The cellular basis for animal regeneration

SUMOylation controls stem cell proliferation and regional cell death through Hedgehog signaling in planarians

Mechanisms underlying anteroposterior body axis differences during adult tissue maintenance and regeneration are poorly understood. Here, we identify that post-translational modifications through the SUMO (Small Ubiquitin-like Modifier) machinery are evolutionarily conserved in the Lophotrocozoan Schmidtea mediterranea. Disruption of SUMOylation in adult animals by RNA-interference of the only SUMO E2 conjugating enzyme Ubc9 leads to a systemic increase in DNA damage and a remarkable regional defect characterized by increased cell death and loss of the posterior half of the body. We identified that Ubc9 is mainly expressed in planarian stem cells (neoblasts) but it is also transcribed in differentiated cells including neurons. Regeneration in Ubc9(RNAi) animals is impaired and associated with low neoblast proliferation. We present evidence indicating that Ubc9-induced regional cell death is preceded by alterations in transcription and spatial expression of repressors and activators of the Hedgehog signaling pathway. Our results demonstrate that SUMOylation acts as a regional-specific cue to regulate cell fate during tissue renewal and regeneration.

Intermediate Developmental Phases During Regeneration

The initial view that regeneration can be a continuum in terms of regulatory mechanisms is gradually changing, and recent evidence points towards the presence of discrete regulatory steps and intermediate phases. Furthermore, regeneration presents an excellent example of a process generating order and pattern, i.e. a self-organization process. It is likely that the process traverses a set of intermediate phases before reaching an endpoint. Although some progress has been made in deciphering the identity of these intermediate phases, a lot more work is needed to derive a comprehensive and complete picture. Here, we discuss the intermediate developmental phases in plant regeneration and compare them with the possible intermediate developmental phases in animal regeneration.

Chemical Amputation and Regeneration of the Pharynx in the Planarian Schmidtea mediterranea

Planarians are flatworms that are extremely efficient at regeneration. They owe this ability to a large number of stem cells that can rapidly respond to any type of injury. Common injury models in these animals remove large amounts of tissue, which damages multiple organs. To overcome this broad tissue damage, we describe here a method to selectively remove a single organ, the pharynx, in the planarian Schmidtea mediterranea. We achieve this by soaking animals in a solution containing the cytochrome oxidase inhibitor sodium azide. Brief exposure to sodium azide causes extrusion of the pharynx from the animal, which we call "chemical amputation." Chemical amputation removes the entire pharynx, and generates a small wound where the pharynx attaches to the intestine. After extensive rinsing, all amputated animals regenerate a fully functional pharynx in approximately one week. Stem cells in the rest of the body drive regeneration of the new pharynx. Here, we provide a detailed protocol for chemical amputation, and describe both histological and behavioral methods to assess successful amputation and regeneration.

Drosophila as a Model System to Study Cell Signaling in Organ Regeneration

Regeneration is a fascinating phenomenon that allows organisms to replace or repair damaged organs or tissues. This ability occurs to varying extents among metazoans. The rebuilding of the damaged structure depends on regenerative proliferation that must be accompanied by proper cell fate respecification and patterning. These cellular processes are regulated by the action of different signaling pathways that are activated in response to the damage. The imaginal discs of Drosophila melanogaster have the ability to regenerate and have been extensively used as a model system to study regeneration. Drosophila provides an opportunity to use powerful genetic tools to address fundamental problems about the genetic mechanisms involved in organ regeneration. Different studies in Drosophila have helped to elucidate the genes and signaling pathways that initiate regeneration, promote regenerative growth, and induce cell fate respecification. Here we review the signaling networks involved in regulating the variety of cellular responses that are required for discs regeneration.

Dicer inactivation stimulates limb regeneration ability in Xenopus laevis

The ontogenetic decline of regeneration capacity in the anuran amphibian Xenopus makes it an excellent model for regeneration studies. However, the cause of the regeneration ability decline is not fully understood. MicroRNAs regulate animal development and have been indicated in various regeneration situations. However, little is known about the role of microRNAs during limb regeneration in Xenopus. This study investigates the effect of Dicer, an enzyme responsible for microRNA maturation, on limb development and regeneration in Xenopus. Dicer is expressed in the developing Xenopus limbs and is up-regulated after limb amputation during both regeneration-competent and regeneration-deficient stages of tadpole development. Inactivation of Dicer in early (NF stage 53) tadpole limb buds leads to shorter tibulare/fibulare formation but does not affect limb regeneration. However, in late-stage, regeneration-deficient tadpole limbs (NF stage 57), Dicer inactivation restores the regeneration blastema and stimulates limb regeneration. Thus, our results demonstrated that Xenopus limb regeneration can be stimulated by the inactivation of Dicer in nonregenerating tadpoles, indicating that microRNAs present in late-stage tadpole limbs may be involved in the ontogenetic decline of limb regeneration in Xenopus.

Comparative regenerative mechanisms across different mammalian tissues

Stimulating regeneration of complex tissues and organs after injury to effect complete structural and functional repair, is an attractive therapeutic option that would revolutionize clinical medicine. Compared to many metazoan phyla that show extraordinary regenerative capacity, which in some instances persists throughout life, regeneration in mammalians, particularly humans, is limited or absent. Here we consider recent insights in the elucidation of molecular mechanisms of regeneration that have come from studies of tissue homeostasis and injury repair in mammalian tissues that span the spectrum from little or no self-renewal, to those showing active cell turnover throughout life. These studies highlight the diversity of factors that constrain regeneration, including immune responses, extracellular matrix composition, age, injury type, physiological adaptation, and angiogenic and neurogenic capacity. Despite these constraints, much progress has been made in elucidating key molecular mechanisms that may provide therapeutic targets for the development of future regenerative therapies, as well as previously unidentified developmental paradigms and windows-of-opportunity for improved regenerative repair.

Functionally Conserved Noncoding Regulators of Cardiomyocyte Proliferation and Regeneration in Mouse and Human

Background:

The adult mammalian heart has little regenerative capacity after myocardial infarction (MI), whereas neonatal mouse heart regenerates without scarring or dysfunction. However, the underlying pathways are poorly defined. We sought to derive insights into the pathways regulating neonatal development of the mouse heart and cardiac regeneration post-MI.

