Cell differentiation and cell fate during urodele tail and limb regeneration

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.

The amazing and anomalous axolotls as scientific models

Ambystoma mexicanum (axolotl) embryos and juveniles have been used as model organisms for developmental and regenerative research for many years. This neotenic aquatic species maintains the unique capability to regenerate most, if not all, of its tissues well into adulthood. With large externally developing embryos, axolotls were one of the original model species for developmental biology. However, increased access to, and use of, organisms with sequenced and annotated genomes, such as Xenopus laevis and tropicalis and Danio rerio, reduced the prevalence of axolotls as models in embryogenesis studies. Recent sequencing of the large axolotl genome opens up new possibilities for defining the recipes that drive the formation and regeneration of tissues like the limbs and spinal cord. However, to decode the large Ambystoma mexicanum genome will take a herculean effort, community resources, and the development of novel techniques. Here, we provide an updated axolotl-staging chart ranging from 1-cell stage to immature adult paired with a perspective on both historical and current axolotl research that spans from their use in early studies of development to the recent cutting-edge research, employment of transgenesis, high resolution imaging, and study of mechanisms deployed in regeneration. This article is protected by copyright. All rights reserved.

The Diverse Manifestations of Regeneration and Why We Need to Study Them

For hundreds of years, the question of why some organisms can regenerate missing body parts while others cannot has remained poorly understood. This has been due in great part to the inability to genetically, molecularly, and cellularly dissect this problem for most of the history of the field. It has only been in the past 20-30 years that important mechanistic advances have been made in methodologies that introduce loss and gain of gene function in animals that can regenerate. However, we still have a very incomplete understanding of how broadly regenerative abilities may be dispersed across species and whether or not such properties share a common evolutionary origin, which may have emerged independently or both. Understanding regeneration, therefore, will require rigorously practiced fundamental, curiosity-driven, discovery research. Expanding the number of research organisms used to study regeneration allows us to uncover aspects of this problem we may not yet know exist and simultaneously increases our chances of solving this long-standing problem of biology.

Amphibian regeneration and mammalian cancer: Similarities and contrasts from an evolutionary biology perspective: Comparing the regenerative potential of mammalian embryos and urodeles to develop effective strategies against human cancer

Here we review and discuss the link between regeneration capacity and tumor suppression comparing mammals (embryos versus adults) with highly regenerative vertebrates. Similar to mammal embryo morphogenesis, in amphibians (essentially newts and salamanders) the reparative process relies on a precise molecular and cellular machinery capable of sensing abnormal signals and actively reprograming or eliminating them. As the embryo's evil twin, tumor also retains common functional attributes. The immune system plays a pivotal role in maintaining a physiological balance to provide surveillance against tumor initiation or to support its initiation and progression. We speculate that susceptibility to cancer development in adult mammals may be determined by the loss of an advanced regenerative capability during evolution and believe that gaining mechanistic insights into how regenerative capacity linked to tumor suppression is postnatally lost in mammals might illuminate an as yet unrecognized route to cancer treatment.

Salamander-like tail regeneration in the West African lungfish

Salamanders, frog tadpoles and diverse lizards have the remarkable ability to regenerate tails. Palaeontological data suggest that this capacity is plesiomorphic, yet when the developmental and genetic architecture of tail regeneration arose is poorly understood. Here, we show morphological and molecular hallmarks of tetrapod tail regeneration in the West African lungfish Protopterus annectens, a living representative of the sister group of tetrapods. As in salamanders, lungfish tail regeneration occurs via the formation of a proliferative blastema and restores original structures, including muscle, skeleton and spinal cord. In contrast with lizards and similar to salamanders and frogs, lungfish regenerate spinal cord neurons and reconstitute dorsoventral patterning of the tail. Similar to salamander and frog tadpoles, Shh is required for lungfish tail regeneration. Through RNA-seq analysis of uninjured and regenerating tail blastema, we show that the genetic programme deployed during lungfish tail regeneration maintains extensive overlap with that of tetrapods, with the upregulation of genes and signalling pathways previously implicated in amphibian and lizard tail regeneration. Furthermore, the lungfish tail blastema showed marked upregulation of genes encoding post-transcriptional RNA processing components and transposon-derived genes. Our results show that the developmental processes and genetic programme of tetrapod tail regeneration were present at least near the base of the sarcopterygian clade and establish the lungfish as a valuable research system for regenerative biology.

Regenerating zebrafish fin epigenome is characterized by stable lineage-specific DNA methylation and dynamic chromatin accessibility

BACKGROUND

Zebrafish can faithfully regenerate injured fins through the formation of a blastema, a mass of proliferative cells that can grow and develop into the lost body part. After amputation, various cell types contribute to blastema formation, where each cell type retains fate restriction and exclusively contributes to regeneration of its own lineage. Epigenetic changes that are associated with lineage restriction during regeneration remain underexplored.

RESULTS

We produce epigenome maps, including DNA methylation and chromatin accessibility, as well as transcriptomes, of osteoblasts and other cells in uninjured and regenerating fins. This effort reveals regeneration as a process of highly dynamic and orchestrated transcriptomic and chromatin accessibility changes, coupled with stably maintained lineage-specific DNA methylation. The epigenetic signatures also reveal many novel regeneration-specific enhancers, which are experimentally validated. Regulatory networks important for regeneration are constructed through integrative analysis of the epigenome map, and a knockout of a predicted upstream regulator disrupts normal regeneration, validating our prediction.

CONCLUSION

Our study shows that lineage-specific DNA methylation signatures are stably maintained during regeneration, and regeneration enhancers are preset as hypomethylated before injury. In contrast, chromatin accessibility is dynamically changed during regeneration. Many enhancers driving regeneration gene expression as well as upstream regulators of regeneration are identified and validated through integrative epigenome analysis.

