Effect of apical epidermal cap on mitotic cycle and cartilage differentiation in regeneration blastemata in the newt, Notophthalmus viridescens

Single-cell RNA sequencing of the holothurian regenerating intestine reveals the pluripotency of the coelomic epithelium

In holothurians, the regenerative process following evisceration involves the development of a "rudiment" or "anlage" at the injured end of the mesentery. This regenerating anlage plays a pivotal role in the formation of a new intestine. Despite its significance, our understanding of the molecular characteristics inherent to the constituent cells of this structure has remained limited. To address this gap, we employed state-of-the-art scRNA-seq and HCR-FISH analyses to discern the distinct cellular populations associated with the regeneration anlage. Through this approach, we successfully identified thirteen distinct cell clusters. Among these, two clusters exhibit characteristics consistent with putative mesenchymal cells, while another four show features akin to coelomocyte cell populations. The remaining seven cell clusters collectively form a large group encompassing the coelomic epithelium of the regenerating anlage and mesentery. Within this large group of clusters, we recognized previously documented cell populations such as muscle precursors, neuroepithelial cells and actively proliferating cells. Strikingly, our analysis provides data for identifying at least four other cellular populations that we define as the precursor cells of the growing anlage. Consequently, our findings strengthen the hypothesis that the coelomic epithelium of the anlage is a pluripotent tissue that gives rise to diverse cell types of the regenerating intestinal organ. Moreover, our results provide the initial view into the transcriptomic analysis of cell populations responsible for the amazing regenerative capabilities of echinoderms.

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.

Epimorphic regeneration in the mammalian tympanic membrane

Adult mammals are generally believed to have limited ability to regenerate complex tissues and instead, repair wounds by forming scars. In humans and across mammalian species, the tympanic membrane (TM) rapidly repairs perforations without intervention. Using mouse models, we demonstrate that the TM repairs itself through a process that bears many hallmarks of epimorphic regeneration rather than typical wound healing. Following injury, the TM forms a wound epidermis characterized by EGFR ligand expression and signaling. After the expansion of the wound epidermis that emerges from known stem cell regions of the TM, a multi-lineage blastema-like cellular mass is recruited. After two weeks, the tissue architecture of the TM is largely restored, but with disorganized collagen. In the months that follow, the organized and patterned collagen framework of the TM is restored resulting in scar-free repair. Finally, we demonstrate that deletion of Egfr in the epidermis results in failure to expand the wound epidermis, recruit the blastema-like cells, and regenerate normal TM structure. This work establishes the TM as a model of mammalian complex tissue regeneration.

Deer antler renewal gives insights into mammalian epimorphic regeneration

Deer antlers are the only known mammalian organ that, once lost, can fully grow back naturally. Hence, the antler offers a unique opportunity to learn how nature has solved the problem of mammalian epimorphic regeneration (EpR). Comprehensive comparisons amongst different types of EpR reveal that antler renewal is fundamentally different from that in lower vertebrates such as regeneration of the newt limb. Surprisingly, antler renewal is comparable to wound healing over a stump of regeneration-incompetent digit/limb, bone fracture repair, and to a lesser extent to digit tip regeneration in mammals. Common to all these mammalian cases of reaction to the amputation/mechanical trauma is the response of the periosteal cells at the distal end/injury site with formation of a circumferential cartilaginous callus (CCC). Interestingly, whether the CCC can proceed to the next stage to transform to a blastema fully depends on the presence of an interactive partner. The actual form of the partner can vary in different cases with the nail organ in digit tip EpR, the opposing callus in bone fracture repair, and the closely associated enveloping skin in antler regeneration. Due to absence of such an interactive partner, the CCC of a mouse/rat digit/limb stump becomes involuted gradually. Based on these discoveries, we created an interactive partner for the rat digit/limb stump through surgically removal of the interposing layers of loose connective tissue and muscle between the resultant CCC and the enveloping skin after amputation and by forcefully bonding two tissue types tightly together. In so doing partial regeneration of the limb stump occurred. In summary, if EpR in humans is to be realized, then I envisage that it would be more likely in a manner akin to antler regeneration rather to that of lower vertebrates such as newt limbs.

