Lizard tail regeneration: regulation of two distinct cartilage regions by Indian hedgehog

Single-cell analysis of lizard blastema fibroblasts reveals phagocyte-dependent activation of Hedgehog-responsive chondrogenesis

Lizards cannot naturally regenerate limbs but are the closest known relatives of mammals capable of epimorphic tail regrowth. However, the mechanisms regulating lizard blastema formation and chondrogenesis remain unclear. Here, single-cell RNA sequencing analysis of regenerating lizard tails identifies fibroblast and phagocyte populations linked to cartilage formation. Pseudotime trajectory analyses suggest spp1+-activated fibroblasts as blastema cell sources, with subsets exhibiting sulf1 expression and chondrogenic potential. Tail blastema, but not limb, fibroblasts express sulf1 and form cartilage under Hedgehog signaling regulation. Depletion of phagocytes inhibits blastema formation, but treatment with pericytic phagocyte-conditioned media rescues blastema chondrogenesis and cartilage formation in amputated limbs. The results indicate a hierarchy of phagocyte-induced fibroblast gene activations during lizard blastema formation, culminating in sulf1+ pro-chondrogenic populations singularly responsive to Hedgehog signaling. These properties distinguish lizard blastema cells from homeostatic and injury-stimulated fibroblasts and indicate potential actionable targets for inducing regeneration in other species, including humans.

On the horizon: Hedgehog signaling to heal broken bones

Uncovering the molecular pathways that drive skeletal repair has been an ongoing challenge. Initial efforts have relied on in vitro assays to identify the key signaling pathways that drive cartilage and bone differentiation. While these assays can provide some clues, assessing specific pathways in animal models is critical. Furthermore, definitive proof that a pathway is required for skeletal repair is best provided using genetic tests. Stimulating the Hh (Hedgehog) pathway can promote cartilage and bone differentiation in cell culture assays. In addition, the application of HH protein or various pathway agonists in vivo has a positive influence on bone healing. Until recently, however, genetic proof that the Hh pathway is involved in bone repair has been lacking. Here, we consider both in vitro and in vivo studies that examine the role of Hh in repair and discuss some of the challenges inherent in their interpretation. We also identify needed areas of study considering a new appreciation for the role of cartilage during repair, the variety of cell types that may have differing roles in repair, and the recent availability of powerful lineage tracing techniques. We are optimistic that emerging genetic tools will make it possible to precisely define when and in which cells promoting Hh signaling can best promote skeletal repair, and thus, the clinical potential for targeting the Hh pathway can be realized.

Lizard Blastema Organoid Model Recapitulates Regenerated Tail Chondrogenesis

(1) Background: Lizard tail regeneration provides a unique model of blastema-based tissue regeneration for large-scale appendage replacement in amniotes. Green anole lizard (Anolis carolinensis) blastemas contain fibroblastic connective tissue cells (FCTCs), which respond to hedgehog signaling to create cartilage in vivo. However, an in vitro model of the blastema has not previously been achieved in culture. (2) Methods: By testing two adapted tissue dissociation protocols and two optimized media formulations, lizard tail FCTCs were pelleted in vitro and grown in a micromass blastema organoid culture. Pellets were analyzed by histology and in situ hybridization for FCTC and cartilage markers alongside staged original and regenerating lizard tails. (3) Results: Using an optimized serum-free media and a trypsin- and collagenase II-based dissociation protocol, micromass blastema organoids were formed. Organoid cultures expressed FCTC marker CDH11 and produced cartilage in response to hedgehog signaling in vitro, mimicking in vivo blastema and tail regeneration. (4) Conclusions: Lizard tail blastema regeneration can be modeled in vitro using micromass organoid culture, recapitulating in vivo FCTC marker expression patterns and chondrogenic potential.

Microscopical observations on the regenerating tail of tsinling dwarf skink (Scincella tsinlingensis)

Although the key steps of tail regeneration in lizards are well understood, further investigations involving skinks can provide the field of regeneration research with additional information. In order to characterize the cytoarchitecture of tail regeneration in Scincella tsinlingensis, an endemic species in China, its histological events and growth trends are investigated. The rate of tail regeneration varies with the season: it proceeds faster in summer and autumn than it does in winter and spring. Tail regeneration of S. tsinlingensis is summarized as wound healing, blastema formation, cell differentiation and tail growth, which can be subdivided into seven stages. Wound healing following tail loss, begins with an obvious outgrowth undergoing re-epithelialization. Numerous proliferating mesenchymal-like cells aggregate near the distal end of the severed spinal cord to form the blastema. The expanding blastema is invaded by blood vessels, nerves and ependyma. A cartilaginous skeleton is formed around the ependymal tube and the muscle starts to differentiate. The keratinization of epidermis coincides with scale formation. Pigmentation eventually occurs in the regenerated tail. Tail regeneration in S. tsinlingensis is an epimorphic kind of regeneration that is also known as blastema-mediated. Structure and composition of the regenerated tail, including its cytoarchitecture, represent a conserved pattern of regeneration also known from other lizards.

Introduction to the Study on Regeneration in Lizards as an Amniote Model of Organ Regeneration

Initial observations on the regeneration of the tail in lizards were recorded in brief notes by Aristotle over 2000 years ago, as reported in his book, History of Animals (cited from [...].