Behavior of Stem-Like Cells, Precursors for Tissue Regeneration in Urodela, Under Conditions of Microgravity

We summarize data from our experiments on stem-like cell-dependent regeneration in amphibians in microgravity. Considering its deleterious effect on many tissues, we asked whether microgravity is compatible with reparative processes, specifically activation and proliferation of source cells. Experiments were conducted using tailed amphibians, which combine profound regenerative capabilities with high robustness, allowing an in vivo study of lens, retina, limb, and tail regeneration in challenging settings of spaceflight. Microgravity promoted stem-like cell proliferation to a varying extent (up to 2-fold), and it seemed to speed up source cell dedifferentiation, as well as sequential differentiation in retina, lens, and limb, leading to formation of bigger and more developed regenerates than in 1g controls. It also promoted proliferation and hypertrophy of Müller glial cells, eliciting a response similar to reactive gliosis. A significant increase in stem-like cell proliferation was mostly beneficial for regeneration and only in rare cases caused moderate tissue growth abnormalities. It is important that microgravity yielded a lasting effect even if applied before operations. We hypothesize on the potential mechanisms of gravity-dependent changes in stem-like cell behavior, including fibroblast growth factor 2 signaling pathway and heat shock proteins, which were affected in our experimental settings. Taken together, our data indicate that microgravity does not disturb the natural regenerative potential of newt stem-like cells, and, depending on the system, even stimulates their dedifferentiation, proliferation, and differentiation. We discuss these data along with publications on mammalian stem cell behavior in vitro and invertebrate regeneration in vivo in microgravity. In vivo data are very scarce and require further research using contemporary methods of cell behavior analysis to elucidate mechanisms of stem cell response to altered gravity. They are relevant for both practical applications, such as managing human reparative responses in spaceflight, and fundamental understanding of stem cell biology.

Changes in cell fate determine the regenerative and functional capacity of the developing kidney before and after release of obstruction

Congenital obstructive nephropathy is a major cause of chronic kidney disease (CKD) in children. The contribution of changes in the identity of renal cells to the pathology of obstructive nephropathy is poorly understood. Using a partial unilateral ureteral obstruction (pUUO) model in genetically modified neonatal mice, we traced the fate of cells derived from the renal stroma, cap mesenchyme, ureteric bud (UB) epithelium, and podocytes using Foxd1Cre, Six2Cre, HoxB7Cre, and Podocyte.Cre mice respectively, crossed with double fluorescent reporter (membrane-targetted tandem dimer Tomato (mT)/membrane-targetted GFP (mG)) mice. Persistent obstruction leads to a significant loss of tubular epithelium, rarefaction of the renal vasculature, and decreased renal blood flow (RBF). In addition, Forkhead Box D1 (Foxd1)-derived pericytes significantly expanded in the interstitial space, acquiring a myofibroblast phenotype. Degeneration of Sine Oculis Homeobox Homolog 2 (Six2) and HoxB7-derived cells resulted in significant loss of glomeruli, nephron tubules, and collecting ducts. Surgical release of obstruction resulted in striking regeneration of tubules, arterioles, interstitium accompanied by an increase in blood flow to the level of sham animals. Contralateral kidneys with remarkable compensatory response to kidney injury showed an increase in density of arteriolar branches. Deciphering the mechanisms involved in kidney repair and regeneration post relief of obstruction has potential therapeutic implications for infants and children and the growing number of adults suffering from CKD.

Homeostatic and regenerative neurogenesis in salamanders

Large-scale regeneration in the adult central nervous system is a unique capacity of salamanders among tetrapods. Salamanders can replace neuronal populations, repair damaged nerve fibers and restore tissue architecture in retina, brain and spinal cord, leading to functional recovery. The underlying mechanisms have long been difficult to study due to the paucity of available genomic tools. Recent technological progress, such as genome sequencing, transgenesis and genome editing provide new momentum for systematic interrogation of regenerative processes in the salamander central nervous system. Understanding central nervous system regeneration also entails designing the appropriate molecular, cellular, and behavioral assays. Here we outline the organization of salamander brain structures. With special focus on ependymoglial cells, we integrate cellular and molecular processes of neurogenesis during developmental and adult homeostasis as well as in various injury models. Wherever possible, we correlate developmental and regenerative neurogenesis to the acquisition and recovery of behaviors. Throughout the review we place the findings into an evolutionary context for inter-species comparisons.

Isolation, Characterization and Osteogenic Potential of Mouse Digit Tip Blastema Cells in Comparison with Bone Marrow-Derived Mesenchymal Stem Cells In Vitro

Objective

Limb regeneration mediated by blastema cells (BlCs) in mammals is limited to the digit tips of neonates. Due to the lack of access to BlCs in adults and the difficulty in isolating and expanding BlCs from neonates, the use of a cellular population with similar features of BlCs would be a valuable strategy to direct a non-regenerative wound towards regeneration. In this study, we have initially isolated and cultured BlCs, and explored their characteristics in vitro. Next, we compared the capability of bone marrow-derived mesenchymal stem cells (BM-MSCs) as an alternative accessible cell source to BlCs for regeneration of appendages.

Materials and Methods

In this experimental study, BM-MSCs were isolated from BM and we obtained BlCs from the neonatal regenerating digit tip of C57B/6 mice. The cells were characterized for expressions of cell surface markers by flow cytometry. Quantitative-reverse transcription polymerase chain reaction (qRT-PCR) and lineage-specific staining were used to assess their ability to differentiate into skeletal cell lineages. The colony forming ability, proliferation, alkaline phosphatase (ALP) activity, calcium content, and osteogenic gene expression were evaluated in both BM- MSCs and BlCs cultures at days 7, 14, and 21.

