Regeneration of the urodele limb: a review

The Wnt/β-catenin/TCF/Sp5/Zic4 Gene Network That Regulates Head Organizer Activity in Hydra Is Differentially Regulated in Epidermis and Gastrodermis

Hydra head formation depends on an organizing center in which Wnt/β-catenin signaling, that plays an inductive role, positively regulates Sp5 and Zic4, with Sp5 limiting Wnt3/β-catenin expression and Zic4 triggering tentacle formation. Using transgenic lines in which the HySp5 promoter drives eGFP expression in either the epidermis or gastrodermis, we show that Sp5 promoter activity is differentially regulated in each epithelial layer. In intact animals, epidermal HySp5:GFP activity is strong apically and weak along the body column, while in the gastrodermis, it is maximal in the tentacle ring region and maintained at a high level along the upper body column. During apical regeneration, HySp5:GFP is activated early in the gastrodermis and later in the epidermis. Alsterpaullone treatment induces a shift in apical HySp5:GFP expression towards the body column where it forms transient circular figures in the epidermis. Upon β-catenin(RNAi), HySp5:GFP activity is down-regulated in the epidermis while bud-like structures expressing HySp5:GFP in the gastrodermis develop. Sp5(RNAi) reveals a negative Sp5 autoregulation in the epidermis, but not in the gastrodermis. These differential regulations in the epidermis and gastrodermis highlight the distinct architectures of the Wnt/β-catenin/TCF/Sp5/Zic4 network in the hypostome, tentacle base and body column of intact animals, as well as in the buds and apical and basal regenerating tips.

Loss of the ability to regenerate body appendages in vertebrates: from side effects of evolutionary innovations to gene loss

The ability to regenerate large body appendages is an ancestral trait of vertebrates, which varies across different animal groups. While anamniotes (fish and amphibians) commonly possess this ability, it is notably restricted in amniotes (reptiles, birds, and mammals). In this review, we explore the factors contributing to the loss of regenerative capabilities in amniotes. First, we analyse the potential negative impacts on appendage regeneration caused by four evolutionary innovations: advanced immunity, skin keratinization, whole-body endothermy, and increased body size. These innovations emerged as amniotes transitioned to terrestrial habitats and were correlated with a decline in regeneration capability. Second, we examine the role played by the loss of regeneration-related enhancers and genes initiated by these innovations in the fixation of an inability to regenerate body appendages at the genomic level. We propose that following the cessation of regenerative capacity, the loss of highly specific regeneration enhancers could represent an evolutionarily neutral event. Consequently, the loss of such enhancers might promptly follow the suppression of regeneration as a side effect of evolutionary innovations. By contrast, the loss of regeneration-related genes, due to their pleiotropic functions, would only take place if such loss was accompanied by additional evolutionary innovations that compensated for the loss of pleiotropic functions unrelated to regeneration, which would remain even after participation of these genes in regeneration was lost. Through a review of the literature, we provide evidence that, in many cases, the loss in amniotes of genes associated with body appendage regeneration in anamniotes was significantly delayed relative to the time when regenerative capability was lost. We hypothesise that this delay may be attributed to the necessity for evolutionary restructuring of developmental mechanisms to create conditions where the loss of these genes was a beneficial innovation for the organism. Experimental investigation of the downregulation of genes involved in the regeneration of body appendages in anamniotes but absent in amniotes offers a promising avenue to uncover evolutionary innovations that emerged from the loss of these genes. We propose that the vast majority of regeneration-related genes lost in amniotes (about 150 in humans) may be involved in regulating the early stages of limb and tail regeneration in anamniotes. Disruption of this stage, rather than the late stage, may not interfere with the mechanisms of limb and tail bud development during embryogenesis, as these mechanisms share similarities with those operating in the late stage of regeneration. Consequently, the most promising approach to restoring regeneration in humans may involve creating analogs of embryonic limb buds using stem cell-based tissue-engineering methods, followed by their transfer to the amputation stump. Due to the loss of many genes required specifically during the early stage of regeneration, this approach may be more effective than attempting to induce both early and late stages of regeneration directly in the stump itself.

Tail and Spinal Cord Regeneration in Urodelean Amphibians

Urodelean amphibians can regenerate the tail and the spinal cord (SC) and maintain this ability throughout their life. This clearly distinguishes these animals from mammals. The phenomenon of tail and SC regeneration is based on the capability of cells involved in regeneration to dedifferentiate, enter the cell cycle, and change their (or return to the pre-existing) phenotype during de novo organ formation. The second critical aspect of the successful tail and SC regeneration is the mutual molecular regulation by tissues, of which the SC and the apical wound epidermis are the leaders. Molecular regulatory systems include signaling pathways components, inflammatory factors, ECM molecules, ROS, hormones, neurotransmitters, HSPs, transcriptional and epigenetic factors, etc. The control, carried out by regulatory networks on the feedback principle, recruits the mechanisms used in embryogenesis and accompanies all stages of organ regeneration, from the moment of damage to the completion of morphogenesis and patterning of all its structures. The late regeneration stages and the effects of external factors on them have been poorly studied. A new model for addressing this issue is herein proposed. The data summarized in the review contribute to understanding a wide range of fundamentally important issues in the regenerative biology of tissues and organs in vertebrates including humans.

Cell proliferation and regeneration in the gill : By

Seminal studies from the early 20th century defined the structural changes associated with development and regeneration of the gills in goldfish at the gross morphological and cellular levels using standard techniques of light and electron microscopy. More recently, investigations using cell lineage tracing, molecular biology, immunohistochemistry and single-cell RNA-sequencing have pushed the field forward and have begun to reveal the cellular and molecular processes that orchestrate cell proliferation and regeneration in the gills. The gill is a multifunctional organ that mediates an array of important physiological functions, including respiration, ion regulation and excretion of waste products. It is comprised of unique cell types, such as pavement cells, ionocytes, chemoreceptors and undifferentiated stem or progenitor cells that regulate growth and replenish cell populations. The gills develop from the embryonic endoderm and are rich in cell types derived from the neural crest. The gills have the capacity to remodel themselves in response to environmental change, such as in the case of ionocytes, chemoreceptors and the interlamellar cell mass, and can completely regenerate gill filaments and lamellae. Both processes of remodeling and regeneration invariably involve cell proliferation. Although gill regeneration has been reported in only a limited number of fish species, the process appears to have many similarities to regeneration of other organs in fish and amphibians. The present article reviews the studies that have described gill development and growth, and that demonstrate a suite of genes, transcription factors and other proteins involved in cell proliferation and regeneration in the gills.