The vascularization, innervation and myogenesis of early regenerated tail in Gekko japonicus

Many species of lizards are capable of tail regeneration. There has been increased interest in the study of lizard tail regeneration in recent years as it is an amenable regeneration model for amniotes. In this study, Gekko japonicus was used as a model to investigate the initiation of vascularization, innervation and myogenesis during tail regeneration. We found that angiogenesis and axon regeneration occurred almost simultaneously within 4 days post amputation. The results showed that the endothelial cells of the original vasculature proliferated and extended into the blastema as capillary vessels, which inter-connected to form a capillary network. The nerve fibers innervated the regenerated tissue from the original spinal cord and dorsal root ganglia, and the fiber bundles increased during 14 days. Regenerating muscle tissues emerged 2 weeks after amputation. PAX3 and PAX7 expression were detected during myogenesis, with PAX7 showing a continuous increase in expression from day 3 until the day 14, whereas PAX3 reached a peak level on day 10 day post amputation, and then declined quickly to level as normal control on day 14. PCNA and PAX3 double-positive satellite cells were observed in the original rostral tissues, indicating the involvement of satellite cell proliferation during tail regeneration. Taken together, these data suggest that tail regeneration in Gekko japonicus involved rapid angiogenesis from the beginning to the day 10 and followed by capillary remodeling. The innervation of regenerated tail was significant on day 4 and increased gradually during regeneration, while the regenerated muscle tissues was obvious on day 14 after amputation.

Review. Limb regeneration in lizards under natural and experimental conditions with considerations on the induction of appendages regeneration in amniotes

BACKGROUND

Study on the failure of limb regeneration in lizards evidences the difficult problems met from amniotes to regenerate organs. Contrary to the tail, limb loss in terrestrial environment is generally fatal and no selection for its regeneration occurred during lizard evolution.

METHODS

Experimentally amputated limbs were fixed and embedded for microscopy.

RESULTS

After limb loss an intense inflammatory reaction occurs and immune cells are recruited underneath a wound epidermis, forming a vascularized granulation tissue. The regenerating epidermis takes 2-3 weeks to cover the limb stump since degenerating long bones must be excised first while a dense connective tissue is formed and no limb growth occurs. Cell proliferation occurs in granulation tissues and wound epidermis during the initial 2-3 weeks of wound healing but disappears later determining the arrest of growth. Transcriptome data indicates that the limb, contrary to the tail, activates numerous genes involved in inflammation, immunity and fibroplasia while down-regulates some proliferative and most myogenic genes. Attempts to stimulate limb regeneration, by implants of nervous tissues or growth factors such as FGFs only maintain proliferation for few weeks but eventually the scarring program prevails and only short outgrowths missing of autopodial elements are regenerated.

CONCLUSIONS

While lizard limbs show the typical scarring outcome of mammals, the comparison of genes activated in the regenerating tail has allowed identifying key genes implicated in organ regeneration in amniotes.

Spinal ganglia and peripheral nerves innervating the regenerating tail and muscles of lizards

The present review summarizes available information on the contribution of regenerating nerves to the process of regeneration in the tail of lizards. From the last three segments of the spinal cord and ganglia proximal to the regenerating tail, motor, sensory somatic and autonomous nerves regenerate and richly innervate the growing blastema. However, experimental studies have indicated that peripheral nerves are not essential for stimulating the regeneration of the tail that instead is mainly sustained by the interaction of the apical ependyma with the wound epidermis. Ganglion neurons innervating the regenerating blastema increase their size and some satellite cells multiply but no ganglion neurons are regenerated. Numerous Schwann cells proliferate to keep pace with nerve regeneration, and they form myelin starting from 3 to 4 weeks of tail regeneration. The hypertrophic ganglion neurons synthesize growth factors and signaling proteins such as FGFs and Wnts that are transported into the regenerating blastema through the regenerating nerves. Nerves form synaptic-like contacts with mesenchymal cells or fibroblasts at the tip of the regenerating blastema but not synaptic boutons. These terminals may discharge stimulating factors that favor cell proliferation but this is not experimentally demonstrated. Most of the innervation is directed to differentiating muscles where nerve endings form cholinergic motor-plates. Transcriptome data on the regenerating blastema-cone detect up-regulation of various genes coding for ionic channels, neurotransmitter receptors and signaling proteins. The latter suggests that the neurotrophic stimulation may control cell proliferation but is most directed to the functionality of regenerating muscles.

The value of the hedgehog signal in osteoblasts in fluoride-induced bone-tissue injury

OBJECTIVE

This study was designed to observe the expression of important hedgehog (Hh) signal factors in the bone tissue of rats with chronic fluorosis and cultured osteoblasts in order to investigate the role and significance of the Hh signal in fluoride-induced bone injury.

METHODS

Healthy Sprague-Dawley (SD) rats were randomly divided into four groups: the control group, the fluorosis group (F Group), the fluoride + blocker group (F + Cycl group: rats were treated with fluoride + cyclopamine), and the fluoride + blocker control group (F + DMSO group). After 6 months of intervention, the urinary fluoride content of rats in each group was detected. The primary osteoblasts of rats were selected for cell experiment, and the experiment was carried out after the cells were passaged from the second to the fourth generation.