Results

qRT-PCR analysis revealed that the cells from both sources readily differentiated into mesodermal lineages. There was significantly higher colony forming ability in BM-MSCs compared to BlCs (P<0.05). Alizarin red staining (ARS), calcium, and the ALP assay showed the same degree of mineral deposition in both BlCs and BM-MSCs. Gene expression levels of osteblastic markers indicated similar bone differentiation capacity for both BlCs and BM-MSCs at all time-points.

Conclusion

Characteristics of BlCs in vitro appear to be similar to BM-MSCs. Therefore, they could be considered as a substitute for BlCs for a regenerative approach with potential use in future clinical settings for regenerating human appendages.

Long-term time-lapse live imaging reveals extensive cell migration during annelid regeneration

BACKGROUND

Time-lapse imaging has proven highly valuable for studying development, yielding data of much finer resolution than traditional "still-shot" studies and allowing direct examination of tissue and cell dynamics. A major challenge for time-lapse imaging of animals is keeping specimens immobile yet healthy for extended periods of time. Although this is often feasible for embryos, the difficulty of immobilizing typically motile juvenile and adult stages remains a persistent obstacle to time-lapse imaging of post-embryonic development.

RESULTS

Here we describe a new method for long-duration time-lapse imaging of adults of the small freshwater annelid Pristina leidyi and use this method to investigate its regenerative processes. Specimens are immobilized with tetrodotoxin, resulting in irreversible paralysis yet apparently normal regeneration, and mounted in agarose surrounded by culture water or halocarbon oil, to prevent dehydration but allowing gas exchange. Using this method, worms can be imaged continuously and at high spatial-temporal resolution for up to 5 days, spanning the entire regeneration process. We performed a fine-scale analysis of regeneration growth rate and characterized cell migration dynamics during early regeneration. Our studies reveal the migration of several putative cell types, including one strongly resembling published descriptions of annelid neoblasts, a cell type suggested to be migratory based on "still-shot" studies and long hypothesized to be linked to regenerative success in annelids.

CONCLUSIONS

Combining neurotoxin-based paralysis, live mounting techniques and a starvation-tolerant study system has allowed us to obtain the most extensive high-resolution longitudinal recordings of full anterior and posterior regeneration in an invertebrate, and to detect and characterize several cell types undergoing extensive migration during this process. We expect the tetrodotoxin paralysis and time-lapse imaging methods presented here to be broadly useful in studying other animals and of particular value for studying post-embryonic development.

The role of stem cells in limb regeneration

Limb regeneration is a complex yet fascinating process observed to some extent in many animal species, though seen in its entirety in urodele amphibians. Accomplished by formation of a morphologically uniform intermediate, the blastema, scientists have long attempted to define the cellular constituents that enable regrowth of a functional appendage. Today, we know that the blastema consists of a variety of multipotent progenitor cells originating from a variety of tissues, and which contribute to limb tissue regeneration in a lineage-restricted manner. By continuing to dissect the role of stem cells in limb regeneration, we can hope to one day modulate the human response to limb amputation and facilitate regrowth of a working replacement.

Ear wound regeneration in the African spiny mouse Acomys cahirinus

While regeneration occurs in a number of taxonomic groups across the Metazoa, there are very few reports of regeneration in mammals, which generally respond to wounding with fibrotic scarring rather than regeneration. A recent report described skin shedding, skin regeneration and extensive ear punch closure in two rodent species, Acomys kempi and Acomys percivali. We examined these striking results by testing the capacity for regeneration of a third species, Acomys cahirinus, and found a remarkable capacity to repair full thickness circular punches in the ear pinna. Four‐millimeter‐diameter wounds closed completely in 2 months in 100% of ear punches tested. Histology showed extensive formation of elastic cartilage, adipose tissue, dermis, epidermis and abundant hair follicles in the repaired region. Furthermore, we demonstrated abundant angiogenesis and unequivocal presence of both muscle and nerve fibers in the reconstituted region; in contrast, similar wounds in C57BL/6 mice simply healed the borders of the cut by fibrotic scarring. Our results confirm the regenerative capabilities of Acomys, and suggest this model merits further attention.

Charting the course of renal cryoinjury

We sought to characterize a minor renal cryoinjury that allows investigation into renal damage processes and subsequent endogenous repair mechanisms. To achieve this, we induced a small cryoinjury to mice, in which the transient superficial application of a liquid nitrogen-cooled cryoprobe to the exposed kidney induces a localized lesion that did not impair renal function. The resulting cryoinjury was examined by immunohistochemistry and Laser-Doppler flowmetry. Within hours of cryoinjury induction, tubular and vascular necrotic damage was observed, while blood flow in the directly injured area was reduced by 65%. The injured area demonstrated a peak in tubular and perivascular cell proliferation at 4 days postinjury, while apoptosis and fibrosis peaked at day 7. Infiltration of macrophages into the injury was first observed at day 4, and peaked at day 7. Vascular density in the direct injured area was lowest at day 7. As compared to the direct injured area, the (peripheral) penumbral region surrounding the directly injured area demonstrated enhanced cellular proliferation (2.5-6-fold greater), vascular density (1.6-2.9 fold greater) and blood perfusion (twofold greater). After 4 weeks, the area of damage was reduced by 73%, fibrosis decreased by 50% and blood flow in the direct injured area was reestablished by 63% with almost complete perfusion restoration in the injury's penumbral region. In conclusion, kidney cryoinjury provides a flexible facile model for the study of renal damage and associated endogenous repair processes.

