regeneration was purely beneficial, why cannot all of members of the flat, round, and ribbon worms regenerate? At that, why cannot all other bilaterians, including humans, regenerate as well? Regeneration capacity is an obvious advantage in accidental, predatory, and parasitic loss of body parts and is also closely intertwined with asexual reproduction strategies. Regeneration is suspected to play a role in life span extension or even rejuvenation. An answer for reduced or missing regeneration capacity in many species may be found in limitations of the body plan, high costs, and inherent dangers of regeneration. Defects in adults and juveniles are shown, and similarities between development and regeneration are pointed out. With a focus on some worms, but also highlighting comparisons with other animal taxa, putative reasons for a limited and an advanced regeneration capacity are discussed in this article.

Regeneration across Metazoan Phylogeny: Lessons from Model Organisms

“…It has often been remarked that regeneration is a primary, ancestral attribute of metazoans that was curtailed or lost secondarily over the course of evolution (Bely and Nyberg, 2010). Extensive parallels between regeneration and other forms of development, especially post-embryonic growth, suggest that regeneration probably arises as a byproduct, or epiphenomenon, of normal development (Egger, 2008). Regeneration-associated cascades may first arise as a method to propagate the species, and only subsequently be coopted for the purpose of adapting to injuries and other natural traumas.…”

From Hydra Regeneration to Human Brain Structural Plasticity: A Long Trip through Narrowing Roads

“…Primitive bilaterians, which start to show an asymmetric regionalization of the nerve net [97], and planarians, which possess a primitive brain structure (a sort of bilobed brain formed by two cephalic ganglia [98]), can still regenerate a functional brain from almost any tiny body fragment [25]. However, although some bilaterally symmetric animals, such as flatworms, annelids, and nemerteans, are renowned for their outstanding regeneration capacity, not all species of these taxa can regenerate equally well and some cannot regenerate at all [99].…”

Rejuvenation in distinct cell populations – What does it mean?

“…To minimize the generation of oxidative stress products, previous work has shown in a variety of mammalian species that caloric restriction significantly contributes to an overall extension in life span [Barzilai and Bartke, 2009]. Although these effects are not necessarily associated with rejuvenation, longevity is also observed in certain animals even after starvation [Egger, 2008] and may imply the risk to develop neoplastic properties. Conversely, a certain threshold of oxidative stress or nutrient restriction can also trigger rejuvenation in certain cell types, whereby the activation of poly(ADPribose)polymerase (PARP) enzymes confers signals during the stress response via associated factors such as the proteasome complex [Harnacke et al, 2006;Selle et al 2007].…”

“…Thus, lost or damaged tissues are reconstituted in a subsequent proliferation and differentiation program [Tsonis, 2008]. Similar types of rejuvenation to preserve the regenerative potential are also observed in some annelids, whereby repeated regeneration in animals following consecutive amputations contributed to a substantially increased life span as compared to untreated animals [Egger, 2008]. However, the mechanisms which regulate cellular rejuvenation and simultaneously limit this process to restore and maintain the previous tissue size and homeostasis remain obscure.…”