RESULTS

The proliferation rate of primary rat osteoblasts presented time-affected and dose-affected relationships in a short time under treatment with a low dose of sodium fluoride (NaF), but the proliferation of osteoblasts was inhibited by long-term and high-dose NaF exposure. In the F group, the alkaline phosphatase (ALP) activity of osteoblasts increased gradually. The ALP activity was lower in the F + Cycl group than in the F group, and there was no significant difference between the F + DMSO group and F group. With the increase in fluoride exposure, the expression of Hh signal factors and osteogenic-related factor proteins increased gradually. The expressions of Indian hedgehog (Ihh), smoothened (Smo), Glioma-associated oncogene homolog (Gli) 2, and Runt-related transcription factor 2 (Runx2)in the F + Cycl group increased with the dose of fluoride but they were significantly inhibited compared with the F group. Compared with the control group, the content of urinary fluoride in the F group was significantly higher (P < 0.05), but there was no significant change in urinary fluoride content in the F + Cycl group and the F + DMSO group. Compared with the control group, the serum bone alkaline phosphatase (BALP) contents of rats in the other groups increased after 6 months' intake of fluoride water (P < 0.05). After drug blocking, the serum BALP content in the F + Cycl group was lower than that in the F + DMSO group (P < 0.05). The BALP content in the F + DMSO group was similar to that in the F group: it did not decrease. The mRNA expressions of Ihh, Smo, Gli2, and Runx2 in bone tissue of the F group were significantly higher than those in the control group (P < 0.05). After cyclopamine blocking, the expressions decreased (P < 0.05), but the differences between the F + DMSO group and F group were not statistically significant.

CONCLUSION

Hh signal plays an important role in fluoride-induced bone injury. The effective inhibition of cyclopamine is expected to be a new target for the treatment of skeletal damage caused by fluorosis.

Anatomical and histological analyses reveal that tail repair is coupled with regrowth in wild-caught, juvenile American alligators (Alligator mississippiensis)

Reptiles are the only amniotes that maintain the capacity to regenerate appendages. This study presents the first anatomical and histological evidence of tail repair with regrowth in an archosaur, the American alligator. The regrown alligator tails constituted approximately 6–18% of the total body length and were morphologically distinct from original tail segments. Gross dissection, radiographs, and magnetic resonance imaging revealed that caudal vertebrae were replaced by a ventrally-positioned, unsegmented endoskeleton. This contrasts with lepidosaurs, where the regenerated tail is radially organized around a central endoskeleton. Furthermore, the regrown alligator tail lacked skeletal muscle and instead consisted of fibrous connective tissue composed of type I and type III collagen fibers. The overproduction of connective tissue shares features with mammalian wound healing or fibrosis. The lack of skeletal muscle contrasts with lizards, but shares similarities with regenerated tails in the tuatara and regenerated limbs in Xenopus adult frogs, which have a cartilaginous endoskeleton surrounded by connective tissue, but lack skeletal muscle. Overall, this study of wild-caught, juvenile American alligator tails identifies a distinct pattern of wound repair in mammals while exhibiting features in common with regeneration in lepidosaurs and amphibia.

Appendage regeneration in anamniotes utilizes genes active during larval-metamorphic stages that have been lost or altered in amniotes: The case for studying lizard tail regeneration

This review elaborates the idea that organ regeneration derives from specific evolutionary histories of vertebrates. Regenerative ability depends on genomic regulation of genes specific to the life-cycles that have differentially evolved in anamniotes and amniotes. In aquatic environments, where fish and amphibians live, one or multiple metamorphic transitions occur before the adult stage is reached. Each transition involves the destruction and remodeling of larval organs that are replaced with adult organs. After organ injury or loss in adult anamniotes, regeneration uses similar genes and developmental process than those operating during larval growth and metamorphosis. Therefore, the broad presence of regenerative capability across anamniotes is possible because generating new organs is included in their life history at metamorphic stages. Soft hyaluronate-rich regenerative blastemas grow in submersed or in hydrated environments, that is, essential conditions for regeneration, like during development. In adult anamniotes, the ability to regenerate different organs decreases in comparison to larval stages and becomes limited during aging. Comparisons of genes activated during metamorphosis and regeneration in anamniotes identify key genes unique to these processes, and include thyroid, wnt and non-coding RNAs developmental pathways. In the terrestrial environment, some genes or developmental pathways for metamorphic transitions were lost during amniote evolution, determining loss of regeneration. Among amniotes, the formation of soft and hydrated blastemas only occurs in lizards, a morphogenetic process that evolved favoring their survival through tail autotomy, leading to a massive although imperfect regeneration of the tail. Deciphering genes activity during lizard tail regeneration would address future attempts to recreate in other amniotes regenerative blastemas that grow into variably completed organs.

Autotomy and Regeneration in Squamate Reptiles (Squamata, Reptilia): Defensive Behavior Strategies and Morphological Characteristics (Using Computer Microtomography Methods)

Abstract

It has been noted that caudal autotomy as a way of defending against predators in recent reptiles is characteristic solely of lepidosaurs and is absent in crocodiles and turtles. It was found that, in the order Rhynchocephalia and in representatives of the majority of families of lizards, intravertebral (IntraVB) autotomy is a widespread phenomenon, whereas agamid lizards and some snakes do not have a break plane, and their tails break between adjacent vertebrae (intervertebral (InterVB) autotomy). The frequencies of occurrence of InterVB autotomy and regeneration in six species of agamas of the genus Paralaudakia were analyzed. Six types of regenerate’s characteristic of the studied group and the anatomical structure of the knob -shaped jagged regenerate are described on the basis of the results of computed microtomography (micro-CT). Phenomena of autotomy and regeneration are discussed in the phylogenetic context.