Adenosine enhances progenitor cell recruitment and nerve growth via its A2B receptor during adult fin regeneration

A major issue in regenerative medicine is the control of progenitor cell mobilisation. Apoptosis has been reported as playing a role in cell plasticity, and it has been recently shown that apoptosis is necessary for organ and appendage regeneration. In this context, we explore its possible mode of action in progenitor cell recruitment during adult regeneration in zebrafish. Here, we show that apoptosis inhibition impairs blastema formation and nerve growth, both of which can be restored by exogenous adenosine acting through its A2B receptor. Moreover, adenosine increases the number of progenitor cells. Purinergic signalling is therefore an early and essential event in the pathway from lesion to blastema formation and provides new targets for manipulating cell plasticity in the adult.

Position-specific induction of ectopic limbs in non-regenerating blastemas on axolotl forelimbs

Ectopic retinoic acid (RA) has been hypothesized to reprogram the positional identity of cells in developing and regenerating limbs to a single positional value corresponding to the posterior‐ventral‐proximal (PVPr) position on the limb. We tested this hypothesis by using RA to reprogram the information of blastema cells that were induced to form at different positions around the limb circumference. We observed that RA treatment of blastemas in anterior and dorsal locations, but not posterior and ventral locations, resulted in the induction of complete ectopic limbs. These position‐specific differences in limb induction are probably due to differences in the positional disparity between the RA‐reprogrammed blastema cells and the cells at the periphery of the wound. These observations are consistent with the hypothesis that RA treatment reprograms the information in blastema cells to the PVPr position on the limb, since anterior and dorsal positions have the largest disparity and posterior and ventral have the smallest disparity from the PVPr identity.

The Hippo pathway regulates stem cells during homeostasis and regeneration of the flatworm Macrostomum lignano

The Hippo pathway orchestrates activity of stem cells during development and tissue regeneration and is crucial for controlling organ size. However, roles of the Hippo pathway in highly regenerative organisms, such as flatworms, are unknown. Here we show that knockdown of the Hippo pathway core genes in the flatworm Macrostomum lignano affects tissue homeostasis and causes formation of outgrowths through hyperproliferation of stem cells (neoblasts), and leads to disruption of allometric scaling during regeneration and increased size of regenerated parts. We further show that Yap, the downstream effector of the Hippo pathway, is a potential neoblast marker gene, as it is expressed in dividing cells in M. lignano and is essential for neoblast self-renewal. The phenotypes we observe in M. lignano upon knockdown of the Hippo pathway core genes and Yap are consistent with the known functions of the pathway in other model organisms and demonstrate that the Hippo pathway is functionally conserved between flatworms and mammals. This work establishes M. lignano as a productive model for investigation of the Hippo pathway.

Measurement of S-phase duration of adult stem cells in the flatworm Macrostomum lignano by double replication labelling and quantitative colocalization analysis

Platyhelminthes are highly attractive models for addressing fundamental aspects of stem cell biology in vivo. These organisms possess a unique stem cell system comprised of neoblasts that are the only proliferating cells during adulthood. We have investigated Ts (S-phase duration) of neoblasts during homoeostasis and regeneration in the flatworm, Macrostomum lignano. A double immunohistochemical technique was used, performing sequential pulses with the thymidine analogues CldU (chlorodeoxyuridine) and IdU (iododeoxyuridine), separated by variable chase times in the presence of colchicine. Owing to the localized nature of the fluorescent signals (cell nuclei) and variable levels of autofluorescence, standard intensity-based colocalization analyses could not be applied to accurately determine the colocalization. Therefore, an object-based colocalization approach was devised to score the relative number of double-positive cells. Using this approach, Ts (S-phase duration) in the main population of neoblasts was ∼13 h. During early regeneration, no significant change in Ts was observed.

Marking the tempo for myogenesis: Pax7 and the regulation of muscle stem cell fate decisions

Post-natal growth and regeneration of skeletal muscle is highly dependent on a population of resident myogenic precursors known as satellite cells. Transcription factors from the Pax gene family, Pax3 and Pax7, are critical for satellite cell biogenesis, survival and potentially self-renewal; however, the underlying molecular mechanisms remain unsolved. This is particularly true in the case of Pax7, which appears to regulate myogenesis at multiple levels. Accordingly, recent data have highlighted the importance of a functional relationship between Pax7 and the MyoD family of muscle regulatory transcription factors during normal muscle formation and disease. Here we will critically review key findings suggesting that Pax7 may play a dual role by promoting resident muscle progenitors to commit to the skeletal muscle lineage while preventing terminal differentiation, thus keeping muscle progenitors poised to differentiate upon environmental cues. In addition, potential regulatory mechanisms for the control of Pax7 activity will be proposed.

Microarray analysis of microRNA expression during axolotl limb regeneration

Among vertebrates, salamanders stand out for their remarkable capacity to quickly regrow a myriad of tissues and organs after injury or amputation. The limb regeneration process in axolotls (Ambystoma mexicanum) has been well studied for decades at the cell-tissue level. While several developmental genes are known to be reactivated during this epimorphic process, less is known about the role of microRNAs in urodele amphibian limb regeneration. Given the compelling evidence that many microRNAs tightly regulate cell fate and morphogenetic processes through development and adulthood by modulating the expression (or re-expression) of developmental genes, we investigated the possibility that microRNA levels change during limb regeneration. Using two different microarray platforms to compare the axolotl microRNA expression between mid-bud limb regenerating blastemas and non-regenerating stump tissues, we found that miR-21 was overexpressed in mid-bud blastemas compared to stump tissue. Mature A. mexicanum (“Amex”) miR-21 was detected in axolotl RNA by Northern blot and differential expression of Amex-miR-21 in blastema versus stump was confirmed by quantitative RT-PCR. We identified the Amex Jagged1 as a putative target gene for miR-21 during salamander limb regeneration. We cloned the full length 3′UTR of Amex-Jag1, and our in vitro assays demonstrated that its single miR-21 target recognition site is functional and essential for the response of the Jagged1 gene to miR-21 levels. Our findings pave the road for advanced in vivo functional assays aimed to clarify how microRNAs such as miR-21, often linked to pathogenic cell growth, might be modulating the redeployment of developmental genes such as Jagged1 during regenerative processes.