Identification of signalling pathways involved in gill regeneration in zebrafish

The occurrence of regeneration of the organs involved in respiratory gas exchange amongst vertebrates is heterogeneous. In some species of amphibians and fishes, the gills regenerate completely following resection or amputation, whereas in mammals, only partial, facultative regeneration of lung tissue occurs following injury. Given the homology between gills and lungs, the capacity of gill regeneration in aquatic species is of major interest in determining the underlying molecular or signalling pathways involved in respiratory organ regeneration. In the present study, we used adult zebrafish (Danio rerio) to characterize signalling pathways involved in the early stages of gill regeneration. Regeneration of the gills was induced by resection of gill filaments and observed over a period of up to 10 days. We screened for the effects on regeneration of the drugs SU5402, dorsomorphin and LY411575, which inhibit FGF, BMP or Notch signalling pathways, respectively. Exposure to each drug for 5 days significantly reduced regrowth of filament tips in regenerating tissue, compared with unresected controls. In separate experiments under normal conditions of regeneration, we used reverse transcription quantitative PCR and observed an increased expression of genes encoding for the bone morphogenetic factor, Bmp2b, fibroblast growth factor, Fgf8a, a transcriptional regulator (Her6) involved in Notch signalling, and Sonic Hedgehog (Shha), in regenerating gills at 10 day post-resection, compared with unresected controls. In situ hybridization confirmed that all four genes were expressed in regenerating gill tissue. This study implicates BMP, FGF, Notch and Shh signalling in gill regeneration in zebrafish.

Downregulation of Yap1 during limb regeneration results in defective bone formation in axolotl

The Hippo pathway plays an imperative role in cellular processes such as differentiation, regeneration, cell migration, organ growth, apoptosis, and cell cycle. Transcription coregulator component of Hippo pathway, YAP1, promotes transcription of genes involved in cell proliferation, migration, differentiation, and suppressing apoptosis. However, its role in epimorphic regeneration has not been fully explored. The axolotl is a well-established model organism for developmental biology and regeneration studies. By exploiting its remarkable regenerative capacity, we investigated the role of Yap1 in the early blastema stage of limb regeneration. Depleting Yap1 using gene-specific morpholinos attenuated the competence of axolotl limb regeneration evident in bone formation defects. To explore the affected downstream pathways from Yap1 down-regulation, the gene expression profile was examined by employing LC-MS/MS technology. Based on the generated data, we provided a new layer of evidence on the putative roles of increased protease inhibition and immune system activities and altered ECM composition in diminished bone formation capacity during axolotl limb regeneration upon Yap1 deficiency. We believe that new insights into the roles of the Hippo pathway in complex structure regeneration were granted in this study.

Cancer in Amphibia, a rare phenomenon?

Compared to other animals, the spontaneous occurrence of tumors in wild amphibians is relatively rare, generally limited to specific populations or species. The number of reports of spontaneous tumors in amphibians known up to 1986 was 491 cases in anurans and about 253 cases in urodeles. Similarly, there have been many, unsuccessful attempts to chemically or biologically induce tumors in amphibians. With these considerations, it is inevitable to wonder: do urodeles and anurans have an inherent resistance to cancer? Here, we review the spontaneous and induced occurrence of tumors in amphibians in a timeline, as well as failed attempts to induce tumors in these amphibians. Indeed, recent studies seem to indicate that there is a relationship between regeneration and cancer because regenerating tissues seem to resist tumorigenesis, as opposed to nonregenerative tissues of the same amphibian models. Although the mechanisms that allow regenerating tissues to resist tumorigenesis have not been elucidated, it is worth to note that, in addition to the apparent relationship between regeneration and cancer, amphibians possess characteristics that could contribute to their ability to resist the development of neoplastic events. The implications of these features in cancer susceptibility are discussed.

An approach for elucidating dermal fibroblast dedifferentiation in amphibian limb regeneration

Urodele amphibians, Pleurodeles waltl and Ambystoma mexicanum, have organ-level regeneration capability, such as limb regeneration. Multipotent cells are induced by an endogenous mechanism in amphibian limb regeneration. It is well known that dermal fibroblasts receive regenerative signals and turn into multipotent cells, called blastema cells. However, the induction mechanism of the blastema cells from matured dermal cells was unknown. We previously found that BMP2, FGF2, and FGF8 (B2FF) could play sufficient roles in blastema induction in urodele amphibians. Here, we show that B2FF treatment can induce dermis-derived cells that can participate in multiple cell lineage in limb regeneration. We first established a newt dermis-derived cell line and confirmed that B2FF treatment on the newt cells provided plasticity in cellular differentiation in limb regeneration. To clarify the factors that can provide the plasticity in differentiation, we performed the interspecies comparative analysis between newt cells and mouse cells and found the Pde4b gene was upregulated by B2FF treatment only in the newt cells. Blocking PDE4B signaling by a chemical PDE4 inhibitor suppressed dermis-to-cartilage transformation and the mosaic knockout animals showed consistent results. Our results are a valuable insight into how dermal fibroblasts acquire multipotency during the early phase of limb regeneration via an endogenous program in amphibian limb regeneration.

The engine initiating tissue regeneration: does a common mechanism exist during evolution?

A successful tissue regeneration is a very complex process that requires a precise coordination of many molecular, cellular and physiological events. One of the critical steps is to convert the injury signals into regeneration signals to initiate tissue regeneration. Although many efforts have been made to investigate the mechanisms triggering tissue regeneration, the fundamental questions remain unresolved. One of the major obstacles is that the injury and the initiation of regeneration are two highly coupled processes and hard to separate from one another. In this article, we review the major events occurring at the early injury/regeneration stage in a range of species, and discuss the possible common mechanisms during initiation of tissue regeneration.

Salamanders: The molecular basis of tissue regeneration and its relevance to human disease

Salamanders are recognized for their ability to regenerate a broad range of tissues. They have also have been used for hundreds of years for classical developmental biology studies because of their large accessible embryos. The range of tissues these animals can regenerate is fascinating, from full limbs to parts of the brain or heart, a potential that is missing in humans. Many promising research efforts are working to decipher the molecular blueprints shared across the organisms that naturally have the capacity to regenerate different tissues and organs. Salamanders are an excellent example of a vertebrate that can functionally regenerate a wide range of tissue types. In this review, we outline some of the significant insights that have been made that are aiding in understanding the cellular and molecular mechanisms of tissue regeneration in salamanders and discuss why salamanders are a worthy model in which to study regenerative biology and how this may benefit research fields like regenerative medicine to develop therapies for humans in the future.