Single Cell Sequencing Analysis of Lizard Phagocytic Cell Populations and Their Role in Tail Regeneration

Lizards are the closest relatives of mammals capable of tail regeneration, but the specific determinants of amniote regenerative capabilities are currently unknown. Macrophages are phagocytic immune cells that play a critical role in wound healing and tissue regeneration in a wide range of species. We hypothesize that macrophages regulate the process of lizard tail regeneration, and that comparisons with mammalian cell populations will yield insight into the role phagocytes play in determining an organism's regenerative potential. Single cell RNA sequencing (scRNAseq) was used to profile lizard immune cells and compare with mouse counterparts to contrast cell types between the two species. Treatment with clodronate liposomes effectively inhibited lizard tail stump tissue ablation and subsequent regeneration, and scRNAseq was used to profile changes in lizard immune cell populations resulting from tail amputation as well as identifying specific cell types affected by clodronate treatment. ScRNAseq analysis of lizard bone marrow, peripheral blood, and tissue-resident phagocyte cell populations was used to trace marker progression during macrophage differentiation and activation. These results indicated that lizard macrophages are recruited to tail amputation injuries faster than mouse populations and express high levels of matrix metalloproteinases (MMPs). In turn, treatment with MMP inhibitors inhibited lizard tail regeneration. These results provide single cell sequencing data sets for evaluating and comparing lizard and mammalian immune cell populations, and identifying macrophage populations that are critical regulators of lizard tail regrowth.

High expressions of the cytoglobin and PGC-1α genes during the tissue regeneration of house gecko (Hemidactylus platyurus) tails

BACKGROUND

The tissue regeneration process requires high oxygen and energy levels. Cytoglobin (Cygb) is a member of the globin family, which has the ability to bind oxygen, plays a role in dealing with oxidative stress, and carries oxygen into the mitochondria. Energy production for tissue regeneration is associated with mitochondria-especially mitochondrial biogenesis. The peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha protein helps to regulate mitochondrial biogenesis. House geckos (Hemidactylus platyurus) are reptiles that have the ability to regenerate the tissue in their tails. House geckos were selected as the animal models for this study in order to analyze the association of Cygb with oxygen supply and the association of PGC-1α with energy production for tissue regeneration.

RESULTS

The growth of house gecko tails showed a slow growth at the wound healing phase, then followed by a fast growth after wound healing phase of the regeneration process. While Cygb mRNA expression reached its peak at the wound healing phase and slowly decreased until the end of the observation. PGC-1α mRNA was expressed and reached its peak earlier than Cygb.

CONCLUSIONS

The expressions of both the Cygb and PGC-1α genes were relatively high compared to the control group. We therefore suggest that Cygb and PGC-1α play an important role during the tissue regeneration process.

Formation, structure, and function of extra-skeletal bones in mammals

This review describes the formation, structure, and function of bony compartments in antlers, horns, ossicones, osteoderm and the os penis/os clitoris (collectively referred to herein as AHOOO structures) in extant mammals. AHOOOs are extra-skeletal bones that originate from subcutaneous (dermal) tissues in a wide variety of mammals, and this review elaborates on the co-development of the bone and skin in these structures. During foetal stages, primordial cells for the bony compartments arise in subcutaneous tissues. The epithelial-mesenchymal transition is assumed to play a key role in the differentiation of bone, cartilage, skin and other tissues in AHOOO structures. AHOOO ossification takes place after skeletal bone formation, and may depend on sexual maturity. Skin keratinization occurs in tandem with ossification and may be under the control of androgens. Both endochondral and intramembranous ossification participate in bony compartment formation. There is variation in gradients of density in different AHOOO structures. These gradients, which vary according to function and species, primarily reduce mechanical stress. Anchorage of AHOOOs to their surrounding tissues fortifies these structures and is accomplished by bone-bone fusion and Sharpey fibres. The presence of the integument is essential for the protection and function of the bony compartments. Three major functions can be attributed to AHOOOs: mechanical, visual, and thermoregulatory. This review provides the first extensive comparative description of the skeletal and integumentary systems of AHOOOs in a variety of mammals.

Immunodetection of ephrin receptors in the regenerating tail of the lizard Podarcis muralis suggests stimulation of differentiation and muscle segmentation

Ephrin receptors are the most common tyrosine kinase effectors operating during development. Ephrin receptor genes are reported to be up-regulated in the regenerating tail of the Podarcis muralis lizard. Thus, in the current study, we investigated immunolocalization of ephrin receptors in the Podarcis muralis tail during regeneration. Weak immunolabelled bands for ephrin receptors were detected at 15-17 kDa, with a stronger band also detected at 60-65 kDa. Labelled cells and nuclei were seen in the basal layer of the apical wound epidermis and ependyma, two key tissues stimulating tail regeneration. Strong nuclear and cytoplasmic labelling were present in the segmental muscles of the regenerating tail, sparse blood vessels, and perichondrium of regenerating cartilage. The immunolocalization of ephrin receptors in muscle that gives rise to large portions of new tail tissue was correlated with their segmentation. This study suggests that the high localization of ephrin receptors in differentiating epidermis, ependyma, muscle, and cartilaginous cells is connected to the regulation of cell proliferation through the activation of programs for cell differentiation in the proximal regions of the regenerating tail. The lower immunolabelling of ephrin receptors in the apical blastema, where signaling proteins stimulating cell proliferation are instead present, helps maintain the continuous growth of this region.