Hedgehog and Wnt coordinate signaling in myogenic progenitors and regulate limb regeneration

Amphibians have a remarkable capacity for limb regeneration. Following a severe injury, there is complete regeneration with restoration of the patterning and cellular architecture of the amputated limb. While studies have focused on the structural anatomical changes during amphibian limb regeneration, the signaling mechanisms that govern cellular dedifferentiation and blastemal progenitors are unknown. Here, we demonstrate the temporal and spatial requirement for hedgehog (Hh) signaling and its hierarchical correlation with respect to Wnt signaling during newt limb regeneration. While the dedifferentiation process of mature lineages does not depend on Hh signaling, the proliferation and the migration of the dedifferentiated cells are dependent on Hh signaling. Temporally controlled chemical inactivation of the Hh pathway indicates that Hh-mediated antero-posterior (AP) specification occurs early during limb regeneration and that Hh is subsequently required for expansion of the blastemal progenitors. Inhibition of Hh signaling results in G0/G1 arrest with a concomitant reduction in S-phase and G2/M population in myogenic progenitors. Furthermore, Hh inhibition leads to reduced Pax7-positive cells and fewer regenerating fibers relative to control tissue. We demonstrate that activation of Wnt signaling rescues the inhibition of Hh pathway mainly by enhancing proliferative signals, possibly mediated through TCF4 activity. Collectively, our results demonstrate coordinated signaling of Hh and Wnt activities in regulating blastemal progenitors and their hierarchical positioning during limb regeneration.

Differentiated skeletal cells contribute to blastema formation during zebrafish fin regeneration

The origin of cells that generate the blastema following appendage amputation has been a long-standing question in epimorphic regeneration studies. The blastema is thought to originate from either stem (or progenitor) cells or differentiated cells of various tissues that undergo dedifferentiation. Here, we investigate the origin of cells that contribute to the regeneration of zebrafish caudal fin skeletal elements. We provide evidence that the process of lepidotrichia (bony rays) regeneration is initiated as early as 24 hours post-amputation and that differentiated scleroblasts acquire a proliferative state, detach from the lepidotrichia surface, migrate distally, integrate into the blastema and dedifferentiate. These findings provide novel insights into the origin of cells in epimorphic appendage regeneration in zebrafish and suggest conservation of regeneration mechanisms between fish and amphibians.

Plasticity of human menstrual blood stem cells derived from the endometrium

Stem cells can be obtained from women's menstrual blood derived from the endometrium. The cells display stem cell markers such as Oct-4, SSEA-4, Nanog, and c-kit (CD117), and have the potent ability to differentiate into various cell types, including the heart, nerve, bone, cartilage, and fat. There has been no evidence of teratoma, ectopic formation, or any immune response after transplantation into an animal model. These cells quickly regenerate after menstruation and secrete many growth factors to display recurrent angiogenesis. The plasticity and safety of the acquired cells have been demonstrated in many studies. Menstrual blood-derived stem cells (MenSCs) provide an alternative source of adult stem cells for research and application in regenerative medicine. Here we summarize the multipotent properties and the plasticities of MenSCs and other endometrial stem cells from recent studies conducted both in vitro and in vivo.

Bone regenerates via dedifferentiation of osteoblasts in the zebrafish fin

While mammals have a limited capacity to repair bone defects, zebrafish can completely regenerate amputated bony structures of their fins. Fin regeneration is dependent on formation of a blastema, a progenitor cell pool accumulating at the amputation plane. It is unclear which cells the blastema is derived from, whether it forms by dedifferentiation of mature cells, and whether blastema cells are multipotent. We show that mature osteoblasts dedifferentiate and form part of the blastema. Osteoblasts downregulate expression of intermediate and late bone differentiation markers and induce genes expressed by bone progenitors. Dedifferentiated osteoblasts proliferate in a FGF-dependent manner and migrate to form part of the blastema. Genetic fate mapping shows that osteoblasts only give rise to osteoblasts in the regenerate, indicating that dedifferentiation is not associated with the attainment of multipotency. Thus, bone can regenerate from mature osteoblasts via dedifferentiation, a finding with potential implications for human bone repair.

Regeneration of cryoinjury induced necrotic heart lesions in zebrafish is associated with epicardial activation and cardiomyocyte proliferation

In mammals, myocardial cell death due to infarction results in scar formation and little regenerative response. In contrast, zebrafish have a high capacity to regenerate the heart after surgical resection of myocardial tissue. However, whether zebrafish can also regenerate lesions caused by cell death has not been tested. Here, we present a simple method for induction of necrotic lesions in the adult zebrafish heart based on cryoinjury. Despite widespread tissue death and loss of cardiomyocytes caused by these lesions, zebrafish display a robust regenerative response, which results in substantial clearing of the necrotic tissue and little scar formation. The cellular mechanisms underlying regeneration appear to be similar to those activated in response to ventricular resection. In particular, the epicardium activates a developmental gene program, proliferates and covers the lesion. Concomitantly, mature uninjured cardiomyocytes become proliferative and invade the lesion. Our injury model will be a useful tool to study the molecular mechanisms of natural heart regeneration in response to necrotic cell death.

Spinal cord repair in regeneration-competent vertebrates: adult teleost fish as a model system

Spinal cord injuries in mammals, including humans, have devastating long-term consequences. Despite substantial research, therapeutic approaches developed in mammalian model systems have had limited success to date. An alternative strategy in the search for treatment of spinal cord lesions is provided by regeneration-competent vertebrates. These organisms, which include fish, urodele amphibians, and certain reptiles, have a spinal cord very similar in structure to that of mammals, but are capable of spontaneous structural and functional recovery after spinal cord injury. The present review aims to provide an overview of the current status of our knowledge of spinal cord regeneration in one of these groups, teleost fish. The findings are discussed from a comparative perspective, with reference to other taxa of regeneration-competent vertebrates, as well as to mammals.