A comparative perspective on lung and gill regeneration

The ability to continuously grow and regenerate the gills throughout life is a remarkable property of fish and amphibians. Considering that gill regeneration was first described over one century ago, it is surprising that the underlying mechanisms of cell and tissue replacement in the gills remain poorly understood. By contrast, the mammalian lung is a largely quiescent organ in adults but is capable of facultative regeneration following injury. In the course of the past decade, it has been recognized that lungs contain a population of stem or progenitor cells with an extensive ability to restore tissue; however, despite recent advances in regenerative biology of the lung, the signaling pathways that underlie regeneration are poorly understood. In this Review, we discuss the common evolutionary and embryological origins shared by gills and mammalian lungs. These are evident in homologies in tissue structure, cell populations, cellular function and genetic pathways. An integration of the literature on gill and lung regeneration in vertebrates is presented using a comparative approach in order to outline the challenges that remain in these areas, and to highlight the importance of using aquatic vertebrates as model organisms. The study of gill regeneration in fish and amphibians, which have a high regenerative potential and for which genetic tools are widely available, represents a unique opportunity to uncover common signaling mechanisms that may be important for regeneration of respiratory organs in all vertebrates. This may lead to new advances in tissue repair following lung disease.

Neural regulation in tooth regeneration of Ambystoma mexicanum

The presence of nerves is an important factor in successful organ regeneration in amphibians. The Mexican salamander, Ambystoma mexicanum, is able to regenerate limbs, tail, and gills when nerves are present. However, the nerve-dependency of tooth regeneration has not been evaluated. Here, we reevaluated tooth regeneration processes in axolotls using a three-dimensional reconstitution method called CoMBI and found that tooth regeneration is nerve-dependent although the dentary bone is independent of nerve presence. The induction and invagination of the dental lamina were delayed by denervation. Exogenous Fgf2, Fgf8, and Bmp7 expression could induce tooth placodes even in the denervated mandible. Our results suggest that the role of nerves is conserved and that Fgf+Bmp signals play key roles in axolotl organ-level regeneration. The presence of nerves is an important factor in successful organ regeneration in amphibians. The Mexican salamander, Ambystoma mexicanum, is able to regenerate limbs, tail, and gills when nerves are present. However, the nervedependency of tooth regeneration has not been evaluated. Here, we reevaluated tooth regeneration processes in axolotls using a three-dimensional reconstitution method called CoMBI and found that tooth regeneration is nerve-dependent although the dentary bone is independent of nerve presence. The induction and invagination of the dental lamina were delayed by denervation. Exogenous Fgf2, Fgf8, and Bmp7 expression could induce tooth placodes even in the denervated mandible. Our results suggest that the role of nerves is conserved and that Fgf+Bmp signals play key roles in axolotl organ-level regeneration.

Cells Isolated from Regenerating Caudal Fin of Sparus aurata Can Differentiate into Distinct Bone Cell Lineages

Teleosts have the ability to regenerate their caudal fin upon amputation. A highly proliferative mass of undifferentiated cells called blastema forms beneath wound epidermis and differentiates to regenerate all missing parts of the fin. To date, the origin and fate of the blastema is not completely understood. However, current hypotheses suggest that the blastema is comprised of lineage-restricted dedifferentiated cells. To investigate the differentiation capacity of regenerating fin-derived cells, primary cultures were initiated from the explants of 2-days post-amputation (dpa) regenerates of juvenile gilthead seabream (Sparus aurata). These cells were subcultured for over 30 passages and were named as BSa2. After 10 passages they were characterized for their ability to differentiate towards different bone cell lineages and mineralize their extracellular matrix, through immunocytochemistry, histology, and RT-PCR. Exogenous DNA was efficiently delivered into these cells by nucleofection. Assessment of lineage-specific markers revealed that BSa2 cells were capable of osteo/chondroblastic differentiation. BSa2 cells were also found to be capable of osteoclastic differentiation, as demonstrated through TRAP-specific staining and pit resorption assay. Here, we describe the development of the first successful cell line viz., BSa2, from S. aurata 2-dpa regenerating caudal fins, which has the ability of multilineage differentiation and is capable of in vitro mineralization. The availability of such in vitro cell systems has the potential to stimulate research on the mechanisms of cell differentiation during fin regeneration and provide new insights into the mechanisms of bone formation.

A histological study of normal and pathological limb regeneration in the Mexican axolotl Ambystoma mexicanum

Salamanders show unparalleled capacities of tissue regeneration amongst tetrapods (four-legged vertebrates), being able to repair and renew lost or damage body parts, such as tails, jaws, and limbs in a seemingly perfect fashion. Despite countless studies on axolotl (Ambystoma mexicanum) regeneration, only a few studies have thus far compared gross morphological and histological features of the original and regenerated limb skeleton. Therein, most studies have focused on nerves or muscles, while even fewer have provided detailed information about bones and cartilage. This study compares skeletal tissue structures of original and regenerated limbs with respect to tissue level histology. Histological serial sections of 55 axolotl larvae were generated, including 29 limbs that were severed by conspecifics, and 26 that were subject to targeted amputations. Amputations were executed in several larval stages (48, 52, and 53) and at different limb positions (humeral midshaft, above the mesopod). In addition, 3D reconstructions were prepared based on X-ray microtomography scans. The results demonstrate that regenerated forelimbs show a diversity of limb and digit abnormalities as a result of imperfect regeneration. Furthermore, abnormalities were more severe and more frequent in regenerated forelimbs caused by natural bites as compared with regenerated forelimbs after amputation. The results indicate that abnormalities occur frequently after regeneration in larval axolotls contradicting the notion of regeneration generally resulting in perfect limbs.