Tail regeneration in Lepidosauria as an exception to the generalized lack of organ regeneration in amniotes

The present review hypothesizes that during the transition from water to land, amniotes lost part of the genetic program for metamorphosis utilized in larvae of their amphibian ancestors, a program that in extant fish and amphibians allows organ regeneration. The direct development of amniotes, with their growth from embryos to adults, occurred with the elimination of larval stages, increases the efficiency of immune responses and the complexity of nervous circuits. In amniotes, T-cells and macrophages likely eliminate embryonic-larval antigens that are replaced with the definitive antigens of adult organs. Among lepidosaurians numerous lizard families during the Permian and Triassic evolved the process of tail autotomy to escape predation, followed by tail regeneration. Autotomy limits inflammation allowing the formation of a regenerative blastema rich in the immunosuppressant and hygroscopic hyaluronic acid. Expression loss of developmental genes for metamorphosis and segmentation in addition to an effective immune system, determined an imperfect regeneration of the tail. Genes involved in somitogenesis were likely lost or are inactivated and the axial skeleton and muscles of the original tail are replaced with a nonsegmented cartilaginous tube and segmental myotomes. Lack of neural genes, negative influence of immune system, and isolation of the regenerating spinal cord within the cartilaginous tube impede the production of nerve and glial cells, and a stratified spinal cord with ganglia. Tissue and organ regeneration in other body regions of lizards and other reptiles is relatively limited, like in the other amniotes, although the cartilage shows a higher regenerative capability than in mammals.

Expression and role of HIF-1α and HIF-2α in tissue regeneration: a study of hypoxia in house gecko tail regeneration

The house gecko (Hemidactylus platyurus) has evolved the ability to autotomize its tail when threatened. The lost part is then regrown via epimorphic regeneration in a process that requires high energy and oxygen levels. Oxygen demand is therefore likely to outstrip supply and this can result in relative hypoxia in the tissues of the regenerating tail. The hypoxic state is stabilized by the Hypoxia Inducible Factor-1α (HIF-1α) and HIF-2α proteins. We induced tail autotomy in 30 mal H. platyurus adults using a standard procedure and then collected samples of the regenerated tail tissue on days 1, 3, 5, 8, 10, 13, 17, 21, 25, and 30 post autotomy. For each sample, mRNA expression was analyzed by qPCR, proteins were analyzed using Western Blot tests and immunohistochemistry, and the histological structure was analyzed using Hematoxylin and Eosin staining. On day 1, HIF-1α mRNA expression increased and the tissue was dominated by leucocyte and erythrocyte cells. HIF-1α mRNA expression peaked on day 3, at which time some cells were actively proliferating, migrating, and differentiating. At the same time as HIF-1α expression decreased, HIF-2α mRNA expression increased, as did overall cellular activity. HIF-2α expression increased more gradually but was present over a longer period of time than HIF-1α. We hypothesize that HIF-1α helps to initially stimulate the tissue regeneration process while HIF-2α functionally takes over the role of HIF-1α after HIF-1α succumbs to the oxygen conditions, but we suspect that both HIF-1α and HIF-2α play a role in overcoming the tissue’s hypoxic state.

Common themes in tetrapod appendage regeneration: a cellular perspective

Complete and perfect regeneration of appendages is a process that has fascinated and perplexed biologists for centuries. Some tetrapods possess amazing regenerative abilities, but the regenerative abilities of others are exceedingly limited. The reasons underlying these differences have largely remained mysterious. A great deal has been learned about the morphological events that accompany successful appendage regeneration, and a handful of experimental manipulations can be reliably applied to block the process. However, only in the last decade has the goal of attaining a thorough molecular and cellular biological understanding of appendage regeneration in tetrapods become within reach. Advances in molecular and genetic tools for interrogating these remarkable events are now allowing for unprecedented access to the fundamental biology at work in appendage regeneration in a variety of species. This information will be critical for integrating the large body of detailed observations from previous centuries with a modern understanding of how cells sense and respond to severe injury and loss of body parts. Understanding commonalities between regenerative modes across diverse species is likely to illuminate the most important aspects of complex tissue regeneration.

Differences in neural stem cell identity and differentiation capacity drive divergent regenerative outcomes in lizards and salamanders

While lizards and salamanders both exhibit the ability to regenerate amputated tails, the outcomes achieved by each are markedly different. Salamanders, such as Ambystoma mexicanum, regenerate nearly identical copies of original tails. Regenerated lizard tails, however, exhibit important morphological differences compared with originals. Some of these differences concern dorsoventral patterning of regenerated skeletal and spinal cord tissues; regenerated salamander tail tissues exhibit dorsoventral patterning, while regrown lizard tissues do not. Additionally, regenerated lizard tails lack characteristically roof plate-associated structures, such as dorsal root ganglia. We hypothesized that differences in neural stem cells (NSCs) found in the ependyma of regenerated spinal cords account for these divergent regenerative outcomes. Through a combination of immunofluorescent staining, RT-PCR, hedgehog regulation, and transcriptome analysis, we analyzed NSC-dependent tail regeneration. Both salamander and lizard Sox2⁺ NSCs form neurospheres in culture. While salamander neurospheres exhibit default roof plate identity, lizard neurospheres exhibit default floor plate. Hedgehog signaling regulates dorsalization/ventralization of salamander, but not lizard, NSCs. Examination of NSC differentiation potential in vitro showed that salamander NSCs are capable of neural differentiation into multiple lineages, whereas lizard NSCs are not, which was confirmed by in vivo spinal cord transplantations. Finally, salamander NSCs xenogeneically transplanted into regenerating lizard tail spinal cords were influenced by native lizard NSC hedgehog signals, which favored salamander NSC floor plate differentiation. These findings suggest that NSCs in regenerated lizard and salamander spinal cords are distinct cell populations, and these differences contribute to the vastly different outcomes observed in tail 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.