Peripheral nerve regeneration in the MRL/MpJ ear wound model

The MRL/MpJ mouse displays an accelerated ability to heal ear punch wounds without scar formation (whereas wounds on the dorsal surface of the trunk heal with scar formation), offering a rare opportunity for studying tissue regeneration in adult mammals. A blastema-like structure develops and subsequently the structure of the wounded ear is restored, including cartilage, skin, hair follicles and adipose tissue. We sought to assess if the MRL/MpJ strain also possessed an enhanced capacity for peripheral nerve regeneration. Female MRL/MpJ and C57BL/6 mice were wounded with a 2-mm excisional biopsy punch to the centre of each ear and two 4-mm excisional biopsy punches to the dorsal skin. Immunohistochemical dual staining of pan-neurofilament and CD31 markers was used to investigate reinnervation and vascularisation of both the dorsal surface of the trunk and ear wounds. The MRL/MpJ mouse ear exhibited a significantly (P > 0.01) higher density of regenerated nerves than C57BL/6 between 10 and 21 days post-wounding when the blastema-like structure was forming. Unlike dorsal skin wounds, nerve regeneration in the ear wound preceded vascularisation, recapitulating early mammalian development. Immunohistochemical data suggest that factors within the blastemal mesenchyme, such as aggrecan, may direct nerve regrowth in the regenerating ear tissue.

Built to rebuild: in search of organizing principles in plant regeneration

Plants are under constant attack from insects, microbes, and other physical assaults that damage or remove body parts. Regeneration is one common strategy among plants to repair their body plan. How do organisms that are proficient at regeneration adapt their developmental programs for repatterning tissues? A new body of research employing high-resolution imaging together with cell-fate markers has led to new insights into the tissues competent to regenerate and the mechanisms that re-establish pattern. In parallel to new findings in metazoan systems, recent work in plants shows that regeneration programs commonly thought to rely on dedifferentiated cells do not need to reprogram to a ground state. Imaging studies that track the expression of regulators of the plant's proliferative centers, meristems, in conjunction with mutant analysis have shed new light on the earliest organizational cues during regenerative organ formation. One promise of plant regeneration studies is to reveal the common design attributes of programs that pattern similar organs in different developmental contexts.

Potential of Macrostomum lignano to recover from gamma-ray irradiation

Stem cells are the only proliferating cells in flatworms and can be eliminated by irradiation with no damage to differentiated cells. We investigated the effect of fractionated irradiation schemes on Macrostomum lignano, namely, on survival, gene expression, morphology and regeneration. Proliferating cells were almost undetectable during the first week post-treatment. Cell proliferation and gene expression were restored within 1 month in a dose-dependent manner following exposure to up to 150 Gy irradiation. During recovery, stem cells did not cross the midline but were restricted within lateral compartments. An accumulated dose of 210 Gy resulted in a lethal phenotype. Our findings demonstrate that M. lignano represents a suitable model system for elucidating the effect of irradiation on the stem cell system in flatworms and for improving our understanding of the recovery potential of severely damaged stem-cell systems.

Immortality and the base of multicellular life: Lessons from cnidarian stem cells

Cnidarians are phylogenetically basal members of the animal kingdom (>600 million years old). Together with plants they share some remarkable features that cannot be found in higher animals. Cnidarians and plants exhibit an almost unlimited regeneration capacity and immortality. Immortality can be ascribed to the asexual mode of reproduction that requires cells with an unlimited self-renewal capacity. We propose that the basic properties of animal stem cells are tightly linked to this archaic mode of reproduction. The cnidarian stem cells can give rise to a number of differentiated cell types including neuronal and germ cells. The genomes of Hydra and Nematostella, representatives of two major cnidarian classes indicate a surprising complexity of both genomes, which is in the range of vertebrates. Recent work indicates that highly conserved signalling pathways control Hydra stem cell differentiation. Furthermore, the availability of genomic resources and novel technologies provide approaches to analyse these cells in vivo. Studies of stem cells in cnidarians will therefore open important insights into the basic mechanisms of stem cell biology. Their critical phylogenetic position at the base of the metazoan branch in the tree of life makes them an important link in unravelling the common mechanisms of stem cell biology between animals and plants.

Extreme makeover: converting one cell into another

Cells of adult mammals can be converted (reprogrammed) to new cells. In one approach, adult cells are converted to pluripotent stem cells, followed by differentiation to regenerate new cell types. Alternatively, adult cells may be directly converted into other mature cells or progenitors. We discuss and compare these two approaches with particular emphasis on the latter and its relevance for regenerative medicine.

Adult stem cells and their trans-differentiation potential--perspectives and therapeutic applications

Stem cells are self-renewing multipotent progenitors with the broadest developmental potential in a given tissue at a given time. Normal stem cells in the adult organism are responsible for renewal and repair of aged or damaged tissue. Adult stem cells are present in virtually all tissues and during most stages of development. In this review, we introduce the reader to the basic information about the field. We describe selected stem cell isolation techniques and stem cell markers for various stem cell populations. These include makers for endothelial progenitor cells (CD146/MCAM/MUC18/S-endo-1, CD34, CD133/prominin, Tie-2, Flk1/KD/VEGFR2), hematopoietic stem cells (CD34, CD117/c-Kit, Sca1), mesenchymal stem cells (CD146/MCAM/MUC18/S-endo-1, STRO-1, Thy-1), neural stem cells (CD133/prominin, nestin, NCAM), mammary stem cells (CD24, CD29, Sca1), and intestinal stem cells (NCAM, CD34, Thy-1, CD117/c-Kit, Flt-3). Separate section provides a concise summary of recent clinical trials involving stem cells directed towards improvement of a damaged myocardium. In the last part of the review, we reflect on the field and on future developments.