Extracellular matrix gene expression during arm regeneration in Amphiura filiformis

Extracellular matrix (ECM) plays a dynamic role during tissue development and re-growth. Body part regeneration efficiency relies also on effective ECM remodelling and deposition. Among invertebrates, echinoderms are well known for their striking regenerative abilities since they can rapidly regenerate functioning complex structures. To gather insights on the involvement of ECM during arm regeneration, the brittle star Amphiura filiformis was chosen as experimental model. Eight ECM genes were identified and cloned, and their spatio-temporal and quantitative expression patterns were analysed by means of whole mount in situ hybridisation and quantitative PCR on early and advanced regenerative stages. Our results show that almost none of the selected ECM genes are expressed at early stages of regeneration, suggesting a delay in their activation that may be responsible for the high regeneration efficiency of these animals, as described for other echinoderms and in contrast to most vertebrates. Moreover, at advanced stages, these genes are spatially and temporally differentially expressed, suggesting that the molecular regulation of ECM deposition/remodelling varies throughout the regenerative process. Phylogenetic analyses of the identified collagen-like genes reveal complex evolutionary dynamics with many rounds of duplications and losses and pinpointed their homologues in selected vertebrates. The study of other ECM genes will allow a better understanding of ECM contribution to brittle star arm regeneration.

A single-cell view of tissue regeneration in plants

With the development of advanced molecular-genetic and computational technologies it becomes possible to tackle individual cells within a regenerating tissue, to define morphogenetic and cellular changes in space and time by live imaging, to acquire transcriptome status with single-cell RNA sequencing (ScRNA-seq), and to delineate the candidate mechanisms by iterative computational and experimental approaches. Here, we review recent findings and current knowledge on tissue regeneration in plants, focusing on four evolutionarily conserved scenarios that a cell may embark on to facilitate the regeneration of a plant tissue structure lost by injury, namely cell death, division, dedifferentiation, and transdifferentiation. Understanding of these scenarios at single-cell resolution, singularly and in combination, could provide an unprecedented view of tissue regeneration in plants.

Stability and plasticity of positional memory during limb regeneration in Ambystoma mexicanum

BACKGROUND

Urodele amphibians are capable of regenerating their organs after severe damage. During such regeneration, participating cells are given differentiation instructions by the surrounding cells. Limb regeneration has been investigated as a representative phenomenon of organ regeneration. Cells known as blastema cells are induced after limb amputation. In this process, dermal fibroblasts are dedifferentiated and become undifferentiated similar to limb bud cells. Just like limb bud cells, the induced blastema cells are positioned along the three limb developmental axes: the dorsoventral, the anteroposterior, and the proximodistal. The accurate developmental axes are essential for reforming the structures correctly. Despite the importance of the developmental axes, the relationship between the newly establishing developmental axes and existing limb axes was not well described with molecular markers.

RESULTS

In this study, we grafted skin from GFP-transgenic axolotls and traced the cell lineage with position-specific gene expressions in order to investigate the correlation of the newly established axes and cellular origin. Shh- and Lmx1b-expressing cells emerged from the posterior skin and dorsal skin, respectively, even though the skin was transplanted to an inconsistent position. Shox2, a posterior marker gene, could be activated in cells derived from distal skin.

CONCLUSIONS

Our results suggest that the location memories on anteroposterior and dorsoventral axes are relatively stable in a regenerating blastema though cellular differentiation is reprogrammed.

Tailored chromatin modulation to promote tissue regeneration

Epigenetic regulation of gene expression is fundamental in the maintenance of cellular identity and the regulation of cellular plasticity during tissue repair. In fact, epigenetic modulation is associated with the processes of cellular de-differentiation, proliferation, and re-differentiation that takes place during tissue regeneration. In here we explore the epigenetic events that coordinate tissue repair in lower vertebrates with high regenerative capacity, and in mammalian adult stem cells, which are responsible for the homeostasis maintenance of most of our tissues. Finally we summarize promising CRISPR-based editing technologies developed during the last years, which look as promising tools to not only study but also promote specific events during tissue regeneration.

TISSUE REPAIR AND EPIMORPHIC REGENERATION: AN OVERVIEW

PURPOSE OF THE REVIEW

This manuscript discusses wound healing as a component of epimorphic regeneration and the role of the immune system in this process.

RECENT FINDINGS

Epimorphic regeneration involves formation of a blastema, a mass of undifferentiated cells capable of giving rise to the regenerated tissues. The apical epithelial cap plays an important role in blastemal formation.

SUMMARY

True regeneration is rarely observed in mammals. With the exception of transgenic strains, tissue repair in mammals usually leads to non-functional fibrotic tissue formation. In contrast, a number of lower order species including planarians, salamanders, and reptiles, have the ability to overcome the burden of scarring and tissue loss through complex adaptations that allow them to regenerate various anatomic structures through epimorphic regeneration. Blastemal cells have been suggested to originate via various mechanisms including de-differentiation, transdifferentiation, migration of pre-existing adult stem cell niches, and combinations of these.

Reactivation of larval keratin gene (krt62.L) in blastema epithelium during Xenopus froglet limb regeneration

Limb regeneration is considered a form of limb redevelopment because of the molecular and morphological similarities. Forming a regeneration blastema is, in essence, creating a developing limb bud in an adult body. This reactivation of a developmental process in a mature body is worth studying. Xenopus laevis has a biphasic life cycle that involves distinct larval and adult stages. These distinct developmental stages are useful for investigating the reactivation of developmental processes in post-metamorphic frogs (froglets). In this study, we focused on the re-expression of a larval gene (krt62.L) during Xenopus froglet limb regeneration. Recently renamed krt62.L, this gene was known as the larval keratin (xlk) gene, which is specific to larval-tadpole stages. During limb regeneration in a froglet, krt62.L was re-expressed in a basal layer of blastema epithelium, where adult-specific keratin (Krt12.6.S) expression was also observable. Nerves produce important regulatory factors for amphibian limb regeneration, and also play a role in blastema formation and maintenance. The effect of nerve function on krt62.L expression could be seen in the maintenance of krt62.L expression, but not in its induction. When an epidermis-stripped limb bud was grafted in a froglet blastema, the grafted limb bud could reach the digit-forming stage. This suggests that krt62.L-positive froglet blastema epithelium is able to support the limb development process. These findings imply that the developmental process is locally reactivated in an postmetamorphic body during limb regeneration.