Methods and Results:

Total RNA-seq of mouse heart through the first 10 days of postnatal life (referred to as P3, P5, P10) revealed a previously unobserved transition in microRNA (miRNA) expression between P3 and P5 associated specifically with altered expression of protein-coding genes on the focal adhesion pathway and cessation of cardiomyocyte cell division. We found profound changes in the coding and noncoding transcriptome after neonatal MI, with evidence of essentially complete healing by P10. Over two-thirds of each of the messenger RNAs, long noncoding RNAs, and miRNAs that were differentially expressed in the post-MI heart were differentially expressed during normal postnatal development, suggesting a common regulatory pathway for normal cardiac development and post-MI cardiac regeneration. We selected exemplars of miRNAs implicated in our data set as regulators of cardiomyocyte proliferation. Several of these showed evidence of a functional influence on mouse cardiomyocyte cell division. In addition, a subset of these miRNAs, miR-144-3p, miR-195a-5p, miR-451a, and miR-6240 showed evidence of functional conservation in human cardiomyocytes.

Conclusions:

The sets of messenger RNAs, miRNAs, and long noncoding RNAs that we report here merit further investigation as gatekeepers of cell division in the postnatal heart and as targets for extension of the period of cardiac regeneration beyond the neonatal period.

Amorphous, Smart, and Bioinspired Polyphosphate Nano/Microparticles: A Biomaterial for Regeneration and Repair of Osteo-Articular Impairments In-Situ

Using femur explants from mice as an in vitro model, we investigated the effect of the physiological polymer, inorganic polyphosphate (polyP), on differentiation of the cells of the bone marrow in their natural microenvironment into the osteogenic and chondrogenic lineages. In the form of amorphous Ca-polyP nano/microparticles, polyP retains its function to act as both an intra- and extracellular metabolic fuel and a stimulus eliciting morphogenetic signals. The method for synthesis of the nano/microparticles with the polyanionic polyP also allowed the fabrication of hybrid particles with the bisphosphonate zoledronic acid, a drug used in therapy of bone metastases in cancer patients. The results revealed that the amorphous Ca-polyP particles promote the growth/viability of mesenchymal stem cells, as well as the osteogenic and chondrogenic differentiation of the bone marrow cells in rat femur explants, as revealed by an upregulation of the expression of the transcription factors SOX9 (differentiation towards osteoblasts) and RUNX2 (chondrocyte differentiation). In parallel to this bone anabolic effect, incubation of the femur explants with these particles significantly reduced the expression of the gene encoding the osteoclast bone-catabolic enzyme, cathepsin-K, while the expression of the tartrate-resistant acid phosphatase remained unaffected. The gene expression data were supported by the finding of an increased mineralization of the cells in the femur explants in response to the Ca-polyP particles. Finally, we show that the hybrid particles of polyP complexed with zoledronic acid exhibit both the cytotoxic effect of the bisphosphonate and the morphogenetic and mineralization inducing activity of polyP. Our results suggest that the Ca-polyP nano/microparticles are not only a promising scaffold material for repairing long bone osteo-articular damages but can also be applied, as a hybrid with zoledronic acid, as a drug delivery system for treatment of bone metastases. The polyP particles are highlighted as genuine, smart, bioinspired nano/micro biomaterials.

Systemic cell cycle activation is induced following complex tissue injury in axolotl

Activation of progenitor cells is crucial to promote tissue repair following injury in adult animals. In the context of successful limb regeneration following amputation, progenitor cells residing within the stump must re-enter the cell cycle to promote regrowth of the missing limb. We demonstrate that in axolotls, amputation is sufficient to induce cell-cycle activation in both the amputated limb and the intact, uninjured contralateral limb. Activated cells were found throughout all major tissue populations of the intact contralateral limb, with internal cellular populations (bone and soft tissue) the most affected. Further, activated cells were additionally found within the heart, liver, and spinal cord, suggesting that amputation induces a common global activation signal throughout the body. Among two other injury models, limb crush and skin excisional wound, only limb crush injuries were capable of inducing cellular responses in contralateral uninjured limbs but did not achieve activation levels seen following limb loss. We found this systemic activation response to injury is independent of formation of a wound epidermis over the amputation plane, suggesting that injury-induced signals alone can promote cellular activation. In mammals, mTOR signaling has been shown to promote activation of quiescent cells following injury, and we confirmed a subset of activated contralateral cells is positive for mTOR signaling within axolotl limbs. These findings suggest that conservation of an early systemic response to injury exists between mammals and axolotls, and propose that a distinguishing feature in species capable of full regeneration is converting this initial activation into sustained and productive growth at the site of regeneration.

EvoRegen in animals: Time to uncover deep conservation or convergence of adult stem cell evolution and regenerative processes

How do animals regenerate specialised tissues or their entire body after a traumatic injury, how has this ability evolved and what are the genetic and cellular components underpinning this remarkable feat? While some progress has been made in understanding mechanisms, relatively little is known about the evolution of regenerative ability. Which elements of regeneration are due to lineage specific evolutionary novelties or have deeply conserved roots within the Metazoa remains an open question. The renaissance in regeneration research, fuelled by the development of modern functional and comparative genomics, now enable us to gain a detailed understanding of both the mechanisms and evolutionary forces underpinning regeneration in diverse animal phyla. Here we review existing and emerging model systems, with the focus on invertebrates, for studying regeneration. We summarize findings across these taxa that tell us something about the evolution of adult stem cell types that fuel regeneration and the growing evidence that many highly regenerative animals harbor adult stem cells with a gene expression profile that overlaps with germline stem cells. We propose a framework in which regenerative ability broadly evolves through changes in the extent to which stem cells generated through embryogenesis are maintained into the adult life history.

The abrogation of condensin function provides independent evidence for defining the self-renewing population of pluripotent stem cells

Heterogeneity of planarian stem cells has been categorised on the basis of single cell expression analyses and subsequent experiments to demonstrate lineage relationships. Some data suggest that despite heterogeneity in gene expression amongst cells in the cell cycle, in fact only one sub-population, known as sigma neoblasts, can self-renew. Without the tools to perform live in vivo lineage analysis, we instead took an alternative approach to provide independent evidence for defining the self-renewing stem cell population. We exploited the role of highly conserved condensin family genes to functionally assay neoblast self-renewal properties. Condensins are involved in forming properly condensed chromosomes to allow cell division to proceed during mitosis, and their abrogation inhibits mitosis and can lead to repeated endoreplication of the genome in cells that make repeated attempts to divide. We find that planarians possess only the condensin I complex, and that this is required for normal stem cell function. Abrogation of condensin function led to rapid stem cell depletion accompanied by the appearance of 'giant' cells with increased DNA content. Using previously discovered markers of heterogeneity we show that enlarged cells are always from the sigma-class of the neoblast population and we never observe evidence for endoreplication for the other neoblast subclasses. Overall, our data establish that condensins are essential for stem cell maintenance and provide independent evidence that only sigma-neoblasts are capable of multiple rounds of cell division and hence self-renewal.

