Taking the challenge to the extreme, Spallanzani investigated regeneration in more-complex animals and departed from experiments on simpler invertebrates to actual tetrapods that resembled human anatomy (Dinsmore, 1996). A similarly radical manifestation of in vivo reprogramming as a response to injury also occurs during newt limb regeneration when postmitotic, multinucleated muscle cells break up into mononucleate progeny, which subsequently re-enter the cell cycle and contribute to the new appendage. We do not capture any email address. They show that Epimorphic Regeneration happens in certain species such as salamanders which have the ability to regenerate limbs, the tail, and even eye lenses, with a deer their antlers can regenerate. However, this accessory blastema eventually regresses unless a piece of skin from the posterior side of the limb is grafted to the anterior wound site. epimorphic regeneration of a number of organs, but not all. We also present arguments about why it is important to study closely related species in regeneration research. This finding might indicate that, in the absence of Pax7, skeletal muscle regeneration is fuelled by dedifferentiation of myofibres in Pleurodeles (Elewa et al., 2017). Earlier studies identified connective tissue, which gives rise to cartilage, bone, tendons, periskeleton and dermal and interstitial fibroblasts, as the major contributor to the blastema during limb regeneration (Muneoka et al., 1986). Genetically modified lines are difficult to establish due to its long generation time and complex life cycle (Abdullayev et al., 2013; Looso et al., 2013). Here we argue the need to move beyond the primary salamander models to investigate regeneration in other species. The questions provoked by salamander regeneration have since been refined to address the regeneration-specific mechanisms involved in sensing which cells, tissues, organs or entire appendages are missing in a mature body, and in triggering the appropriate regenerative response to recreate the original structure. Limb regeneration depends on the formation of a blastema, from which the new appendage develops. When a salamander is faced with a predator, it may simply cause its tail to fall off, which flops around distracting the predator while the salamander scampers away. In this Primer, we provide an overview of salamanders as a model for the study of regeneration. (E) The plethodontid Bolitoglossa ramosi is a fully terrestrial, direct developer (no larval stage) for which a limb regeneration transcriptome has been reported (Arenas Gomez et al., 2017, 2018). Longevity Views, — Did I have a COVID-19 infection? Salamanders are much better at regeneration, in every way, but at least we know mammals aren’t completely left out of the regeneration game. Key processes during limb regeneration. The molecular and cellular choreography of appendage regeneration, Xenopus microRNA genes are predominantly located within introns and are differentially expressed in adult frog tissues via post-transcriptional regulation, The wound epithelium of regenerating limbs of Pleurodeles waltl and Notophthalmus viridescens: studies with mAbs WE3 and WE4, phalloidin, and DNase 1, Salamander spinal cord regeneration: The ultimate positive control in vertebrate spinal cord regeneration, Spatio-temporal neural stem cell behavior leads to both perfect and imperfect structural brain regeneration in adult newts, Speciation, species boundaries and phylogeography of amphibians, Skeletal muscle dedifferentiation during salamander limb regeneration, Changes in regenerative capacity through lifespan, A primer for generating and using transcriptome data and gene sets, From pluripotency to totipotency: an experimentalist's guide to cellular potency, https://dev.biologists.org/collection/regeneration_models, Salamander species: variations on a theme, Insights from studying regeneration in salamanders, Read & Publish participation extends worldwide, Imaging Development, Stem Cells and Regeneration, The Immune System in Development and Regeneration, © 2019. Each article provides background information about the phylogenetic position of the species, its life-cycle and habitat, the different organs and tissues that regenerate, and the experimental tools and techniques that are available for studying these organisms in a regenerative context. Regeneration: if they can do it, why can't we? An exception from this rule is the HoxA gene cluster: despite the general increase in intron length, the sizes of introns in the axolotl HoxA locus are very similar to those in other vertebrates (Nowoshilow et al., 2018). Using unbiased profiling and clustering of over 25,000 cells, Leigh and colleagues described the variety of immune cells that localize to the wound site and infiltrate the developing blastema. 