Heat shock response and shape regulation during newt tail regeneration

Regenerating newt tail has recently been found to react to hypergravity in a stable and reproducible way - by curving downwards. Such morphogenetic effect of non-specific physical factor applied to a complex structure of an adult animal is a rare phenomenon with unknown molecular basis. For the first steps of unraveling this basis we've chosen heat shock proteins (HSPs) as promising candidates. Morphometrical analysis of tail regeneration was performed in aquarium (control), on substrate (relative hypergravity) and in aquarium under weekly application of heat shock. HSPs were inhibited pharmacologically during regeneration in aquarium and on substrate. Hsp70, 90 gene expression and protein localization were analyzed in the studied conditions. Weekly application of heat shock to newts regenerating tails in otherwise normal conditions led to development of curved tails (both upwards and downwards), suggesting that similar mechanisms are at play in both hypergravity-altered and heat shock-altered morphogenesis. Heat shock protein inhibitor KNK437 didn't affect tail shape during normal regeneration, but prevented the formation of tail curve in appropriate conditions. It was shown that HSP70 and HSP90 proteins are present in muscle and connective tissue of intact tails as well as regenerates, but only appear in epidermis in hypergravity-altered regenerates and heated tails. Based on our data, we hypothesize, that different external factors (e.g. hypergravity and heat shock) are received, analyzed and transmitted further to affect morphogenesis by similar mechanisms that utilize a set of HSP in epidermal cells.

Novel axolotl cardiac function analysis method using magnetic resonance imaging

The salamander axolotl is capable of complete regeneration of amputated heart tissue. However, non-invasive imaging tools for assessing its cardiac function were so far not employed. In this study, cardiac magnetic resonance imaging is introduced as a non-invasive technique to image heart function of axolotls. Three axolotls were imaged with magnetic resonance imaging using a retrospectively gated Fast Low Angle Shot cine sequence. Within one scanning session the axolotl heart was imaged three times in all planes, consecutively. Heart rate, ejection fraction, stroke volume and cardiac output were calculated using three techniques: (1) combined long-axis, (2) short-axis series, and (3) ultrasound (control for heart rate only). All values are presented as mean ± standard deviation. Heart rate (beats per minute) among different animals was 32.2±6.0 (long axis), 30.4±5.5 (short axis) and 32.7±4.9 (ultrasound) and statistically similar regardless of the imaging method (p > 0.05). Ejection fraction (%) was 59.6±10.8 (long axis) and 48.1±11.3 (short axis) and it differed significantly (p = 0.019). Stroke volume (μl/beat) was 133.7±33.7 (long axis) and 93.2±31.2 (short axis), also differed significantly (p = 0.015). Calculations were consistent among the animals and over three repeated measurements. The heart rate varied depending on depth of anaesthesia. We described a new method for defining and imaging the anatomical planes of the axolotl heart and propose one of our techniques (long axis analysis) may prove useful in defining cardiac function in regenerating axolotl hearts.

Mechanisms of urodele limb regeneration

This review explores the historical and current state of our knowledge about urodele limb regeneration. Topics discussed are (1) blastema formation by the proteolytic histolysis of limb tissues to release resident stem cells and mononucleate cells that undergo dedifferentiation, cell cycle entry and accumulation under the apical epidermal cap. (2) The origin, phenotypic memory, and positional memory of blastema cells. (3) The role played by macrophages in the early events of regeneration. (4) The role of neural and AEC factors and interaction between blastema cells in mitosis and distalization. (5) Models of pattern formation based on the results of axial reversal experiments, experiments on the regeneration of half and double half limbs, and experiments using retinoic acid to alter positional identity of blastema cells. (6) Possible mechanisms of distalization during normal and intercalary regeneration. (7) Is pattern formation is a self-organizing property of the blastema or dictated by chemical signals from adjacent tissues? (8) What is the future for regenerating a human limb?

Chemical activation of Wnt/β-catenin signalling inhibits innervation and causes skeletal tissue malformations during axolotl limb regeneration

Limb regeneration involves several interrelated physiological processes in which a particular signalling pathway may play a variety of functions. Blocking the function of Wnt/β-catenin signalling during limb regeneration inhibits regeneration in axolotls (Ambystoma mexicanum). Limb development shares many features with limb regeneration, and Wnt/β-catenin activation has different effects depending on the developmental stage. The aim of this study was to evaluate whether Wnt/β-catenin signalling activation during axolotl limb regeneration has different effects when activated at different stages of regeneration. To evaluate this hypothesis, we treated amputated axolotls with a Wnt agonist chemical at different stages of limb regeneration. The results showed that limb regeneration was inhibited when the treatment began before blastema formation. Under these conditions, blastema formation was hindered, possibly due to the lack of innervation. On the other hand, when axolotls were treated after blastema formation and immediately before the onset of morphogenesis, we observed structural disorganization in skeletal formation. In conclusion, we found that limb regeneration was differentially affected depending on the stage at which the Wnt signalling pathway was activated.

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.

Tetrapod limb and sarcopterygian fin regeneration share a core genetic programme

Salamanders are the only living tetrapods capable of fully regenerating limbs. The discovery of salamander lineage-specific genes (LSGs) expressed during limb regeneration suggests that this capacity is a salamander novelty. Conversely, recent paleontological evidence supports a deeper evolutionary origin, before the occurrence of salamanders in the fossil record. Here we show that lungfishes, the sister group of tetrapods, regenerate their fins through morphological steps equivalent to those seen in salamanders. Lungfish de novo transcriptome assembly and differential gene expression analysis reveal notable parallels between lungfish and salamander appendage regeneration, including strong downregulation of muscle proteins and upregulation of oncogenes, developmental genes and lungfish LSGs. MARCKS-like protein (MLP), recently discovered as a regeneration-initiating molecule in salamander, is likewise upregulated during early stages of lungfish fin regeneration. Taken together, our results lend strong support for the hypothesis that tetrapods inherited a bona fide limb regeneration programme concomitant with the fin-to-limb transition.

Analysis of embryonic development in the unsequenced axolotl: Waves of transcriptomic upheaval and stability

The axolotl (Ambystoma mexicanum) has long been the subject of biological research, primarily owing to its outstanding regenerative capabilities. However, the gene expression programs governing its embryonic development are particularly underexplored, especially when compared to other amphibian model species. Therefore, we performed whole transcriptome polyA+ RNA sequencing experiments on 17 stages of embryonic development. As the axolotl genome is unsequenced and its gene annotation is incomplete, we built de novo transcriptome assemblies for each stage and garnered functional annotation by comparing expressed contigs with known genes in other organisms. In evaluating the number of differentially expressed genes over time, we identify three waves of substantial transcriptome upheaval each followed by a period of relative transcriptome stability. The first wave of upheaval is between the one and two cell stage. We show that the number of differentially expressed genes per unit time is higher between the one and two cell stage than it is across the mid-blastula transition (MBT), the period of zygotic genome activation. We use total RNA sequencing to demonstrate that the vast majority of genes with increasing polyA+ signal between the one and two cell stage result from polyadenylation rather than de novo transcription. The first stable phase begins after the two cell stage and continues until the mid-blastula transition, corresponding with the pre-MBT phase of transcriptional quiescence in amphibian development. Following this is a peak of differential gene expression corresponding with the activation of the zygotic genome and a phase of transcriptomic stability from stages 9-11. We observe a third wave of transcriptomic change between stages 11 and 14, followed by a final stable period. The last two stable phases have not been documented in amphibians previously and correspond to times of major morphogenic change in the axolotl embryo: gastrulation and neurulation. These results yield new insights into global gene expression during early stages of amphibian embryogenesis and will help to further develop the axolotl as a model species for developmental and regenerative biology.