Wound healing, cellular regeneration and plasticity: the elegans way

Regeneration and wound healing are complex processes that allow organs and tissues to regain their integrity and functionality after injury. Wound healing, a key property of epithelia, involves tissue closure that in some cases leads to scar formation. Regeneration, a process rather limited in mammals, is the capacity to regrow (parts of) an organ or a tissue, after damage or amputation. What are the properties of organs and the features of tissue permitting functional regrowth and repair? What are the cellular and molecular mechanisms underlying these processes? These questions are crucial both in fundamental and applied contexts, with important medical implications. The mechanisms and cells underlying tissue repair have thus been the focus of intense investigation. The last decades have seen rapid progress in the domain and new models emerging. Here, we review the fundamental advances and the perspectives that the use of C. elegans as a model have brought to the mechanisms of wound healing and cellular plasticity, axon regeneration and transdifferentiation in vivo.

Stem Cells, Patterning and Regeneration in Planarians: Self-Organization at the Organismal Scale

The establishment of size and shape remains a fundamental challenge in biological research that planarian flatworms uniquely epitomize. Planarians can regenerate complete and perfectly proportioned animals from tiny and arbitrarily shaped tissue pieces; they continuously renew all organismal cell types from abundant pluripotent stem cells, yet maintain shape and anatomy in the face of constant turnover; they grow when feeding and literally degrow when starving, while scaling form and function over as much as a 40-fold range in body length or an 800-fold change in total cell numbers. This review provides a broad overview of the current understanding of the planarian stem cell system, the mechanisms that pattern the planarian body plan and how the interplay between patterning signals and cell fate choices orchestrates regeneration. What emerges is a conceptual framework for the maintenance and regeneration of the planarian body plan on basis of the interplay between pluripotent stem cells and self-organizing patterns and further, the general utility of planarians as model system for the mechanistic basis of size and shape.

Amorphous polyphosphate, a smart bioinspired nano-/bio-material for bone and cartilage regeneration: towards a new paradigm in tissue engineering

Physiological amorphous polyphosphate nano/micro-particles, injectable and implantable, attract and stimulate MSCs into implants for tissue regeneration.

Cross-limb communication during Xenopus hind-limb regenerative response: non-local bioelectric injury signals

Regeneration of damaged body-parts requires coordination of size, shape, location, and orientation of tissue with the rest of the body. It is not currently known how far injury sites communicate with the remaining soma during repair, or what information may emanate from the injury site to other regions. We examined the bioelectric properties (resting potential gradients in the epidermis) of Xenopus froglets undergoing hind-limb amputation and observed that the contralateral (un-damaged) limb exhibits apparent depolarization signals immediately after the opposite hind-limb is amputated. The pattern of depolarization matches that of the amputated limb and is correlated to the position and type of injury, revealing that information about damage is available to remote body tissues and is detectable non-invasively in vivo by monitoring of the bioelectric state. These data extend knowledge about the electrophysiology of regenerative response, identify a novel communication process via long-range spread of injury signaling, a phenomenon which we call bioelectric injury mirroring (BIM), and suggests revisions to regenerative medicine and diagnostic strategies focused entirely on the wound site and to the use of contralateral limbs as controls.

Planarians and the History of Animal Regeneration: Paradigm Shifts and Key Concepts in Biology

Regeneration has captured human imagination for much of recorded history. Its sociological influence is evident in ancient and modern folklore, art, politics, and even language. In many ways, the study of regeneration helped establish the field of biology as a legitimate scientific discipline. Furthermore, regeneration research yielded critical insights that challenged flawed scientific models and uncovered fundamental principles underpinning the workings of life on this planet. This chapter details some ways in which the study of animal regeneration-with special emphasis on planarian regeneration-influenced the evolution of thought in biology. This includes contributions to the discovery of stem cells, the nature of heredity, and key concepts in pattern formation.

Solitary Ascidians as Model Organisms in Regenerative Biology Studies

Regeneration, the process of replacing lost or damaged body parts, has long captured human imagination and is a key feature among all animal phyla. Due to their close phylogenetic relationship to vertebrates and their high regenerative abilities, ascidians (Chordata, Ascidiacea) are often used as models to shed light on the cellular and genetic process involved in tissue regeneration. Surprisingly, ascidian regeneration studies are based on only a few model species. In this chapter, we point out the important potential of solitary ascidians in regenerative and stem cell studies. We review recent studies of regeneration among solitary ascidians and discuss the cellular mechanism of tissue regeneration and the possible involvement of circulating cells in these processes. New data regarding the relationship between age and regeneration abilities of the solitary ascidian Polycarpa mytiligera (Stolidobranchia, Styelidae) are presented. The unique regeneration abilities found in P. mytiligera following evisceration of its digestive system and following amputation of its neural complex and siphon-associated structures and nerves imply on its potential to serve as a novel model system for understanding tissue regeneration.

The planarian TCF/LEF factor Smed-tcf1 is required for the regeneration of dorsal-lateral neuronal subtypes

The adult brain of the planarian Schmidtea mediterranea (a freshwater flatworm) is a dynamic structure with constant cell turnover as well as the ability to completely regenerate de novo. Despite this, function and pattern is achieved in a reproducible manner from individual to individual in terms of the correct spatial and temporal production of specific neuronal subtypes. Although several signaling molecules have been found to be key to scaling and cell turnover, the mechanisms by which specific neural subtypes are specified remain largely unknown. Here we performed a 6 day RNAseq time course on planarians that were regenerating either 0, 1, or 2 heads in order to identify novel regulators of brain regeneration. Focusing on transcription factors, we identified a TCF/LEF factor, Smed-tcf1, which was required to correctly pattern the dorsal-lateral cell types of the regenerating brain. The most severely affected neurons in Smed-tcf1(RNAi) animals were the dorsal GABAergic neurons, which failed to regenerate, leading to an inability of the animals to phototaxis away from light. Together, Smed-tcf1 is a critical regulator, required to pattern the dorsal-lateral region of the regenerating planarian brain.