Nerves and Proliferation of Progenitor Cells in Limb Regeneration

Nerves, in conjunction with the apical epidermal cap (AEC), play an important role in the proliferation of the mesenchymal progenitor cells comprising the blastema of regenerating urodele amphibian limbs. Reinnervation after amputation requires factors supplied by the forming blastema, and neurotrophic factors must be present at or above a quantitative threshold for mitosis of the blastema cells. The AEC forms independently of nerves, but requires nerves to be maintained. Urodele limb buds are independent of nerves for regeneration, but innervation imposes a regenerative requirement for nerve factors on their cells as they differentiate. There are three main ideas on the functional relationship between nerves, AEC, and blastema cells: (1) nerves and AEC produce factors with different roles in maintaining progenitor status and mitosis; (2) the AEC produces the factors that promote blastema cell mitosis, but requires nerves to express them; (3) blastema cells, nerves, and AEC all produce the same factor(s) that additively attain the required threshold for mitosis.

The blastema and epimorphic regeneration in mammals

Studying regeneration in animals where and when it occurs is inherently interesting and a challenging research topic within developmental biology. Historically, vertebrate regeneration has been investigated in animals that display enhanced regenerative abilities and we have learned much from studying organ regeneration in amphibians and fish. From an applied perspective, while regeneration biologists will undoubtedly continue to study poikilothermic animals (i.e., amphibians and fish), studies focused on homeotherms (i.e., mammals and birds) are also necessary to advance regeneration biology. Emerging mammalian models of epimorphic regeneration are poised to help link regenerative biology and regenerative medicine. The regenerating rodent digit tip, which parallels human fingertip regeneration, and the regeneration of large circular defects through the ear pinna in spiny mice and rabbits, provide tractable, experimental systems where complex tissue structures are regrown through blastema formation and morphogenesis. Using these models as examples, we detail similarities and differences between the mammalian blastema and its classical counterpart to arrive at a broad working definition of a vertebrate regeneration blastema. This comparison leads us to conclude that regenerative failure is not related to the availability of regeneration-competent progenitor cells, but is most likely a function of the cellular response to the microenvironment that forms following traumatic injury. Recent studies demonstrating that targeted modification of this microenvironment can restrict or enhance regenerative capabilities in mammals helps provide a roadmap for eventually pushing the limits of human regeneration.

Mechanisms of urodele limb regeneration

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

Regeneration and Regrowth Potentials of Digit Tips in Amphibians and Mammals

Tissue regeneration and repair have received much attention in the medical field over the years. The study of amphibians, such as newts and salamanders, has uncovered many of the processes that occur in these animals during full-limb/digit regeneration, a process that is highly limited in mammals. Understanding these processes in amphibians could shed light on how to develop and improve this process in mammals. Amputation injuries in mammals usually result in the formation of scar tissue with limited regrowth of the limb/digit; however, it has been observed that the very tips of digits (fingers and toes) can partially regrow in humans and mice under certain conditions. This review will summarize and compare the processes involved in salamander limb regeneration, mammalian wound healing, and digit regeneration in mice and humans.

Regenerative Medicine: lessons from Mother Nature

Regenerative medicine strategies for the restoration of functional tissue have evolved from the concept of ex vivo creation of engineered tissue toward the broader concept of in vivo induction of functional tissue reconstruction. Multidisciplinary approaches are being investigated to achieve this goal using evolutionarily conserved principles of stem cell biology, developmental biology and immunology, current methods of engineering and medicine. This evolution from ex vivo tissue engineering to the manipulation of fundamental in vivo tenets of development and regeneration has the potential to capitalize upon the incredibly complex and only partially understood ability of cells to adapt, proliferate, self-organize and differentiate into functional tissue.