FGF and BMP derived from dorsal root ganglia regulate blastema induction in limb regeneration in Ambystoma mexicanum

Urodele amphibians have a remarkable organ regeneration ability that is regulated by neural inputs. The identification of these neural inputs has been a challenge. Recently, Fibroblast growth factor (Fgf) and Bone morphogenic protein (Bmp) were shown to substitute for nerve functions in limb and tail regeneration in urodele amphibians. However, direct evidence of Fgf and Bmp being secreted from nerve endings and regulating regeneration has not yet been shown. Thus, it remained uncertain whether they were the nerve factors responsible for successful limb regeneration. To gather experimental evidence, the technical difficulties involved in the usage of axolotls had to be overcome. We achieved this by modifying the electroporation method. When Fgf8-AcGFP or Bmp7-AcGFP was electroporated into the axolotl dorsal root ganglia (DRG), GFP signals were detectable in the regenerating limb region. This suggested that Fgf8 and Bmp7 synthesized in neural cells in the DRG were delivered to the limbs through the long axons. Further knockdown experiments with double-stranded RNA interference resulted in impaired limb regeneration ability. These results strongly suggest that Fgf and Bmp are the major neural inputs that control the organ regeneration ability.

Directing lineage specification of human mesenchymal stem cells by decoupling electrical stimulation and physical patterning on unmodified graphene

The organization and composition of the extracellular matrix (ECM) have been shown to impact the propagation of electrical signals in multiple tissue types. To date, many studies with electroactive biomaterial substrates have relied upon passive electrical stimulation of the ionic media to affect cell behavior. However, development of cell culture systems in which stimulation can be directly applied to the material - thereby isolating the signal to the cell-material interface and cell-cell contracts - would provide a more physiologically-relevant paradigm for investigating how electrical cues modulate lineage-specific stem cell differentiation. In the present study, we have employed unmodified, directly-stimulated, (un)patterned graphene as a cell culture substrate to investigate how extrinsic electrical cycling influences the differentiation of naïve human mesenchymal stem cells (hMSCs) without the bias of exogenous biochemicals. We first demonstrated that cyclic stimulation does not deteriorate the cell culture media or result in cytotoxic pH, which are critical experiments for correct interpretation of changes in cell behavior. We then measured how the expression of osteogenic and neurogenic lineage-specific markers were altered simply by exposure to electrical stimulation and/or physical patterns. Expression of the early osteogenic transcription factor RUNX2 was increased by electrical stimulation on all graphene substrates, but the mature marker osteopontin was only modulated when stimulation was combined with physical patterns. In contrast, the expression of the neurogenic markers MAP2 and β3-tubulin were enhanced in all electrical stimulation conditions, and were less responsive to the presence of patterns. These data indicate that specific combinations of non-biological inputs - material type, electrical stimulation, physical patterns - can regulate hMSC lineage specification. This study represents a substantial step in understanding how the interplay of electrophysical stimuli regulate stem cell behavior and helps to clarify the potential for graphene substrates in tissue engineering applications.

Making muscle: Morphogenetic movements and molecular mechanisms of myogenesis in Xenopus laevis

Xenopus laevis offers unprecedented access to the intricacies of muscle development. The large, robust embryos make it ideal for manipulations at both the tissue and molecular level. In particular, this model system can be used to fate map early muscle progenitors, visualize cell behaviors associated with somitogenesis, and examine the role of signaling pathways that underlie induction, specification, and differentiation of muscle. Several characteristics that are unique to X. laevis include myogenic waves with distinct gene expression profiles and the late formation of dermomyotome and sclerotome. Furthermore, myogenesis in the metamorphosing frog is biphasic, facilitating regeneration studies. In this review, we describe the morphogenetic movements that shape the somites and discuss signaling and transcriptional regulation during muscle development and regeneration. With recent advances in gene editing tools, X. laevis remains a premier model organism for dissecting the complex mechanisms underlying the specification, cell behaviors, and formation of the musculature system.

How Somatic Adult Tissues Develop Organizer Activity

The growth and patterning of anatomical structures from specific cellular fields in developing organisms relies on organizing centers that instruct surrounding cells to modify their behavior, namely migration, proliferation, and differentiation. We discuss here how organizers can form in adult organisms, a process of utmost interest for regenerative medicine. Animals like Hydra and planarians, which maintain their shape and fitness thanks to a highly dynamic homeostasis, offer a useful paradigm to study adult organizers in steady-state conditions. Beside the homeostatic context, these model systems also offer the possibility to study how organizers form de novo from somatic adult tissues. Both extracellular matrix remodeling and caspase activation play a key role in this transition, acting as promoters of organizer formation in the vicinity of the wound. Their respective roles and the crosstalk between them just start to be deciphered.

Maintaining Eastern newts (Notophthalmus viridescens) for regeneration research

The adult Eastern newt, Notophthalmus viridescens, has long served as a model for appendage as well as heart muscle regeneration studies. Newt tissues include all major cell types known in other vertebrates and mammals, including bone, cartilage, tendon, muscle, nerves, dermis, and epidermis. Therefore, these aquatic salamanders make an excellent model for studying the regeneration of complex tissues. Regeneration of adult tissues requires the integration of new tissues with preexisting tissues to form a functioning unit through a process that is not yet well understood. Scale is also an issue, because the regenerating tissues or structures are magnitudes larger than their embryonic counterparts during development, and therefore, it is likely that different physics and mechanics apply. Regardless, regeneration recapitulates to some degree developmental processes. In this chapter, we will describe basic methods for maintaining adult Eastern newts in the laboratory for the study of regeneration. To determine similarities and differences between development and regeneration at the cellular and molecular level, there is also a need for embryonic newt tissue. We therefore also outline a relatively simple way to produce and raise newt embryos in the laboratory.