Drug Discovery in Fish, Flies, and Worms

Nonmammalian model organisms such as the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the zebrafish Danio rerio provide numerous experimental advantages for drug discovery including genetic and molecular tractability, amenability to high-throughput screening methods and reduced experimental costs and increased experimental throughput compared to traditional mammalian models. An interdisciplinary approach that strategically combines the study of nonmammalian and mammalian animal models with diverse experimental tools has and will continue to provide deep molecular and genetic understanding of human disease and will significantly enhance the discovery and application of new therapies to treat those diseases. This review will provide an overview of C. elegans, Drosophila, and zebrafish biology and husbandry and will discuss how these models are being used for phenotype-based drug screening and for identification of drug targets and mechanisms of action. The review will also describe how these and other nonmammalian model organisms are uniquely suited for the discovery of drug-based regenerative medicine therapies.

Efficient transgenesis and annotated genome sequence of the regenerative flatworm model Macrostomum lignano

Regeneration-capable flatworms are informative research models to study the mechanisms of stem cell regulation, regeneration, and tissue patterning. However, the lack of transgenesis methods considerably hampers their wider use. Here we report development of a transgenesis method for Macrostomum lignano, a basal flatworm with excellent regeneration capacity. We demonstrate that microinjection of DNA constructs into fertilized one-cell stage eggs, followed by a low dose of irradiation, frequently results in random integration of the transgene in the genome and its stable transmission through the germline. To facilitate selection of promoter regions for transgenic reporters, we assembled and annotated the M. lignano genome, including genome-wide mapping of transcription start regions, and show its utility by generating multiple stable transgenic lines expressing fluorescent proteins under several tissue-specific promoters. The reported transgenesis method and annotated genome sequence will permit sophisticated genetic studies on stem cells and regeneration using M. lignano as a model organism.

Regeneration capable flatworms have emerged as powerful models for studying stem cell biology and patterning, however their study has been hindered by the lack of transgenesis methods. Here, the authors describe a transgenesis method for Macrostomum lignano, as well as a new annotated genome sequence.

Investigation into the cellular origins of posterior regeneration in the annelid Capitella teleta

Many animals can regenerate, although there is great diversity in regenerative capabilities. A major question in regenerative biology is determining the cellular source of newly formed tissue. The polychaete annelid, Capitella teleta, can regenerate posterior segments following transverse amputation. However, the source, behavior and molecular characteristics of the cells that form new tissue during regeneration are largely unknown. Using an indirect cell tracking method involving 5′‐ethynyl‐2′‐deoxyuridine (EdU) incorporation, we show that cell migration occurs during C. teleta posterior regeneration. Expression of the multipotency/germ line marker CapI‐vasa led us to hypothesize that stem cells originate from a multipotent progenitor cell (MPC) cluster, migrate through the coelomic cavity, and contribute to regeneration of tissue. We show that the capacity for posterior regeneration and segment formation is greater with than without the MPC cluster. Finally, we propose a working model of posterior regeneration in C. teleta. This work is the first in C. teleta that addresses the potential source of cells contributing to posterior regeneration, and may provide clues as to why some animals are highly successful regenerators.

Annotation of nerve cord transcriptome in earthworm Eisenia fetida

In annelid worms, the nerve cord serves as a crucial organ to control the sensory and behavioral physiology. The inadequate genome resource of earthworms has prioritized the comprehensive analysis of their transcriptome dataset to monitor the genes express in the nerve cord and predict their role in the neurotransmission and sensory perception of the species. The present study focuses on identifying the potential transcripts and predicting their functional features by annotating the transcriptome dataset of nerve cord tissues prepared by Gong et al., 2010 from the earthworm Eisenia fetida. Totally 9762 transcripts were successfully annotated against the NCBI nr database using the BLASTX algorithm and among them 7680 transcripts were assigned to a total of 44,354 GO terms. The conserve domain analysis indicated the over representation of P-loop NTPase domain and calcium binding EF-hand domain. The COG functional annotation classified 5860 transcript sequences into 25 functional categories. Further, 4502 contig sequences were found to map with 124 KEGG pathways. The annotated contig dataset exhibited 22 crucial neuropeptides having considerable matches to the marine annelid Platynereis dumerilii, suggesting their possible role in neurotransmission and neuromodulation. In addition, 108 human stem cell marker homologs were identified including the crucial epigenetic regulators, transcriptional repressors and cell cycle regulators, which may contribute to the neuronal and segmental regeneration. The complete functional annotation of this nerve cord transcriptome can be further utilized to interpret genetic and molecular mechanisms associated with neuronal development, nervous system regeneration and nerve cord function.

Fibroblast growth factors: key players in regeneration and tissue repair

Tissue injury initiates a complex repair process, which in some organisms can lead to the complete regeneration of a tissue. In mammals, however, the repair of most organs is imperfect and results in scar formation. Both regeneration and repair are orchestrated by a highly coordinated interplay of different growth factors and cytokines. Among the key players are the fibroblast growth factors (FGFs), which control the migration, proliferation, differentiation and survival of different cell types. In addition, FGFs influence the expression of other factors involved in the regenerative response. Here, we summarize current knowledge on the roles of endogenous FGFs in regeneration and repair in different organisms and in different tissues and organs. Gaining a better understanding of these FGF activities is important for appropriate modulation of FGF signaling after injury to prevent impaired healing and to promote organ regeneration in humans.

Regeneration Genetics

Understanding how and why animals regenerate complex tissues has the potential to transform regenerative medicine. Here we present an overview of genetic approaches that have recently been applied to dissect mechanisms of regeneration. We describe new advances that relate to central objectives of regeneration biologists researching different tissues and species, focusing mainly on vertebrates. These objectives include defining the cellular sources and key cell behaviors in regenerating tissue, elucidating molecular triggers and brakes for regeneration, and defining the earliest events that control the presence of these molecular factors.

Spinal cord self-repair during tail regeneration in Polypedates maculatus and putative role of FGF1 as a neurotrophic factor

Spinal cord injury could be fatal in man and often results in irreversible medical conditions affecting mobility. However, anuran amphibians win over such pathological condition by the virtue of regeneration abilities. The tail of anuran tadpoles therefore allures researchers to study spinal cord injury and self- repair process. In the present study, we inflicted injury to the spinal cord by means of surgical transection of the tail and investigated the self-repair activity in the tadpoles of the Indian tree frog Polypedates maculatus. We also demonstrate for the first time by immunofluorescence localization the expression pattern of Fibroblast Growth Factor1 (FGF1) during spinal cord regeneration which has not been documented earlier in anurans. FGF1, bearer of the mitogenic and neurotrophic properties seems to be expressed by progenitor cells that facilitate regeneration. Spinal cord during tail regeneration in P. maculatus attains functional recovery within a span of 2 weeks thus enabling the organism to survive in an aquatic medium till metamorphosis. Moreover, during the course of spinal cord regeneration in the regenerating tail, melanocytes showed an interesting behaviour as these neural crest derivatives were missing near the early regenerates until their reappearance where they were positioned in close proximity with the regenerated spinal cord as in the normal tail.