Beryllium nitrate inhibits fibroblast migration to disrupt epimorphic regeneration

Epimorphic regeneration proceeds with or without formation of a blastema, as observed for the limb and skin, respectively. Inhibition of epimorphic regeneration provides a means to interrogate the cellular and molecular mechanisms that regulate it. In this study, we show that exposing amputated limbs to beryllium nitrate disrupts blastema formation and causes severe patterning defects in limb regeneration. In contrast, exposing full-thickness skin wounds to beryllium only causes a delay in skin regeneration. By transplanting full-thickness skin from ubiquitous GFP-expressing axolotls to wild-type hosts, we demonstrate that beryllium inhibits fibroblast migration during limb and skin regeneration in vivo Moreover, we show that beryllium also inhibits cell migration in vitro using axolotl and human fibroblasts. Interestingly, beryllium did not act as an immunostimulatory agent as it does in Anurans and mammals, nor did it affect keratinocyte migration, proliferation or re-epithelialization, suggesting that the effect of beryllium is cell type-specific. While we did not detect an increase in cell death during regeneration in response to beryllium, it did disrupt cell proliferation in mesenchymal cells. Taken together, our data show that normal blastema organogenesis cannot occur without timely infiltration of local fibroblasts and highlights the importance of positional information to instruct pattern formation during regeneration. In contrast, non-blastemal-based skin regeneration can occur despite early inhibition of fibroblast migration and cell proliferation.

Signalling by Transforming Growth Factor Beta Isoforms in Wound Healing and Tissue Regeneration

Transforming growth factor beta (TGFβ) signalling is essential for wound healing, including both non-specific scar formation and tissue-specific regeneration. Specific TGFβ isoforms and downstream mediators of canonical and non-canonical signalling play different roles in each of these processes. Here we review the role of TGFβ signalling during tissue repair, with a particular focus on the prototypic isoforms TGFβ1, TGFβ2, and TGFβ3. We begin by introducing TGFβ signalling and then discuss the role of these growth factors and their key downstream signalling mediators in determining the balance between scar formation and tissue regeneration. Next we discuss examples of the pleiotropic roles of TGFβ ligands during cutaneous wound healing and blastema-mediated regeneration, and how inhibition of the canonical signalling pathway (using small molecule inhibitors) blocks regeneration. Finally, we review various TGFβ-targeting therapeutic strategies that hold promise for enhancing tissue repair.

Comparative analysis of ear-hole closure identifies epimorphic regeneration as a discrete trait in mammals

Why mammals have poor regenerative ability has remained a long-standing question in biology. In regenerating vertebrates, injury can induce a process known as epimorphic regeneration to replace damaged structures. Using a 4-mm ear punch assay across multiple mammalian species, here we show that several Acomys spp. (spiny mice) and Oryctolagus cuniculus completely regenerate tissue, whereas other rodents including MRL/MpJ 'healer' mice heal similar injuries by scarring. We demonstrate ear-hole closure is independent of ear size, and closure rate can be modelled with a cubic function. Cellular and genetic analyses reveal that injury induces blastema formation in Acomys cahirinus. Despite cell cycle re-entry in Mus musculus and A. cahirinus, efficient cell cycle progression and proliferation only occurs in spiny mice. Together, our data unite blastema-mediated regeneration in spiny mice with regeneration in other vertebrates such as salamanders, newts and zebrafish, where all healthy adults regenerate in response to injury.

Drug-induced regeneration in adult mice

Whereas amphibians regenerate lost appendages spontaneously, mammals generally form scars over the injury site through the process of wound repair. The MRL mouse strain is an exception among mammals because it shows a spontaneous regenerative healing trait and so can be used to investigate proregenerative interventions in mammals. We report that hypoxia-inducible factor 1α (HIF-1α) is a central molecule in the process of regeneration in adult MRL mice. The degradation of HIF-1α protein, which occurs under normoxic conditions, is mediated by prolyl hydroxylases (PHDs). We used the drug 1,4-dihydrophenonthrolin-4-one-3-carboxylic acid (1,4-DPCA), a PHD inhibitor, to stabilize constitutive expression of HIF-1α protein. A locally injectable hydrogel containing 1,4-DPCA was designed to achieve controlled delivery of the drug over 4 to 10 days. Subcutaneous injection of the 1,4-DPCA/hydrogel into Swiss Webster mice that do not show a regenerative phenotype increased stable expression of HIF-1α protein over 5 days, providing a functional measure of drug release in vivo. Multiple peripheral subcutaneous injections of the 1,4-DPCA/hydrogel over a 10-day period led to regenerative wound healing in Swiss Webster mice after ear hole punch injury. Increased expression of the HIF-1α protein may provide a starting point for future studies on regeneration in mammals.