Developmental and adult-specific processes contribute to de novo neuromuscular regeneration in the lizard tail

Peripheral nerves exhibit robust regenerative capabilities in response to selective injury among amniotes, but the regeneration of entire muscle groups following volumetric muscle loss is limited in birds and mammals. In contrast, lizards possess the remarkable ability to regenerate extensive de novo muscle after tail loss. However, the mechanisms underlying reformation of the entire neuromuscular system in the regenerating lizard tail are not completely understood. We have tested whether the regeneration of the peripheral nerve and neuromuscular junctions (NMJs) recapitulate processes observed during normal neuromuscular development in the green anole, Anolis carolinensis. Our data confirm robust axonal outgrowth during early stages of tail regeneration and subsequent NMJ formation within weeks of autotomy. Interestingly, NMJs are overproduced as evidenced by a persistent increase in NMJ density 120 and 250 days post autotomy (DPA). Substantial Myelin Basic Protein (MBP) expression could also be detected along regenerating nerves indicating that the ability of Schwann cells to myelinate newly formed axons remained intact. Overall, our data suggest that the mechanism of de novo nerve and NMJ reformation parallel, in part, those observed during neuromuscular development. However, the prolonged increase in NMJ number and aberrant muscle differentiation hint at processes specific to the adult response. An examination of the coordinated exchange between peripheral nerves, Schwann cells, and newly synthesized muscle of the regenerating neuromuscular system may assist in the identification of candidate molecules that promote neuromuscular recovery in organisms incapable of a robust regenerative response.

Cartilage and Muscle Cell Fate and Origins during Lizard Tail Regeneration

Introduction

Human cartilage is an avascular tissue with limited capacity for repair. By contrast, certain lizards are capable of musculoskeletal tissue regeneration following tail loss throughout all stages of their lives. This extraordinary ability is the result of a complex process in which a blastema forms and gives rise to the tissues of the regenerate. Blastemal cells have been shown to originate either from dedifferentiated tissues or from existing progenitor cells in various species, but their origin has not been determined in lizards. As reptiles, lizards are the closest relatives to mammals with enhanced regenerative potential, and the origin of blastemal cells has important implications for the regenerative process. Hence, the aim of this study is to determine the cellular origin of regenerated cartilage and muscle tissues in reptiles using the mourning gecko lizard as the regenerative model.

Methods

To trace the fate and differentiation potential of cartilage during tail regeneration, cartilage cells pre-labeled with the fluorescent tracer Dil were injected into lizard tails, and the contribution of cartilage cells to regenerated tail tissues was assessed by histologic examination at 7, 14, and 21 days post-tail amputation. The contribution of muscle cells to regenerated tail tissues was evaluated using muscle creatine kinase promoter-driven Cre recombinase in conjunction with the Cre-responsive green-to-red fluorescence shift construct CreStoplight. 21 days after amputation, tail tissues were analyzed by histology for red fluorescent protein (RFP)-positive cells.

Results

At 7 days post-amputation, Dil-labeled cartilage cells localized to the subapical space contributing to the blastema. At 14 and 21 days post-amputation, Dil-labeled cells remained in the subapical space and colocalized with Collagen type II (Col2) staining in the cartilage tube and myosin heavy chain (MHC) staining in regenerated muscle. Lineage tracing of myocytes showed colocalization of RFP with Col2 and MHC in differentiated tissues at 21 days post-amputation.

Conclusion

This study demonstrates that differentiated cartilage cells contribute to both regenerated muscle and cartilage tissues following tail loss, and in turn, differentiated muscle cells contribute to both tissue types as well. These findings suggest that dedifferentiation and/or transdifferentiation are at least partially responsible for the regenerative outcome in the mourning gecko.

Reverse engineering development: Crosstalk opportunities between developmental biology and tissue engineering

The fields of developmental biology and tissue engineering have been revolutionized in recent years by technological advancements, expanded understanding, and biomaterials design, leading to the emerging paradigm of "developmental" or "biomimetic" tissue engineering. While developmental biology and tissue engineering have long overlapping histories, the fields have largely diverged in recent years at the same time that crosstalk opportunities for mutual benefit are more salient than ever. In this perspective article, we will use musculoskeletal development and tissue engineering as a platform on which to discuss these emerging crosstalk opportunities and will present our opinions on the bright future of these overlapping spheres of influence. The multicellular programs that control musculoskeletal development are rapidly becoming clarified, represented by shifting paradigms in our understanding of cellular function, identity, and lineage specification during development. Simultaneously, advancements in bioartificial matrices that replicate the biochemical, microstructural, and mechanical properties of developing tissues present new tools and approaches for recapitulating development in tissue engineering. Here, we introduce concepts and experimental approaches in musculoskeletal developmental biology and biomaterials design and discuss applications in tissue engineering as well as opportunities for tissue engineering approaches to inform our understanding of fundamental biology. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:2356-2368, 2017.