Osteoblast Production by Reserved Progenitor Cells in Zebrafish Bone Regeneration and Maintenance

Mammals cannot re-form heavily damaged bones as in large fracture gaps, whereas zebrafish efficiently regenerate bones even after amputation of appendages. However, the source of osteoblasts that mediate appendage regeneration is controversial. Several studies in zebrafish have shown that osteoblasts are generated by dedifferentiation of existing osteoblasts at injured sites, but other observations suggest that de novo production of osteoblasts also occurs. In this study, we found from cell-lineage tracing and ablation experiments that a group of cells reserved in niches serves as osteoblast progenitor cells (OPCs) and has a significant role in fin ray regeneration. Besides regeneration, OPCs also supply osteoblasts for normal bone maintenance. We further showed that OPCs are derived from embryonic somites, as is the case with embryonic osteoblasts, and are replenished from mesenchymal precursors in adult zebrafish. Our findings reveal that reserved progenitors are a significant and complementary source of osteoblasts for zebrafish bone regeneration.

Neural stem/progenitor cells are activated during tail regeneration in the leopard gecko (Eublepharis macularius)

As for many lizards, the leopard gecko (Eublepharis macularius) can self-detach its tail to avoid predation and then regenerate a replacement. The replacement tail includes a regenerated spinal cord with a simple morphology: an ependymal layer surrounded by nerve tracts. We hypothesized that cells within the ependymal layer of the original spinal cord include populations of neural stem/progenitor cells (NSPCs) that contribute to the regenerated spinal cord. Prior to tail loss, we performed a bromodeoxyuridine pulse-chase experiment and found that a subset of ependymal layer cells (ELCs) were label-retaining after a 140-day chase period. Next, we conducted a detailed spatiotemporal characterization of these cells before, during, and after tail regeneration. Our findings show that SOX2, a hallmark protein of NSPCs, is constitutively expressed by virtually all ELCs before, during, and after regeneration. We also found that during regeneration, ELCs express an expanded panel of NSPC and lineage-restricted progenitor cell markers, including MSI-1, SOX9, and TUJ1. Using electron microscopy, we determined that multiciliated, uniciliated, and biciliated cells are present, although the latter was only observed in regenerated spinal cords. Our results demonstrate that cells within the ependymal layer of the original, regenerating and fully regenerate spinal cord represent a heterogeneous population. These include radial glia comparable to Type E and Type B cells, and a neuronal-like population of cerebrospinal fluid-contacting cells. We propose that spinal cord regeneration in geckos represents a truncation of the restorative trajectory observed in some urodeles and teleosts, resulting in the formation of a structurally distinct replacement.

Oral-Facial Tissue Reconstruction in the Regenerative Axolotl

Absence of large amounts of orofacial tissues caused by cancerous resections, congenital defects, or trauma results in sequelae such as dysphagia and noticeable scars. Oral-neck tissue regeneration was studied in the axolotl (regenerative amphibian) following a 2.5-mm punch biopsy that simultaneously removed skin, connective tissue, muscle, and cartilage in the tongue and intermandibular region. The untreated wound was studied macroscopically and histologically at 17 different time points ranging from 0 to180 days (N = 120 axolotls). At 12 hr, the wound's surface was smoothened and within 1mm, internal lingual muscular modifications occurred; at the same distance, between days 4-7 lingual muscle degradation was complete. Immunofluorescence indicates complete keratinocytes migration by 48 hr. These cells with epidermal Leydig cells, appearing yellow, lead the chin's deep tissue outgrowth until its closure on the 14th day. Regeneration speeds varied and peaked in time for each tissue, (1) deep chin 84.3 μm/hr from 24 to 96 hr, (2) superficial chin 71.1 μm/hr from 7-14 days, and (3) tongue 86.0 μm/hr between 48 hr and 7 days. Immunofluorescence to Col IV showed basement membrane reconnected between days 30-45 coinciding with the chin's dermal tissue's surface area recovery. New muscle appeared at 21 days and was always preceded by the formation of a collagen bed. Both chin tissues regain all surface area and practically all components while the lingual structure lacks some content but is generally similar to the original. The methodology and high-resolution observations described here are the first of its kind for this animal model and could serve as a basis for future studies in oral and facial regenerative research.

Imaginal disc regeneration takes flight

Drosophila imaginal discs, the larval precursors of adult structures such as the wing and leg, are capable of regenerating after damage. During the course of regeneration, discs can sometimes generate structures that are appropriate for a different type of disc, a phenomenon termed transdetermination. Until recently, these phenomena were studied by physically fragmenting discs and then transplanting them into the abdomens of adult female flies. This field has experienced a renaissance following the development of genetic ablation systems that can damage precisely defined regions of the disc without the need for surgery. Together with more traditional approaches, these newer methods have generated many novel insights into wound healing, the mechanisms that drive regenerative growth, plasticity during regeneration and systemic effects of tissue damage and regeneration.

Epithelial-mesenchymal transition transcription factors control pluripotent adult stem cell migration in vivo in planarians

Migration of stem cells underpins the physiology of metazoan animals. For tissues to be maintained, stem cells and their progeny must migrate and differentiate in the correct positions. This need is even more acute after tissue damage by wounding or pathogenic infection. Inappropriate migration also underpins metastasis. Despite this, few mechanistic studies address stem cell migration during repair or homeostasis in adult tissues. Here, we present a shielded X-ray irradiation assay that allows us to follow stem cell migration in planarians. We demonstrate the use of this system to study the molecular control of stem cell migration and show that snail-1, snail-2 and zeb-1 EMT transcription factor homologs are necessary for cell migration to wound sites and for the establishment of migratory cell morphology. We also observed that stem cells undergo homeostatic migration to anterior regions that lack local stem cells, in the absence of injury, maintaining tissue homeostasis. This requires the polarity determinant notum Our work establishes planarians as a suitable model for further in-depth study of the processes controlling stem cell migration in vivo.