Hedgehog Signaling during Appendage Development and Regeneration

Regulatory networks that govern embryonic development have been well defined. While a common hypothesis supports the notion that the embryonic regulatory cascades are reexpressed following injury and tissue regeneration, the mechanistic regulatory pathways that mediate the regenerative response in higher organisms remain undefined. Relative to mammals, lower vertebrates, including zebrafish and newts, have a tremendous regenerative capacity to repair and regenerate a number of organs including: appendages, retina, heart, jaw and nervous system. Elucidation of the pathways that govern regeneration in these lower organisms may provide cues that will enhance the capacity for the regeneration of mammalian organs. Signaling pathways, such as the hedgehog pathway, have been shown to play critical functions during development and during regeneration in lower organisms. These signaling pathways have been shown to modulate multiple processes including cellular origin, positional identity and cellular maturation. The present review will focus on the cellular and molecular regulation of the hedgehog (HH) signaling pathway and its interaction with other signaling factors during appendage development and regeneration.

Proteomic analysis of fibroblastema formation in regenerating hind limbs of Xenopus laevis froglets and comparison to axolotl

Background

To gain insight into what differences might restrict the capacity for limb regeneration in Xenopus froglets, we used High Performance Liquid Chromatography (HPLC)/double mass spectrometry to characterize protein expression during fibroblastema formation in the amputated froglet hindlimb, and compared the results to those obtained previously for blastema formation in the axolotl limb.

Results

Comparison of the Xenopus fibroblastema and axolotl blastema revealed several similarities and significant differences in proteomic profiles. The most significant similarity was the strong parallel down regulation of muscle proteins and enzymes involved in carbohydrate metabolism. Regenerating Xenopus limbs differed significantly from axolotl regenerating limbs in several ways: deficiency in the inositol phosphate/diacylglycerol signaling pathway, down regulation of Wnt signaling, up regulation of extracellular matrix (ECM) proteins and proteins involved in chondrocyte differentiation, lack of expression of a key cell cycle protein, ecotropic viral integration site 5 (EVI5), that blocks mitosis in the axolotl, and the expression of several patterning proteins not seen in the axolotl that may dorsalize the fibroblastema.

Conclusions

We have characterized global protein expression during fibroblastema formation after amputation of the Xenopus froglet hindlimb and identified several differences that lead to signaling deficiency, failure to retard mitosis, premature chondrocyte differentiation, and failure of dorsoventral axial asymmetry. These differences point to possible interventions to improve blastema formation and pattern formation in the froglet limb.

Experimentally induced metamorphosis in axolotls reduces regenerative rate and fidelity

While most tetrapods are unable to regenerate severed body parts, amphibians display a remarkable ability to regenerate an array of structures. Frogs can regenerate appendages as larva, but they lose this ability around metamorphosis. In contrast, salamanders regenerate appendages as larva, juveniles, and adults. However, the extent to which fundamental traits (e.g., metamorphosis, body size, aging, etc.) restrict regenerative ability remains contentious. Here we utilize the ability of normally paedomorphic adult axolotls (Ambystoma mexicanum) to undergo induced metamorphosis by thyroxine exposure to test how metamorphosis and body size affects regeneration in age‐matched paedomorphic and metamorphic individuals. We show that body size does not affect regeneration in adult axolotls, but metamorphosis causes a twofold reduction in regeneration rate, and lead to carpal and digit malformations. Furthermore, we find evidence that metamorphic blastemal cells may take longer to traverse the cell cycle and display a lower proliferative rate. This study identifies the axolotl as a powerful system to study how metamorphosis restricts regeneration independently of developmental stage, body size, and age; and more broadly how metamorphosis affects tissue‐specific changes.