In vivo reprogramming of adult pancreatic exocrine cells to beta-cells

One goal of regenerative medicine is to instructively convert adult cells into other cell types for tissue repair and regeneration. Although isolated examples of adult cell reprogramming are known, there is no general understanding of how to turn one cell type into another in a controlled manner. Here, using a strategy of re-expressing key developmental regulators in vivo, we identify a specific combination of three transcription factors (Ngn3 (also known as Neurog3) Pdx1 and Mafa) that reprograms differentiated pancreatic exocrine cells in adult mice into cells that closely resemble beta-cells. The induced beta-cells are indistinguishable from endogenous islet beta-cells in size, shape and ultrastructure. They express genes essential for beta-cell function and can ameliorate hyperglycaemia by remodelling local vasculature and secreting insulin. This study provides an example of cellular reprogramming using defined factors in an adult organ and suggests a general paradigm for directing cell reprogramming without reversion to a pluripotent stem cell state.

Common cellular events occur during wound healing and organ regeneration in the sea cucumber Holothuria glaberrima

BACKGROUND

All animals possess some type of tissue repair mechanism. In some species, the capacity to repair tissues is limited to the healing of wounds. Other species, such as echinoderms, posses a striking repair capability that can include the replacement of entire organs. It has been reported that some mechanisms, namely extracellular matrix remodeling, appear to occur in most repair processes. However, it remains unclear to what extent the process of organ regeneration, particularly in animals where loss and regeneration of complex structures is a programmed natural event, is similar to wound healing. We have now used the sea cucumber Holothuria glaberrima to address this question.

RESULTS

Animals were lesioned by making a 3-5 mm transverse incision between one of the longitudinal muscle pairs along the bodywall. Lesioned tissues included muscle, nerve, water canal and dermis. Animals were allowed to heal for up to four weeks (2, 6, 12, 20, and 28 days post-injury) before sacrificed. Tissues were sectioned in a cryostat and changes in cellular and tissue elements during repair were evaluated using classical dyes, immmuohistochemistry and phalloidin labeling. In addition, the temporal and spatial distribution of cell proliferation in the animals was assayed using BrdU incorporation. We found that cellular events associated with wound healing in H. glaberrima correspond to those previously shown to occur during intestinal regeneration. These include: (1) an increase in the number of spherule-containing cells, (2) remodeling of the extracellular matrix, (3) formation of spindle-like structures that signal dedifferentiation of muscle cells in the area flanking the lesion site and (4) intense cellular division occurring mainly in the coelomic epithelium after the first week of regeneration.

CONCLUSION

Our data indicate that H. glaberrima employs analogous cellular mechanisms during wound healing and organ regeneration. Thus, it is possible that regenerative limitations in some organisms are due either to the absence of particular mechanisms associated with repair or the inability of activating the repair process in some tissues or stages.

Activin-βA Signaling Is Required for Zebrafish Fin Regeneration

Vertebrate limb regeneration occurs in anamniotes such as newts, salamanders, and zebrafish. After appendage amputation, the resection site is covered by a wound epidermis capping the underlying mature tissues of the stump from which the blastema emerges. The blastema is a mass of progenitor cells that constitute an apical growth zone. During outgrowth formation, the proximal blastemal cells progressively leave the zone and undergo the differentiation that results in the replacement of the amputated structures. Little is known about the mechanisms triggering regenerative events after injury. The zebrafish caudal fin provides a valuable model to study the mechanisms of regeneration. Zebrafish blastemal cells express specific genes, such as the homeobox-containing transcription factors msxB and msxC, and secreted signal FGF20a. In this study, we set out to identify signals that are transcriptionally upregulated after fin amputation and before blastema formation. Accordingly, a gene encoding a TGFbeta-related ligand, activin-betaA (actbetaA), was found to be strongly induced within 6 hr after fin amputation at the wound margin, and later in the blastema. Inhibition of Activin signaling through two specific chemical inhibitors, SB431542 and SB505124, lead to the early and complete block of regeneration. The morpholino knockdown of actbetaA and its receptor alk4 impaired the progression of regeneration. Closer examination of the phenotype revealed that Activin signaling is necessary for cell migration during wound healing and blastemal proliferation. These findings reveal a role of Activin-betaA signaling in the tissue repair after injury and subsequent outgrowth formation during epigenetic regeneration of the vertebrate appendage.

Regeneration of neural crest derivatives in the Xenopus tadpole tail

Background

After amputation of the Xenopus tadpole tail, a functionally competent new tail is regenerated. It contains spinal cord, notochord and muscle, each of which has previously been shown to derive from the corresponding tissue in the stump. The regeneration of the neural crest derivatives has not previously been examined and is described in this paper.

Results

Labelling of the spinal cord by electroporation, or by orthotopic grafting of transgenic tissue expressing GFP, shows that no cells emigrate from the spinal cord in the course of regeneration.

There is very limited regeneration of the spinal ganglia, but new neurons as well as fibre tracts do appear in the regenerated spinal cord and the regenerated tail also contains abundant peripheral innervation.

The regenerated tail contains a normal density of melanophores. Cell labelling experiments show that melanophores do not arise from the spinal cord during regeneration, nor from the mesenchymal tissues of the skin, but they do arise by activation and proliferation of pre-existing melanophore precursors. If tails are prepared lacking melanophores, then the regenerates also lack them.

Conclusion

On regeneration there is no induction of a new neural crest similar to that seen in embryonic development. However there is some regeneration of neural crest derivatives. Abundant melanophores are regenerated from unpigmented precursors, and, although spinal ganglia are not regenerated, sufficient sensory systems are produced to enable essential functions to continue.