Zebrafish heart regeneration: 15 years of discoveries

Cardiovascular disease is the leading cause of death worldwide. Compared to other organs such as the liver, the adult human heart lacks the capacity to regenerate on a macroscopic scale after injury. As a result, myocardial infarctions are responsible for approximately half of all cardiovascular related deaths. In contrast, the zebrafish heart regenerates efficiently upon injury through robust myocardial proliferation. Therefore, deciphering the mechanisms that underlie the zebrafish heart's endogenous regenerative capacity represents an exciting avenue to identify novel therapeutic strategies for inducing regeneration of the human heart. This review provides a historical overview of adult zebrafish heart regeneration. We summarize 15 years of research, with a special focus on recent developments from this fascinating field. We discuss experimental findings that address fundamental questions of regeneration research. What is the origin of regenerated muscle? How is regeneration controlled from a genetic and molecular perspective? How do different cell types interact to achieve organ regeneration? Understanding natural models of heart regeneration will bring us closer to answering the ultimate question: how can we stimulate myocardial regeneration in humans?

Distinct regenerative potential of trunk and appendages of Drosophila mediated by JNK signalling

The Drosophila body comprises a central part, the trunk, and outgrowths of the trunk, the appendages. Much is known about appendage regeneration, but little about the trunk. As the wing imaginal disc contains a trunk component, the notum, and a wing appendage, we have investigated the response to ablation of these two components. We find that, in contrast with the strong regenerative response of the wing, the notum does not regenerate. Nevertheless, the elimination of the wing primordium elicits a proliferative response of notum cells, but they do not regenerate wing; they form a notum duplicate. Conversely, the wing cells cannot regenerate an ablated notum; they overproliferate and generate a hinge overgrowth. These results suggest that trunk and appendages cannot be reprogrammed to generate each other. Our experiments demonstrate that the proliferative response is mediated by JNK signalling from dying cells, but JNK functions differently in the trunk and the appendages, which may explain their distinct regenerative potential.

Lineage tracing of genome-edited alleles reveals high fidelity axolotl limb regeneration

Salamanders are unparalleled among tetrapods in their ability to regenerate many structures, including entire limbs, and the study of this ability may provide insights into human regenerative therapies. The complex structure of the limb poses challenges to the investigation of the cellular and molecular basis of its regeneration. Using CRISPR/Cas, we genetically labelled unique cell lineages within the developing axolotl embryo and tracked the frequency of each lineage within amputated and fully regenerated limbs. This allowed us, for the first time, to assess the contributions of multiple low frequency cell lineages to the regenerating limb at once. Our comparisons reveal that regenerated limbs are high fidelity replicas of the originals even after repeated amputations.

Trends in tissue repair and regeneration

The 6th EMBO conference on the Molecular and Cellular Basis of Regeneration and Tissue Repair took place in Paestum (Italy) on the 17th-21st September, 2016. The 160 scientists who attended discussed the importance of cellular and tissue plasticity, biophysical aspects of regeneration, the diverse roles of injury-induced immune responses, strategies to reactivate regeneration in mammals, links between regeneration and ageing, and the impact of non-mammalian models on regenerative medicine.

Bioelectric gene and reaction networks: computational modelling of genetic, biochemical and bioelectrical dynamics in pattern regulation

Gene regulatory networks (GRNs) describe interactions between gene products and transcription factors that control gene expression. In combination with reaction-diffusion models, GRNs have enhanced comprehension of biological pattern formation. However, although it is well known that biological systems exploit an interplay of genetic and physical mechanisms, instructive factors such as transmembrane potential (Vmem) have not been integrated into full GRN models. Here we extend regulatory networks to include bioelectric signalling, developing a novel synthesis: the bioelectricity-integrated gene and reaction (BIGR) network. Using in silico simulations, we highlight the capacity for Vmem to alter steady-state concentrations of key signalling molecules inside and out of cells. We characterize fundamental feedbacks where Vmem both controls, and is in turn regulated by, biochemical signals and thereby demonstrate Vmem homeostatic control, Vmem memory and Vmem controlled state switching. BIGR networks demonstrating hysteresis are identified as a mechanisms through which more complex patterns of stable Vmem spots and stripes, along with correlated concentration patterns, can spontaneously emerge. As further proof of principle, we present and analyse a BIGR network model that mechanistically explains key aspects of the remarkable regenerative powers of creatures such as planarian flatworms. The functional properties of BIGR networks generate the first testable, quantitative hypotheses for biophysical mechanisms underlying the stability and adaptive regulation of anatomical bioelectric pattern.

Complexity of the Wnt/β‑catenin pathway: Searching for an activation model

Wnt signaling refers to a conserved signaling pathway, widely studied due to its roles in cellular communication, cell fate decisions, development and cancer. However, the exact mechanism underlying inhibition of the GSK phosphorylation towards β-catenin and activation of the pathway after biding of Wnt ligand to its cognate receptors at the plasma membrane remains unclear. Wnt target genes are widely spread over several animal phyla. They participate in a plethora of functions during the development of an organism, from axial specification, gastrulation and organogenesis all the way to regeneration and repair in adults. Temporal and spatial oncogenetic re-activation of Wnt signaling almost certainly leads to cancer. Wnt signaling components have been extensively studied as possible targets in anti-cancer therapies. In this review we will discuss one of the most intriguing questions in this field, that is how β-catenin, a major component in this pathway, escapes the destruction complex, gets stabilized in the cytosol and it is translocated to the nucleus where it acts as a co-transcription factor. Four major models have evolved during the past 20years. We dissected each of them along with current views and future perspectives on this pathway. This review will focus on the molecular mechanisms by which Wnt proteins modulate β-catenin cytoplasmic levels and the relevance of this pathway for the development and cancer.

Regeneration of the digestive system in the crinoid Himerometra robustipinna occurs by transdifferentiation of neurosecretory-like cells

The structure and regeneration of the digestive system in the crinoid Himerometra robustipinna (Carpenter, 1881) were studied. The gut comprises a spiral tube forming radial lateral processes, which gives it a five-lobed shape. The digestive tube consists of three segments: esophagus, intestine, and rectum. The epithelia of these segments have different cell compositions. Regeneration of the gut after autotomy of the visceral mass progresses very rapidly. Within 6 h after autotomy, an aggregation consisting of amoebocytes, coelomic epithelial cells and juxtaligamental cells (neurosecretory neurons) forms on the inner surface of the skeletal calyx. At 12 h post-autotomy, transdifferentiation of the juxtaligamental cells starts. At 24 h post-autotomy these cells undergo a mesenchymal-epithelial-like transition, resulting in the formation of the luminal epithelium of the gut. Specialization of the intestinal epithelial cells begins on day 2 post-autotomy. At this stage animals acquire the mouth and anal opening. On day 4 post-autotomy the height of both the enterocytes and the visceral mass gradually increases. Proliferation does not play any noticeable role in gut regeneration. The immersion of animals in a 10-7 M solution of colchicine neither stopped formation of the lost structures nor caused accumulation of mitoses in tissues. Weakly EdU-labeled nuclei were observed in the gut only on day 2 post-autotomy and were not detected at later regeneration stages. Single mitotically dividing cells were recorded during the same period. It is concluded that juxtaligamental cells play a major role in gut regeneration in H. robustipinna. The main mechanisms of morphogenesis are cell migration and transdifferentiation.