Skin shedding and tissue regeneration in African spiny mice (Acomys)

Evolutionary modification has produced a spectrum of animal defense traits to escape predation, including the ability to autotomize body parts to elude capture^1,2. Following autotomy, the missing part is either replaced through regeneration (e.g. urodeles, lizards, arthropods, crustaceans) or is permanently lost (mammals). While most autotomy involves the loss of appendages (e.g. leg, cheliped, antennae, tail), skin autotomy can occur in certain taxa of scincid and gekkonid lizards³. Here we report the first demonstration of skin autotomy in Mammalia (African spiny mice, Acomys). Mechanical testing revealed a propensity for skin to tear under very low tension and the absence of a fracture plane. Following skin loss, rapid wound contraction was followed by hair follicle regeneration in dorsal skin wounds. Surprisingly, we found regenerative capacity in Acomys extended to ear holes where they exhibited complete regeneration of hair follicles, sebaceous glands, dermis, and cartilage. Salamanders capable of limb regeneration form a blastema (a mass of lineage-restricted progenitor cells⁴) following limb loss, and our findings suggest that ear tissue regeneration in Acomys may proceed through assembly of a similar structure. This study underscores the importance of investigating regenerative phenomena outside of traditional model organisms and suggests that mammals may retain a higher capacity for regeneration than previously believed. As re-emergent interest in regenerative medicine seeks to isolate molecular pathways controlling tissue regeneration in mammals, Acomys may prove useful in identifying mechanisms to promote regeneration in lieu of fibrosis and scarring.

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.

The role of peripheral nerves in urodele limb regeneration

Nerve axons and the apical epidermal cap (AEC) are both essential for the formation of an accumulation blastema by amputated limbs of urodele salamanders. The AEC forms in the absence of axons, but is not maintained, and blastema formation fails. Growth stages of the blastema become nerve-independent for morphogenesis, but remain dependent on the nerve for blastema growth. Denervated growth stage blastemas form smaller than normal skeletal parts, owing to diminished mitosis, but form the full proximodistal array of skeletal elements. This difference in nerve dependency of morphogenesis and proliferation is hypothesized to be the result of a dependence of the AEC on nerves for blastema cell proliferation but not for blastema morphogenesis. Regenerating axons induce the synthesis and secretion of the anterior gradient protein (AGP) by distal Schwann cells during dedifferentiation and by the gland cells of the AEC during blastema growth stages. AGP promotes the regeneration of a denervated limb to digit stages when electroporated into the limb during dedifferentiation. Once a critical mass of blastema cells has been attained, the blastema can undergo morphogenesis in the absence of the nerve, but the regenerate will be a miniature, because the nerve is no longer inducing the AEC to carry out its AGP-mediated proliferative function. AGP expression by both Schwann cells and the AEC is induced by axons, but the nature of the inductive agent is unclear.

The relationship between inflammation and regeneration in the MRL mouse: potential relevance for putative human regenerative(scarless wound healing) capacities?

The matrix metalloproteinases (MMPs) have been implicated in the regenerative response in amphibians and various mammalian models of regeneration. The neutrophil response is known to bring MMPs and other proteases to the wound to promote bacterial elimination and tissue remodeling. These issues in relation to what is occurring in the MRL mouse model of regeneration/wound healing are discussed, followed by speculation as to their possible relevance for examples of the putative scarless wound healing described by some medical anthropologists and clinicians.

Timing the commitment to a wound-healing response of denervated limb stumps in the adult newt, Notophthalmus viridescens

Adult newt limbs that are denervated 1 day after amputation undergo a wound-healing response and, although they become reinnervated, will not regenerate unless reinjured. Experiments were designed to determine when denervated limb stumps of adult newts become committed to a wound-healing response. In Experiment I, limbs were amputated and denervated 1 day after amputation. On days 7, 14, 21, 28, and 35 days after the initial amputation and denervation, stumps were reamputated to remove the distal tip. This design varied the time the distal stump was devoid of nerves before reamputation. None of the limbs reamputated at 7 days regenerated. About half of the limbs reamputated at 14 days regenerated and almost all of those reamputated at days 21, 28, and 35 regenerated. In Experiment II, limbs were denervated and then amputated on day 7 or 14, at two different levels. Limbs with a short stump became innervated earlier and regeneration occurred more frequently at both levels of amputation. The results of these experiments show that denervated limb stumps become committed to a wound-healing response between days 7 and 14 after amputation/denervation. If sufficient nerves arrive before day 7, a regeneration response is initiated. If the stump is denervated for 14 days or longer, commitment to wound healing occurs and ingrowing nerves cannot initiate a regeneration response.