5). Please log in to add an alert for this article. Both established and emerging species are shown, highlighting regenerative organs/tissues and major resources available for each species. Injury-responsive neurogenesis is fuelled by reactivation of quiescent resident neuronal progenitor cells, the ependymoglial cells, which are the equivalent of glial cells in mammals. Finally, we outline the variety of mechanisms deployed during salamander regeneration, highlight how these mechanisms are currently being investigated and how their study is informing us more broadly about regenerative mechanisms and capabilities. The capacity for lens regeneration also changes: whereas axolotls can regenerate a lens only during early developmental stages, newts sustain this ability throughout adulthood (Eguchi et al., 2011). The larvae are ferocious zooplankton hunters that undergo metamorphosis prior to leaving the aquatic milieu and becoming terrestrial juveniles (termed efts), which seasonally return to water to breed after they reach sexual maturity. The only unique […] Published by The Company of Biologists Ltd. Within the context of this debate, Lazzaro Spallanzani (1729-1799) investigated salamander regeneration. The salamander is not unique in its capability to grow new appendages. Developmental geneticist Kathryn Anderson passed away at home on 30 November 2020. Instead, complete amputation, or a wound that covers the entire circumference, is necessary for limb regrowth. Additionally, severed nerves at the site of amputation are necessary for blastema cell proliferation, as denervation prevents blastema growth and consequently limb regeneration (Farkas and Monaghan, 2017). The cells then start dividing to form the missing tissue. For example, lens regeneration in newts is dependent on iris pigmented epithelial cells that dedifferentiate and proliferate, and the subsequent transdifferentiation of a subset of these cells into a new lens (Eguchi et al., 2011; Sousounis et al., 2014). Conversely, just because newts undergo metamorphosis, it does not necessarily mean that they would lose all embryonic features as adults. Notophthalmus and Cynops have very complex life cycles, with both aquatic and terrestrial phases, which makes them cumbersome to breed under laboratory conditions. The hope for limb regeneration is worthy of science fiction. The salamander limb contains all typical structural elements of tetrapods. About 550 existing species of amphibians are categorized under the common name of Salamander. In the case of lentectomy (lens removal, Fig. For example, genes responsible for patterning and morphogenesis are re-activated during limb regeneration, although their precise regulation is not a complete recapitulation of embryonic development (Stocum, 2017). After the completion of wound healing, the basal epidermis also expresses agr2a. The overall cytoarchitecture of the brain, with its multiple neuronal subpopulations arranged in spatially defined domains, is shared among all vertebrates, including salamanders, even though the salamander brain has undergone secondary simplification during evolution (reviewed by Joven and Simon, 2018). What Is Epimorphic Regeneration? During the Enlightenment, two opposing views attempted to explain how animals develop: preformation and epigenesis. Although lens regeneration is not affected by repeated removal or by ageing in newts, regenerative capacity is lost in the axolotl 2 weeks after hatching (Eguchi et al., 2011; Henry and Hamilton, 2018; Sousounis et al., 2014). 1). The damaged axons then regrow through permissive channels formed by the extensions of ependymoglial cells (which are the counterparts of radial glial cells in mammals) that line the central canal, allowing for rewiring of the damaged circuitry (reviewed by Joven and Simon, 2018). Whether these differences between newts and axolotl in lens regeneration capacity and in myogenic dedifferentiation reflect a higher degree of cell plasticity in newts remains to be determined. We outline the varieties of mechanisms deployed during salamander regeneration, and discuss how these mechanisms are currently being explored and how they have advanced our understanding of animal regeneration. A typical salamander life cycle (exemplified here by that of Notophthalmus viridescens) involves both terrestrial and aquatic stages. 