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

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

Immunohistochemical and western blot analysis suggest that the soluble forms of FGF1-2 and FGFR1-2 sustain tail regeneration in the lizard

Fibroblast Growth Factors 1-2 (FGF1-2) stimulate tail regeneration in lizards and therefore the distribution of their receptors, FGFR1-2, in the regenerating tail of the lizard. Podarcis muralis has been studied using immunofluorescence and western blotting. Immunoreactive protein bands at 15-16kDa for FGF1-2 in addition to those at 50-65kDa are detected in the regenerating epidermis, but weak bands at 35, 45 and 50kDa appear from the regenerating connective tissues. Strongly immunolabeled bands for FGFR1 at 32, 60, and 80kDa and less intense for FGFR2 only appear in the regenerating tail. In normal tail epidermis and dermis, higher MW forms are present at 80 and 115-140kDa, respectively, but they disappear in the regenerating epidermis and dermis where low MW forms of FGFR1-2 are found at 50-70kDa. Immunolocalization confirms that most FGFR1-2 are present in the wound epidermis, Apical Epidermal Peg, ependymal tube while immunolabeling lowers in regenerating muscles, blastema cells, cartilage and connectives tissues. The likely release of FGFs from the Apical Epidermal Peg and ependyma and the presence of their receptors in these tissues may determine the autocrine stimulation of proliferation and a paracrine stimulation of the blastema cells through their FGF Receptors.

Tail regeneration and other phenomena of wound healing and tissue restoration in lizards

Wound healing is a fundamental evolutionary adaptation with two possible outcomes: scar formation or reparative regeneration. Scars participate in re-forming the barrier with the external environment and restoring homeostasis to injured tissues, but are well understood to represent dysfunctional replacements. In contrast, reparative regeneration is a tissue-specific program that near-perfectly replicates that which was lost or damaged. Although regeneration is best known from salamanders (including newts and axolotls) and zebrafish, it is unexpectedly widespread among vertebrates. For example, mice and humans can replace their digit tips, while many lizards can spontaneously regenerate almost their entire tail. Whereas the phenomenon of lizard tail regeneration has long been recognized, many details of this process remain poorly understood. All of this is beginning to change. This Review provides a comparative perspective on mechanisms of wound healing and regeneration, with a focus on lizards as an emerging model. Not only are lizards able to regrow cartilage and the spinal cord following tail loss, some species can also regenerate tissues after full-thickness skin wounds to the body, transections of the optic nerve and even lesions to parts of the brain. Current investigations are advancing our understanding of the biological requirements for successful tissue and organ repair, with obvious implications for biomedical sciences and regenerative medicine.

Genomic and phenotypic signatures of climate adaptation in an Anolis lizard

Integrated knowledge on phenotype, physiology, and genomic adaptations is required to understand the effects of climate on evolution. The functional genomic basis of organismal adaptation to changes in the abiotic environment, its phenotypic consequences, and its possible convergence across vertebrates are still understudied. In this study, we use a comparative approach to verify predicted gene functions for vertebrate thermal adaptation with observed functions underlying repeated genomic adaptations in response to elevation in the lizard Anolis cybotes. We establish a direct link between recurrently evolved phenotypes and functional genomics of altitude-related climate adaptation in three highland and lowland populations in the Dominican Republic. We show that across vertebrates, genes contained in this interactome are expressed within the brain, the endocrine system, and during development. These results are relevant to elucidate the effect of global climate change across vertebrates and might aid in furthering insight into gene-environment relationships under disturbances to homeostasis.

Potential Involvement of Snail Members in Neuronal Survival and Astrocytic Migration during the Gecko Spinal Cord Regeneration

Certain regenerative vertebrates such as fish, amphibians and reptiles are capable of regenerating spinal cord after injury. Most neurons of spinal cord will survive from the injury and regrow axons to repair circuits with an absence of glial scar formation. However, the underlying mechanisms of neuronal anti-apoptosis and glia-related responses have not been fully clarified during the regenerative process. Gecko has becoming an inspiring model to address spinal cord regeneration in amniotes. In the present study, we investigated the regulatory roles of Snail family members, the important transcriptional factors involved in both triggering of the cell migration and cell survival, during the spontaneous spinal cord regeneration. Both Snail1 and Snail3 have been shown to promote neuronal survival and astrocytic migration via anti-apoptotic and GTPases signaling following gecko tail amputation. Transforming growth factor-beta (TGFβ), together with other cytokines were involved in inducing expression of Snail protein. Our data indicate a conserved function of Snail proteins in embryonic development and tissue regeneration, which may provide clues for CNS repair in the mammals.

Wnt-1 immunodetection in the regenerating tail of lizard suggests it is involved in the proliferation and distal growth of the blastema

Lizard tail regeneration depends from growth of the apical tip and autonomous regeneration of a new spinal cord, cartilaginous tube and muscles. The presence of embryonic signaling pathways is likely involved and we have focused on immunolocalization of Wnt1 protein in regenerating tissues, a protein promoting proliferation and tumorigenesis. Western blot indicates some immunoreactive bands in the expected range at 46 and 33kDa in the regenerating tail. Immunolocalization indicates that Wnt1 is prevalently detected in the apical wound epidermis, blastema, and ependyma ampulla of the regenerating tail while it lowers in other tissues of more proximal regions close to the original tail stump. Although a gradient for Wnt1 was not detected, the higher immunofluorescence present in the apical region of the blastema and around the regenerating spinal cord indicates that the protein could be secreted from the apical wound epidermis and the ependyma and might influence cell proliferation in the blastema, the distal-most growing center of the new tail. The present observations suggest the involvement of the Wnt pathway to direct the process of tail regeneration in lizard. The stimulation of proliferation of epidermal and mesenchymal cells in the apical blastema by Wnt proteins remains to be experimentally validated.