Different Tissue-Derived Stem Cells: A Comparison of Neural Differentiation Capability

BACKGROUND

Stem cells are capable of self-renewal and differentiation into a wide range of cell types with multiple clinical and therapeutic applications. Stem cells are providing hope for many diseases that currently lack effective therapeutic methods, including strokes, Huntington's disease, Alzheimer's and Parkinson's disease. However, the paucity of suitable cell types for cell replacement therapy in patients suffering from neurological disorders has hampered the development of this promising therapeutic approach.

AIM

The innovative aspect of this study has been to evaluate the neural differentiation capability of different tissue-derived stem cells coming from different tissue sources such as bone marrow, umbilical cord blood, human endometrium and amniotic fluid, cultured under the same supplemented media neuro-transcription factor conditions, testing the expression of neural markers such as GFAP, Nestin and Neurofilaments using the immunofluorescence staining assay and some typical clusters of differentiation such as CD34, CD90, CD105 and CD133 by using the cytofluorimetric test assay.

RESULTS

Amniotic fluid derived stem cells showed a more primitive phenotype compared to the differentiating potential demonstrated by the other stem cell sources, representing a realistic possibility in the field of regenerative cell therapy suitable for neurodegenerative diseases.

Identification of Conserved and Novel MicroRNAs during Tail Regeneration in the Mexican Axolotl

The Mexican axolotl salamander (Ambystoma mexicanum) is one member of a select group of vertebrate animals that have retained the amazing ability to regenerate multiple body parts. In addition to being an important model system for regeneration, the axolotl has also contributed extensively to studies of basic development. While many genes known to play key roles during development have now been implicated in various forms of regeneration, much of the regulatory apparatus controlling the underlying molecular circuitry remains unknown. In recent years, microRNAs have been identified as key regulators of gene expression during development, in many diseases and also, increasingly, in regeneration. Here, we have used deep sequencing combined with qRT-PCR to undertake a comprehensive identification of microRNAs involved in regulating regeneration in the axolotl. Specifically, among the microRNAs that we have found to be expressed in axolotl tissues, we have identified 4564 microRNA families known to be widely conserved among vertebrates, as well as 59,811 reads of putative novel microRNAs. These findings support the hypothesis that microRNAs play key roles in managing the precise spatial and temporal patterns of gene expression that ensures the correct regeneration of missing tissues.

Effects of p21 Gene Down-Regulation through RNAi on Antler Stem Cells In Vitro

Cell cycle is an integral part of cell proliferation, and consists mainly of four phases, G1, S, G2 and M. The p21 protein, a cyclin dependent kinase inhibitor, plays a key role in regulating cell cyclevia G1 phase control. Cells capable of epimorphic regeneration have G2/M accumulation as their distinctive feature, whilst the majority of somatic cells rest at G1 phase. To investigate the role played byp21 in antler regeneration, we studied the cell cycle distribution of antler stem cells (ASCs), via down-regulation of p21 in vitro using RNAi. The results showed that ASCs had high levels of p21 mRNA expression and rested at G1 phase, which was comparable to the control somatic cells. Down-regulation of p21 did not result in ASC cell cycle re-distribution toward G2/M accumulation, but DNA damage and apoptosis of the ASCs significantly increased and the process of cell aging was slowed. These findings suggest that the ASCs may have evolved to use an alternative, p21-independent cell cycle regulation mechanism. Also a unique p21-dependent inhibitory effect may control DNA damage as a protective mechanism to ensure the fast proliferating ASCs do not become dysplastic/cancerous. Understanding of the mechanism underlying the role played by p21 in the ASCs could give insight into a mammalian system where epimorphic regeneration is initiated whilst the genome stability is effectively maintained.

Regeneration inducers in limb regeneration

Limb regeneration ability, which can be observed in amphibians, has been investigated as a representative phenomenon of organ regeneration. Recently, an alternative experimental system called the accessory limb model was developed to investigate early regulation of amphibian limb regeneration. The accessory limb model contributed to identification of limb regeneration inducers in urodele amphibians. Furthermore, the accessory limb model may be applied to other species to explore universality of regeneration mechanisms. This review aims to connect the insights recently gained to emboss universality of regeneration mechanisms among species. The defined molecules (BMP7 (or2) + FGF2 + FGF8) can transform skin wound healing to organ (limb) regeneration responses. The same molecules can initiate regeneration responses in some species.

Early evolution of limb regeneration in tetrapods: evidence from a 300-million-year-old amphibian

Salamanders are the only tetrapods capable of fully regenerating their limbs throughout their entire lives. Much data on the underlying molecular mechanisms of limb regeneration have been gathered in recent years allowing for new comparative studies between salamanders and other tetrapods that lack this unique regenerative potential. By contrast, the evolution of animal regeneration just recently shifted back into focus, despite being highly relevant for research designs aiming to unravel the factors allowing for limb regeneration. We show that the 300-million-year-old temnospondyl amphibian Micromelerpeton, a distant relative of modern amphibians, was already capable of regenerating its limbs. A number of exceptionally well-preserved specimens from fossil deposits show a unique pattern and combination of abnormalities in their limbs that is distinctive of irregular regenerative activity in modern salamanders and does not occur as variants of normal limb development. This demonstrates that the capacity to regenerate limbs is not a derived feature of modern salamanders, but may be an ancient feature of non-amniote tetrapods and possibly even shared by all bony fish. The finding provides a new framework for understanding the evolution of regenerative capacity of paired appendages in vertebrates in the search for conserved versus derived molecular mechanisms of limb regeneration.

During Drosophila disc regeneration, JAK/STAT coordinates cell proliferation with Dilp8-mediated developmental delay

Regeneration of fragmented Drosophila imaginal discs occurs in an epimorphic manner involving local cell proliferation at the wound site. After disc fragmentation, cells at the wound site activate a restoration program through wound healing, regenerative cell proliferation, and repatterning of the tissue. However, the interplay of signaling cascades driving these early reprogramming steps is not well-understood. Here, we profiled the transcriptome of regenerating cells in the early phase within 24 h after wounding. We found that JAK/STAT signaling becomes activated at the wound site and promotes regenerative cell proliferation in cooperation with Wingless (Wg) signaling. In addition, we showed that the expression of Drosophila insulin-like peptide 8 (dilp8), which encodes a paracrine peptide to delay the onset of pupariation, is controlled by JAK/STAT signaling in early regenerating discs. Our findings suggest that JAK/STAT signaling plays a pivotal role in coordinating regenerative disc growth with organismal developmental timing.