The Drosophila Duox maturation factor is a key component of a positive feedback loop that sustains regeneration signaling

Regenerating tissue must initiate the signaling that drives regenerative growth, and sustain that signaling long enough for regeneration to complete. How these key signals are sustained is unclear. To gain a comprehensive view of the changes in gene expression that occur during regeneration, we performed whole-genome mRNAseq of actively regenerating tissue from damaged Drosophila wing imaginal discs. We used genetic tools to ablate the wing primordium to induce regeneration, and carried out transcriptional profiling of the regeneration blastema by fluorescently labeling and sorting the blastema cells, thus identifying differentially expressed genes. Importantly, by using genetic mutants of several of these differentially expressed genes we have confirmed that they have roles in regeneration. Using this approach, we show that high expression of the gene moladietz (mol), which encodes the Duox-maturation factor NIP, is required during regeneration to produce reactive oxygen species (ROS), which in turn sustain JNK signaling during regeneration. We also show that JNK signaling upregulates mol expression, thereby activating a positive feedback signal that ensures the prolonged JNK activation required for regenerative growth. Thus, by whole-genome transcriptional profiling of regenerating tissue we have identified a positive feedback loop that regulates the extent of regenerative growth.

Regenerating tissue must initiate the signaling that drives regenerative growth, and then sustain that signaling long enough for regeneration to complete. Drosophila imaginal discs, the epithelial structures in the larva that will form the adult animal during metamorphosis, have been an important model system for tissue repair and regeneration for over 60 years. Here we show that damage-induced JNK signaling leads to the upregulation of a gene called moladietz, which encodes a co-factor for an enzyme, NADPH dual oxidase (Duox), that generates reactive oxygen species (ROS), a key tissue-damage signal. High expression of moladietz induces continuous production of ROS in the regenerating tissue. The sustained production of ROS then continues to activate JNK signaling throughout the course of regeneration, ensuring maximal tissue regrowth.

New neurons in adult brain: distribution, molecular mechanisms and therapies

"Are new neurons added in the adult mammalian brain?" "Do neural stem cells activate following CNS diseases?" "How can we modulate their activation to promote recovery?" Recent findings in the field provide novel insights for addressing these questions from a new perspective. In this review, we will summarize the current knowledge about adult neurogenesis and neural stem cell niches in healthy and pathological conditions. We will first overview the milestones that have led to the discovery of the classical ventricular and hippocampal neural stem cell niches. In adult brain, new neurons originate from proliferating neural precursors located in the subventricular zone of the lateral ventricles and in the subgranular zone of the hippocampus. However, recent findings suggest that new neuronal cells can be added to the adult brain by direct differentiation (e.g., without cell proliferation) from either quiescent neural precursors or non-neuronal cells undergoing conversion or reprogramming to neuronal fate. Accordingly, in this review we will also address critical aspects of the newly described mechanisms of quiescence and direct conversion as well as the more canonical activation of the neurogenic niches and neuroblast reservoirs in pathological conditions. Finally, we will outline the critical elements involved in neural progenitor proliferation, neuroblast migration and differentiation and discuss their potential as targets for the development of novel therapeutic drugs for neurodegenerative diseases.

Eye Absence Does Not Regulate Planarian Stem Cells during Eye Regeneration

Dividing cells called neoblasts contain pluripotent stem cells and drive planarian flatworm regeneration from diverse injuries. A long-standing question is whether neoblasts directly sense and respond to the identity of missing tissues during regeneration. We used the eye to investigate this question. Surprisingly, eye removal was neither sufficient nor necessary for neoblasts to increase eye progenitor production. Neoblasts normally increase eye progenitor production following decapitation, facilitating regeneration. Eye removal alone, however, did not induce this response. Eye regeneration following eye-specific resection resulted from homeostatic rates of eye progenitor production and less cell death in the regenerating eye. Conversely, large head injuries that left eyes intact increased eye progenitor production. Large injuries also non-specifically increased progenitor production for multiple uninjured tissues. We propose a model for eye regeneration in which eye tissue production by planarian stem cells is not directly regulated by the absence of the eye itself.

Hematological Analysis of the Ascidian Botrylloides leachii (Savigny, 1816) During Whole-Body Regeneration

Whole-body regeneration (WBR)-the formation of an entire adult from only a small fragment of its own tissue-is extremely rare among chordates. Exceptionally, in the colonial ascidian Botrylloides leachii (Savigny, 1816) a fully functional adult is formed from their common vascular system after ablation of all adults from the colony in just 10 d, thanks to their high blastogenetic potential. While previous studies have identified key genetic markers and morphological changes, no study has yet focused on the hematological aspects of regeneration despite the major involvement of the remaining vascular system and the contained hemocytes in this process. To dissect this process, we analyzed colony blood flow patterns using time-lapse microscopy to obtain a quantitative description of the velocity, reversal pattern, and average distance traveled by hemocytes. We also observed that flows present during regeneration are powered by temporally and spatially synchronized contractions of the terminal ampullae. In addition, we revised previous studies of B. leachii hematology as well as asexual development using histological sectioning and compared the role played by hemocytes during WBR. We found that regeneration starts with a rapid healing response characterized by hemocyte aggregation and infiltration of immunocytes, followed by increased activity of hemoblasts, recruitment of macrophage-like cells for clearing the tissues of debris, and their subsequent disappearance from the circulation concomitant with the maturation of a single regenerated adult. Overall, we provide a detailed account of the hematological properties of regenerating B. leachii colonies, providing novel lines of inquiry toward the decipherment of regeneration in chordates.