EST-based identification of genes expressed in brain and spinal cord of Gekko japonicus, a species demonstrating intrinsic capacity of spinal cord regeneration

In the present study, a brain and spinal cord cDNA library of Gekko japonicus was constructed, and 2349 unique expressed sequence tags (ESTs), composed of 494 clusters and 1855 singletons, were determined from 4108 randomly selected clones. Of these sequences, 54.8% (1287/2349) ESTs are identified as known genes and 45.2% (1062/2349), as unknown genes. The identified ESTs were classified with respect to their functional categorization; both the housekeeping genes and the nervous system-related genes were well represented in the library. The complete open reading frame (ORF)-containing clones derived from the library were identified as well; 303 clones were obtained to date. Finally, two clones with sequence homology to endothelial differentiation-related factor 1 (EDF-1) and to myelin-associated glycoprotein (MAG) precursor were selected, and their expression patterns in normal and regenerating spinal cord were analyzed by RT-PCR. Both of them were expressed in the spinal cord, and the expression level decreased significantly after tail amputation. This study provides a preliminary view of the gene expression profiles of the central nervous system in gecko, and it might accelerate the process of gene discovery and function analysis in this organism, which demonstrates an intrinsic capacity of spinal cord regeneration.

Transdetermination: Drosophila imaginal disc cells exhibit stem cell-like potency

Drosophila imaginal discs, the primordia of the adult fly appendages, are an excellent system for studying developmental plasticity. Cells in the imaginal discs are determined for their disc-specific fate (wingness, legness) during embryogenesis. Disc cells maintain their determination during larval development, a time of extensive growth and proliferation. Only when prompted to regenerate do disc cells exhibit lability in their determined identity. Regeneration in the disc is mediated by a localized region of cell division, known as the regeneration blastema. Most regenerating disc cells strictly adhere to their disc-specific identity; some cells however, switch fate in a phenomenon known as transdetermination. Similar regeneration and transdetermination events can be induced in situ by misexpression of the signaling molecule wingless. Recent studies indicate that the plasticity of disc cells during regeneration is associated with high morphogen activity and the reorganization of chromatin structure. Here we provide both a historical perspective of imaginal disc transdetermination, as well as discuss recent findings on how imaginal disc cells acquire developmental plasticity and multipotency. We also highlight how an understanding of imaginal disc transdetermination can enhance an understanding of developmental potency exhibited by stem cells.

Clonally cultured differentiated pigment cells can dedifferentiate and generate multipotent progenitors with self-renewing potential

The differentiation of a given cell should be irreversible in order to ensure cell-type-specific function and stability of resident tissue. However, under stimulation in vitro or during regeneration, differentiated cells may recover properties of immature cells. Yet the mechanisms whereby differentiated cells can change fate or reverse to precursor cells are poorly understood. We show here that neural crest (NC)-derived pigment cells that have differentiated in quail embryo, when isolated from the skin and clonally cultured in vitro, are able to generate glial and myofibroblastic cells. The phenotypic reprogramming involves dedifferentiation of dividing pigment cells into cells that re-express NC early marker genes Sox10, FoxD3, Pax3 and Slug. Single melanocytes generate multipotent progenitors able to self-renew along serial subcloning, thus exhibiting stem cell properties. The presence of endothelin 3 promotes the emergence and maintenance of multipotent progenitors in melanocyte progeny. These multipotent cells are heterogeneous with respect to marker identity, including pigmented cells and dedifferentiated cells that have reacquired expression of the early NC marker HNK1. These data provide evidence that, when removed from their niche and subjected to appropriate culture conditions, pigment cells are phenotypically unstable and can reverse to their NC-like ancestors endowed with self-renewal capacity.

Wnt/beta-catenin signaling regulates vertebrate limb regeneration

The cellular and molecular bases allowing tissue regeneration are not well understood. By performing gain- and loss-of-function experiments of specific members of the Wnt pathway during appendage regeneration, we demonstrate that this pathway is not only necessary for regeneration to occur, but it is also able to promote regeneration in axolotl, Xenopus, and zebrafish. Furthermore, we show that changes in the spatiotemporal distribution of beta-catenin in the developing chick embryo elicit apical ectodermal ridge and limb regeneration in an organism previously thought not to regenerate. Our studies may provide valuable insights toward a better understanding of adult tissue regeneration.

Mechano-sensory organ regeneration in adults: the zebrafish lateral line as a model

In this report, we present a study of regeneration of the lateral line, a collection of mechano-sensory organ, in the adult zebrafish caudal fin. As all neuromasts are innervated by axon fibers, neuronal regeneration is a key issue in the regenerating process. We first show that support cells from the last neuromast adjacent to the amputation plane divide and migrate to colonize the blastema in order to reform the missing part of the lateral line. We then show that nerve re-growth takes place later than neuromast progenitor cell migration. We also provide evidence that new growth cones form at the amputation plane and subsequently follow the migrating placode-like structure to re-innervate regenerated neuromasts as they differentiate. Altogether, our observations indicate that caudal lateral line regeneration is not a mere recapitulation of the ontogenic process.

The chemokine SDF-1 regulates blastema formation during zebrafish fin regeneration

The work presented in this study focuses on blastema formation in epimorphic regeneration. We describe the expression pattern of Sdf1a and Sdf1b (the chemokines stromal-cell-derived factor-1a and 1b) and their two receptors Cxcr4a and Cxcr4b during zebrafish fin regeneration. We demonstrate that Sdf1a/Cxcr4a plays a critical role in fin regeneration and more precisely in epidermal cell proliferation, an important process for blastema formation. In mammals, a single cxcr4 gene is involved both in chemotaxis and cell proliferation and survival; we discuss in this study a possible functional division of the two cxcr4 zebrafish genes.