Lizard tail regeneration as an instructive model of enhanced healing capabilities in an adult amniote

The ability to regenerate damaged or lost tissues has remained the lofty goal of regenerative medicine. Unfortunately, humans, like most mammals, suffer from very minimal natural regenerative capabilities. Certain non-mammalian animal species, however, are not so limited in their healing capabilities, and several have attracted the attention of researchers hoping to recreate enhanced healing responses in humans. This review focuses on one such animal group with remarkable regenerative abilities, the lizards. As the closest relatives of mammals that exhibit enhanced regenerative abilities as adults, lizards potentially represent the most relevant model for direct comparison and subsequent improvement of mammalian healing. Lizards are able to regenerate amputated tails and exhibit adaptations that both limit tissue damage in response to injury and initiate coordinated regenerative responses. This review summarizes the salient aspects of lizard tail regeneration as they relate to the overall regenerative process and also presents the relevant information pertaining to regrowth of specific tissues, including skeletal, muscular, nervous, and vascular tissues. The goal of this review is to introduce the topic of lizard tail regeneration to new audiences with the hope of expanding the knowledge base of this underutilized but potentially powerful model organism.

Lizard tail skeletal regeneration combines aspects of fracture healing and blastema-based regeneration

Lizards are amniotes with the remarkable ability to regenerate amputated tails. The early regenerated lizard tail forms a blastema, and the regenerated skeleton consists of a cartilage tube (CT) surrounding the regenerated spinal cord. The proximal, but not distal, CT undergoes hypertrophy and ossifies. We hypothesized that differences in cell sources and signaling account for divergent cartilage development between proximal and distal CT regions. Exogenous spinal cord implants induced ectopic CT formation in lizard (Anolis carolinensis) blastemas. Regenerated spinal cords expressed Shh, and cyclopamine inhibited CT induction. Blastemas containing vertebrae with intact spinal cords formed CTs with proximal hypertrophic regions and distal non-hypertrophic regions, whereas removal of spinal cords resulted in formation of proximal CT areas only. In fate-mapping studies, FITC-labeled vertebra periosteal cells were detected in proximal, but not distal, CT areas. Conversely, FITC-labeled blastema cells were restricted to distal CT regions. Proximal cartilage formation was inhibited by removal of periosteum and could be recapitulated in vitro by periosteal cells treated with Ihh and BMP-2. These findings suggest that proximal CTs are directly derived from vertebra periosteal cells in response to BMP and Ihh signaling, whereas distal CTs form from blastema cells in response to Shh signals from regenerated spinal cords.

Immunocalization of telomerase in cells of lizard tail after amputation suggests cell activation for tail regeneration

Tail amputation (autotomy) in most lizards elicits a remarkable regenerative response leading to a new although simplified tail. No information on the trigger mechanism following wounding is known but cells from the stump initiate to proliferate and form a regenerative blastema. The present study shows that telomerases are mainly activated in the nuclei of various connective and muscle satellite cells of the stump, and in other tissues, probably responding to the wound signals. Western blotting detection also indicates that telomerase positive bands increases in the regenerating blastema in comparison to the normal tail. Light and ultrastructural immunocytochemistry localization of telomerase shows that 4-14 days post-amputation in lizards immunopositive nuclei of sparse cells located among the wounded tissues are accumulating into the forming blastema. These cells mainly include fibroblasts and fat cells of the connective tissue and satellite cells of muscles. Also some immature basophilic and polychromatophilic erytroblasts, lymphoblasts and myelocytes present within the Bone Marrow of the vertebrae show telomerase localization in their nuclei, but their contribution to the formation of the regenerative blastema remains undetermined. The study proposes that one of the initial mechanisms triggering cell proliferation for the formation of the blastema in lizards involve gene activation for the production of telomerase that stimulates the following signaling pathways for cell division and migration.

Molecular footprinting of skeletal tissues in the catshark Scyliorhinus canicula and the clawed frog Xenopus tropicalis identifies conserved and derived features of vertebrate calcification

Understanding the evolutionary emergence and subsequent diversification of the vertebrate skeleton requires a comprehensive view of the diverse skeletal cell types found in distinct developmental contexts, tissues, and species. To date, our knowledge of the molecular nature of the shark calcified extracellular matrix, and its relationships with osteichthyan skeletal tissues, remain scarce. Here, based on specific combinations of expression patterns of the Col1a1, Col1a2, and Col2a1 fibrillar collagen genes, we compare the molecular footprint of endoskeletal elements from the chondrichthyan Scyliorhinus canicula and the tetrapod Xenopus tropicalis. We find that, depending on the anatomical location, Scyliorhinus skeletal calcification is associated to cell types expressing different subsets of fibrillar collagen genes, such as high levels of Col1a1 and Col1a2 in the neural arches, high levels of Col2a1 in the tesserae, or associated to a drastic Col2a1 downregulation in the centrum. We detect low Col2a1 levels in Xenopus osteoblasts, thereby revealing that the osteoblastic expression of this gene was significantly reduced in the tetrapod lineage. Finally, we uncover a striking parallel, from a molecular and histological perspective, between the vertebral cartilage calcification of both species and discuss the evolutionary origin of endochondral ossification.