Roles of Hippo signaling pathway in size control of organ regeneration

Animals have an intrinsic regeneration ability for injured tissues and organs. Species that have high regeneration ability such as newts can regenerate an organ with exactly the same size and shape as those of the original one. It has been unclear how a regenerating organ grows and ceases growth at an appropriate size. Organ size control in regeneration is seen in various organs of various species that have high regeneration ability. In animal species that do not have sufficient regeneration ability, a wound heals (the injury is closed, but lost parts are not regenerated), but an organ cannot be restored to its original size. On the other hand, perturbation of regeneration sometimes results in oversized or extra structures. In this sense, organ size control plays essential roles in proper regeneration. In this article, we introduce the concept of size control in organ regeneration regulated by the Hippo signaling pathway. We focused on the transcriptional regulator Yap, which shuttles between the nuclei and cytoplasm to exert a regulatory function in a context-dependent manner. The Yap-mediated Hippo pathway is thought to sense cell density, extracellular matrix (ECM) contact and cell position and to regulate gene expression for control of organ size. This mechanism can reasonably explain size control of organ regeneration.

Regenerated leg segment patterns are regulated epigenetically by histone H3K27 methylation in the cricket Gryllus bimaculatus

Hemimetabolous insects such as the cricket Gryllus bimaculatus regenerate lost tissue parts using blastemal cells, which is a population of dedifferentiated-proliferating cells. The gene expression of several epigenetic factors is upregulated in the blastema compared with the expression in differentiated tissue, suggesting that epigenetic changes in gene expression may control the differentiation status of blastema cells during regeneration. To clarify the molecular basis of epigenetic regulation during regeneration, we focused on the function of the Gryllus Enhancer of zeste (Gb’E(z)) and Ubiquitously-transcribed tetratricopeptide repeat gene on the X chromosome (Gb’Utx) homologues that regulate the methylation and demethylation on histone H3 27th lysine residue (H3K27), respectively. Methylated histone H3K27 in the regenerating leg was diminished by Gb’E(z)RNAi and was increased by Gb’UtxRNAi. Regenerated Gb’E(z)RNAi cricket legs exhibited extra leg segment formation between the tibia and tarsus, and regenerated Gb’UtxRNAi cricket legs showed leg joint formation defects in the tarsus. In the Gb’E(z)RNAi-regenerating leg, the Gb’dac expression domain expanded in the tarsus. In contrast, in the Gb’UtxRNAi-regenerating leg, Gb’Egfr expression in the middle of the tarsus was diminished. These results suggest that regulation of the histone H3K27 methylation state is involved in the repatterning process during leg regeneration among cricket species via the epigenetic regulation of leg patterning gene expression.

Denervation impairs regeneration of amputated zebrafish fins

BACKGROUND

Zebrafish are able to regenerate many of its tissues and organs after damage. In amphibians this process is regulated by nerve fibres present at the site of injury, which have been proposed to release factors into the amputated limbs/fins, promoting and sustaining the proliferation of blastemal cells. Although some candidate factors have been proposed to mediate the nerve dependency of regeneration, the molecular mechanisms involved in this process remain unclear.

RESULTS

We have used zebrafish as a model system to address the role of nerve fibres in fin regeneration. We have developed a protocol for pectoral fin denervation followed by amputation and analysed the regenerative process under this experimental conditions. Upon denervation fins were able to close the wound and form a wound epidermis, but could not establish a functional apical epithelial cap, with a posterior failure of blastema formation and outgrowth, and the accumulation of several defects. The expression patterns of genes known to be key players during fin regeneration were altered upon denervation, suggesting that nerves can contribute to the regulation of the Fgf, Wnt and Shh pathways during zebrafish fin regeneration.

CONCLUSIONS

Our results demonstrate that proper innervation of the zebrafish pectoral fin is essential for a successful regenerative process, and establish this organism as a useful model to understand the molecular and cellular mechanisms of nerve dependence, during vertebrate regeneration.

Lipomatous Lumbar Mass with an Attached Digit and Associated Split Cord Malformation

Background:

A male infant was born with a digit attached to a skin-covered lumbar lipomatous mass and an underlying split cord malformation.

Methods:

Surgical removal of the mass was performed at four months-of-age. By this time the digit had grown a nail and imaging and histology showed ongoing development of articulated phalanges.

Results:

The lipomatous mass contained a long bone, a clavicle-and scapula-like structure and a variety of other mature germ layer derivatives. These features raised a number of diagnostic considerations, including: mature teratoma, hamartoma, rudimentary parasitic twin, lipomyelomeningocele and dorsal accessory limb.

Conclusions:

Based on review of the literature, the authors hypothesize that there is a pathogenetically related spectrum of skin-covered dorsal mass lesions, often associated with spinal dysraphism. These consist of a major lipomatous component and a variety of mature germ layer derivatives that can vary widely in their degree of anatomical organization from case to case.

Limb regeneration in salamanders and digital tip regeneration in experimental mice: implications for the hand surgeon

Several lessons and observations from limb regeneration in animals could open new insights to direct related research in the field of hand surgery. This article briefly reviews the biology of limb regeneration in salamanders and experimental mice, with special emphasis on implications for hand surgery.

LEVEL OF EVIDENCE

5.

Co-operative Bmp- and Fgf-signaling inputs convert skin wound healing to limb formation in urodele amphibians

Urodele amphibians have remarkable organ regeneration capability, and their limb regeneration capability has been investigated as a representative phenomenon. In the early 19th century, nerves were reported to be an essential tissue for the successful induction of limb regeneration. Nerve substances that function in the induction of limb regeneration responses have long been sought. A new experimental system called the accessory limb model (ALM) has been established to identify the nerve factors. Skin wounding in urodele amphibians results in skin wound healing but never in limb induction. However, nerve deviation to the wounded skin induces limb formation in ALM. Thus, nerves can be considered to have the ability to transform skin wound healing to limb formation. In the present study, co-operative Bmp and Fgf application, instead of nerve deviation, to wounded skin transformed skin wound healing to limb formation in two urodele amphibians, axolotl (Ambystoma mexicanum) and newt (Pleurodeles waltl). Our findings demonstrate that defined factors can induce homeotic transformation in postembryonic bodies of urodele amphibians. The combination of Bmp and Fgf(s) may contribute to the development of novel treatments for organ regeneration.