A Brain Unfixed: Unlimited Neurogenesis and Regeneration of the Adult Planarian Nervous System

Powerful genetic tools in classical laboratory models have been fundamental to our understanding of how stem cells give rise to complex neural tissues during embryonic development. In contrast, adult neurogenesis in our model systems, if present, is typically constrained to one or a few zones of the adult brain to produce a limited subset of neurons leading to the dogma that the brain is primarily fixed post-development. The freshwater planarian (flatworm) is an invertebrate model system that challenges this dogma. The planarian possesses a brain containing several thousand neurons with very high rates of cell turnover (homeostasis), which can also be fully regenerated de novo from injury in just 7 days. Both homeostasis and regeneration depend on the activity of a large population of adult stem cells, called neoblasts, throughout the planarian body. Thus, much effort has been put forth to understand how the flatworm can continually give rise to the diversity of cell types found in the adult brain. Here we focus on work using single-cell genomics and functional analyses to unravel the cellular hierarchies from stem cell to neuron. In addition, we will review what is known about how planarians utilize developmental signaling to maintain proper tissue patterning, homeostasis, and cell-type diversity in their brains. Together, planarians are a powerful emerging model system to study the dynamics of adult neurogenesis and regeneration.

Learning from Mother Nature: Innovative Tools to Boost Endogenous Repair of Critical or Difficult-to-Heal Large Tissue Defects

For repair of chronic or difficult-to-heal tissue lesions and defects, major constraints exist to a broad application of cell therapy and tissue engineering approaches, i.e., transplantation of “ex vivo” expanded autologous stem/progenitor cells, alone or associated with carrier biomaterials. To enable a large number of patients to benefit, new strategies should be considered. One of the main goals of contemporary regenerative medicine is to develop new regenerative therapies, inspired from Mother Nature. In all injured tissues, when platelets are activated by tissue contact, their released factors promote innate immune cell migration to the wound site. Platelet-derived factors and factors secreted by migrating immune cells create an inflammatory microenvironment, in turn, causing the activation of angiogenesis and vasculogenesis processes. Eventually, repair or regeneration of the injured tissue occurs via paracrine signals activating, mobilizing or recruiting to the wound site cells with healing potential, such as stem cells, progenitors, or undifferentiated cells derived from the reprogramming of tissue differentiated cells. This review, largely based on our studies, discusses the identification of new tools, inspired by cellular and molecular mechanisms overseeing physiological tissue healing, that could reactivate dormant endogenous regeneration mechanisms lost during evolution and ontogenesis.

Pancreatic β-cell regeneration: Facultative or dedicated progenitors?

The adult pancreas is only capable of limited regeneration. Unlike highly regenerative tissues such as the skin, intestinal crypts and hematopoietic system, no dedicated adult stem cells or stem cell niche have so far been identified within the adult pancreas. New β cells have been shown to form in the adult pancreas, in response to high physiological demand or experimental β-cell ablation, mostly by replication of existing β cells. The possibility that new β cells are formed from other sources is currently a point of major controversy. Under particular injury conditions, fully differentiated pancreatic duct and acinar cells have been shown to dedifferentiate into a progenitor-like state, however the extent, to which ductal, acinar or other endocrine cells contribute to restoring pancreatic β-cell mass remains to be resolved. In this review we focus on regenerative events in the pancreas with emphasis on the restoration of β-cell mass. We present an overview of regenerative responses noted within the different pancreatic lineages, following injury. We also highlight the intrinsic plasticity of the adult pancreas that allows for inter-conversion of fully differentiated pancreatic lineages through manipulation of few genes or growth factors. Taken together, evidence from a number of studies suggest that differentiated pancreatic lineages could act as facultative progenitor cells, but the extent to which these contribute to β-cell regeneration in vivo is still a matter of contention.

Lessons from the swamp: developing small molecules that confer salamander muscle cellularization in mammals

The ability of salamanders, such as newts, to regenerate damaged tissues has been studied for centuries. A prominent example of this regenerative power is the ability to re-grow entire amputated limbs. One important step in this regeneration process is skeletal muscle cellularization, in which the muscle fibers break down into dedifferentiated, mononuclear cells that proliferate and form new muscle in the replacement limb. In contrast, mammalian skeletal muscle does not undergo cellularization after injury. A significant proportion of research about tissue regeneration in salamanders aims to characterize regulatory genes that may have mammalian homologs. A less mainstream approach is to develop small molecule compounds that induce regeneration-related mechanisms in mammals. In this commentary, we discuss progress in discovering small molecules that induce cellularization in mammalian muscle. New research findings using these compounds has also shed light on cellular processes that regulate cellularization, such as apoptotic signaling. Although formidable technical hurdles remain, this progress increases our understanding of tissue regeneration and provide opportunities for developing small molecules that may enhance tissue repair in humans.

The Molecular and Cellular Choreography of Appendage Regeneration

Recent advances in limb regeneration are revealing the molecular events that integrate growth control, cell fate programming, and positional information to yield the exquisite replacement of the amputated limb. Parallel progress in several invertebrate and vertebrate models has provided a broader context for understanding the mechanisms and the evolution of regeneration. Together, these discoveries provide a foundation for describing the principles underlying regeneration of complex, multi-tissue structures. As such these findings should provide a wealth of ideas for engineers seeking to reconstitute regeneration from constituent parts or to elicit full regeneration from partial regeneration events.

Use of Xenopus Frogs to Study Renal Development/Repair

The Xenopus genus includes several members of aquatic frogs native to Africa but is perhaps best known for the species Xenopus laevis and Xenopus tropicalis. These species were popularized as model organisms from as early as the 1800s and have been instrumental in expanding several biological fields including cell biology, environmental toxicology, regenerative biology, and developmental biology. In fact, much of what we know about the formation and maturation of the vertebrate renal system has been acquired by examining the intricate genetic and morphological patterns that epitomize nephrogenesis in Xenopus. From these numerous reports, we have learned that the process of kidney development is as unique among organs as it is conserved among vertebrates. While development of most organs involves increases in size at a single location, development of the kidney occurs through a series of three increasingly complex nephric structures that are temporally distinct from one another and which occupy discrete spatial locales within the body. These three renal systems all serve to provide homeostatic, osmoregulatory, and excretory functions in animals. Importantly, the kidneys in amphibians, such as Xenopus, are less complex and more easily accessed than those in mammals, and thus tadpoles and frogs provide useful models for understanding our own kidney development. Several descriptive and mechanistic studies conducted with the Xenopus model system have allowed us to elucidate the cellular and molecular mediators of renal patterning and have also laid the foundation for our current understanding of kidney repair mechanisms in vertebrates. While some species-specific responses to renal injury have been observed, we still recognize the advantage of the Xenopus system due to its distinctive similarity to mammalian wound healing, reparative, and regenerative responses. In addition, the first evidence of renal regeneration in an amphibian system was recently demonstrated in Xenopus laevis. As genetic and molecular tools continue to advance, our appreciation for and utilization of this amphibian model organism can only intensify and will certainly provide ample opportunities to further our understanding of renal development and repair.