4), pigment cells are the principal source of a new lens: in this case, also referred to as Wolffian regeneration, fully differentiated pigmented epithelial cells dedifferentiate and proliferate, but only the cells that originated from the dorsal iris transdifferentiate into a new lens (Eguchi et al., 2011). This question is for testing whether or not you are a human visitor and to prevent automated spam submissions. The first phase is referred to as the preparation phase and begins immediately following amputation with the formation of a wound epithelium (WE) over the amputation plane. This kind of ‘add‐on’ regeneration is called ‘epimorphic regeneration’ (Suzuki et al. While miR-427 expression and function was thought to be limited to early embryogenesis, its expression in adult newts after injury suggests that a similar mRNA clearance event might occur during newt regeneration, perhaps as a component of cellular reprogramming. Developmental biologists have long pondered on the mysteries of this very limb regeneration. Bones, dermis & cartilage just beneath the site of amputation contribute to regenerating blstema. In summary, salamanders have finally entered the post-genomic era, following the sequencing of two salamander genomes and with a growing toolbox for cell type-specific molecular interrogation. In mammals, one very important family of growth factors implicated in the control of almost all aspects of wound healing is the transforming growth factor-beta family (TGF-β).” – “Our results also demonstrate the presence of multiple components of the TGF-β signaling machinery in axolotl (salamander) cells. Underneath the wound epidermis, stump cells begin to populate the blastema. What drives variation in plethodontid salamander species richness over space and time? Repeated elements are often located in introns whose median size in the axolotl is on average an order of magnitude longer than introns in the human genome. It could have immense implications in our current society. Put yet another way “Epimorphic regeneration following limb amputation involves wound healing, followed shortly by a phase of dedifferentiation that leads to the formation of a regeneration blastema. These structures include the jaws, spinal cord, heart ventricles, some eye structures, and most notably their limbs (1). In amphibians, after limb loss, the amputation area heals forming an epidermal layer. Many salamander species are entirely land living without an aquatic larval stage. salamander regeneration process has been shown to reverse tumorigenicity. For example, it has been shown that limbs can regrow in the absence of nerves upon forced expression of the gene anterior gradient (AG). All salamanders demonstrate the potential to regenerate complex structures: they can regrow, among other parts, entire limbs, a tail, ocular tissues, substantial parts of their central nervous system and heart (Joven and Simon, 2018; Tanaka, 2016). Mechanisms of nuclear reprogramming by eggs and oocytes: a deterministic process? Another stream of similarity between human wound healing and salamander limb regeneration involves TGF, , transforming growth factor beta. Brain regeneration in salamanders. The first was based on the ancient observation that lizards can regrow their tails, a phenomenon that gained renewed interest in the late 17th century. Molecular studies have identified links between nerve dependence and positional memory, notably between positional cues along both the proximo-distal and anterior-posterior axis. We demonstrate that skeletal muscle dedifferentiation involves satellite cell activation and that these cells can contribute to new limb tissues. In other words, an amputation through the upper arm will lead to regeneration of a limb from elbow to hand, while an amputation through the lower arm will not regenerate a second upper elbow but only the more-distal structures (the wrist and hand). By the second week after amputation, the blastema has grown noticeably, and by the third week the initial stages of an elbow bend and a flattening hand palette can be discerned. Every species is capable of regeneration, from bacteria to humans. (Bottom) Studies on red-spotted newts have showed that dopaminergic and cholinergic (not shown) neurons regenerate in several brain regions after the selective ablation of individual neuronal subtypes. After spinal cord transection in salamanders, a process of wound healing at each side of the injury leads to restoration of the central canal, followed by the production of new neurons and axons. (D) The Japanese fire-bellied newt, Cynops pyrrhogaster, has been used to study eye, limb, jaw and brain regeneration. (A) The Mexican axolotl, Ambystoma mexicanum, is a paedomorphic salamander that retains fully aquatic features throughout its entire life cycle. However, in lieu of mutagenesis, comparative genomics now allow us to contrast different responses to injury and to identify gene expression signatures that correlate with efficient regeneration. In sharp contrast, the Pleurodeles genome harbours both Pax3 and Pax7. The most complex part of the CNS – the brain – can also regenerate in salamanders. Our results show that the complex phenomenon of limb regeneration can be achieved without complete dedifferentiation to a pluripotent state, a conclusion with important implications for regenerative medicine(ref).”  This work relates to the salamander Ambystoma mexicanum (the axolotl). Nerve-derived factors are required for subsequent blastema cell proliferation and outgrowth. Indeed, compared with their tailless peers, salamanders excel in regenerating damaged or lost body parts throughout their entire life. also described the heterogeneity of wound epidermis cells during axolotl limb homeostasis and regeneration, and identified markers for ionocytes, Langerhans cells, apical, intermediate and basal epidermis and small secretory cells. When a salamander is faced with a predator, it may simply cause its tail to fall off, which flops around distracting the predator while the salamander scampers away. The Iberian ribbed newt (Pleurodeles waltl) fulfils all necessary criteria: these animals are easy to breed in the laboratory because they do not require a terrestrial habitat after metamorphosis, they have a generation time similar to the axolotl (of 9-12 months) and they possess the same regeneration spectrum as other newts (Chevallier et al., 2004; Joven et al., 2015; Tassava et al., 1993; Urata et al., 2018). This may reflect the juxtaposition that occurs upon amputation: as the blastema grows, the flat transection of an amputation site becomes a dome-shaped protrusion, at the tip of which cells from distant regions (e.g. (C) The Iberian ribbed newt, Pleurodeles waltl, is a highly regenerative, emerging model species. Imaging Development, Stem Cells and Regeneration Their pseudotime analysis also identified a trajectory for wound epidermis differentiation in which basal epidermal cells provide a reservoir of progenitor cells that connect the basal epidermis to outer small secretory cells via the layer of intermediate epidermis (Leigh et al., 2018). By using a specific pharmacological inhibitor of TGF-β type I receptor, SB-431542, we show that TGF-β signaling is required for axolotl limb regeneration(ref).”. Regeneration of ocular tissues. Neuronal diversity is restored but projections are not a faithful replication of the original. Finally, recent scRNA-seq studies have increased the resolution by which we can study limb regeneration and have offered insights into immune cell participation, wound epidermis signalling and the extent to which cells revert to an embryonic-like state (Gerber et al., 2018; Leigh et al., 2018). 3). Submission deadline: 1 September 2021 Two types of injury models have demonstrated significant restorative processes in the salamander brain, both in terms of tissue regeneration as well as behavioural recovery. Recent research says Yes, and this blog entry tells another part of the story. “–genetic studies on mouse digit tip regeneration have identified signaling pathways required for the regeneration response that parallel those known to be important for regeneration in lower vertebrates. Loss of Pax3 in Pleurodeles leads to severe abnormalities, including skeletal muscle agenesis, again as occurs in other vertebrates (Elewa et al., 2017). For example, some models have removed parts of the telencephalon or dorsal midbrain, showing that this is followed by wound closure, massive proliferation, re-appearance of neuronal diversity and the formation of new inter-neuronal connexions (Fig. Blastemas only generate structures distal to their origin unless treated with retinoic acid (RA), which results in proximodistal (PD) limb duplications. “Multiple authors have recently highlighted the similarities between the early phases of mammalian wound healing and urodele (, amphibians of the order Caudata, including salamanders and newts, ) limb regeneration. A weblog on the sciences and practices of living healthily very long – perhaps hundreds of years. 5). With methods on targeting a wide variety of structures, ranging from the limb to the heart and to the brain, and methods for studying genetically modified organisms and tools for mining in the genomic databases. This gene is also found in multiple copies in the genomes of Xenopus (Tang and Maxwell, 2008) and zebrafish (Chen et al., 2005), where it is known as mir-430. Salamanders display complex life cycles in both terrestrial and aquatic habitats. (B) The Eastern red spotted newt, Notophthalmus viridescens, has contributed significantly to our understanding of multiple regeneration processes with reference transcriptomes available. In fact, this pondering has been going on for… Salamander Limb Regeneration. In this Primer, we cover the evolutionary context in which salamanders … AG is a ligand of the cell surface receptor, Prod1, the overexpression of which confers blastemal cells a proximal identity (Kumar et al., 2007). This is exemplified by regeneration of the eye lens throughout the entire life-span of newts. The crucial role of nerves in limb regeneration, and their link to positional cues, was also demonstrated in the accessory limb model. 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