Conjecture: Can continuous regeneration lead to immortality? Studies in the MRL mouse

A particular mouse strain, the MRL mouse, has been shown to have unique healing properties that show normal replacement of tissue without scarring. The serendipitous discovery that the MRL mouse has a profound capacity for regeneration in some ways rivaling the classic newt and axolotl species raises the possibility that humans, too, may have an innate regenerative ability. We propose this mouse as a model for continuous regeneration with possible life-extending properties. We will use the classical "immortal" organism, the hydra, for comparison and examine those key phenotypes that contribute to their immortality as they are expressed in the MRL mouse versus control mouse strains. The phenotypes to be examined include the rate of proliferation and the rate of cell death, which leads to a continual turnover in cells without an increase in mass.

Expression profiles of elastase1 (NvElastaseI) and secretory leukocyte protease inhibitor (NvSLPI) during forelimb regeneration in adult Notophthalmus viridescens suggest a role in epithelial remodeling and delamination

Extracellular proteases and their inhibitors may regulate a number of important processes involved in forelimb regeneration in the adult newt, including epithelial remodeling, breakdown of extracellular matrix, and dedifferentiation. We have identified a newt homologue of human ElastaseI (NvElastaseI) and its potential inhibitor, SLPI (NvSLPI), and evaluated their spatial and temporal expression during limb regeneration. NvElastaseI is upregulated early in regeneration and is associated with subdermal and wound epithelial cells, suggesting an involvement in wound healing and the generation of the wound epithelium. Up until 15 days post-amputation, NvElastaseI is also scattered throughout the developing blastema and may have a role in the dedifferentiation of stump tissues. NvSLPI is found at the interface between the intact skin and the wound epithelium, and may limit NvElastaseI activity. NvSLPI is also expressed in dermal glands, and is likely involved in anti-microbial activity or function. Quite apart from regeneration, complementary patterns of expression of NvElastaseI and NvSLPI are associated with newt epithelial sloughing.

The scarless heart and the MRL mouse

The ability to regenerate tissues and limbs in its most robust form is seen in many non-mammalian species. The serendipitous discovery that the MRL mouse has a profound capacity for regeneration in some ways rivalling the classic newt and axolotl species raises the possibility that humans, too, may have an innate regenerative ability. The adult MRL mouse regrows cartilage, skin, hair follicles and myocardium with near perfect fidelity and without scarring. This is seen in the ability to close through-and-through ear holes, which are generally used for lifelong identification of mice, and the anatomic and functional recovery of myocardium after a severe cryo-injury. We present histological, biochemical and genetic data indicating that the enhanced breakdown of scar-like tissue may be an underlying factor in the MRL regenerative response. Studies as to the source of the cells in the regenerating MRL tissue are discussed. Such studies appear to support multiple mechanisms for cell replacement.

Amphibian regeneration and stem cells

Larval and adult urodeles and anuran tadpoles readily regenerate their limbs via a process of histolysis and dedifferentiation of mature cells local to the amputation surface that accumulate under the wound epithelium as a blastema of stem cells. These stem cells require growth and trophic factors from the apical epidermal cap (AEC) and the nerves that re-innervate the blastema for their survival and proliferation. Members of the fibroblast growth factor (FGF) family synthesized by both AEC and nerves, and glial growth factor, substance P, and transferrin of nerves are suspected survival and proliferation factors. Stem cells derived from fibroblasts and muscle cells can transdifferentiate into other cell types during regeneration. The regeneration blastema is a self-organizing system based on positional information inherited from parent limb cells. Retinoids, which act through nuclear receptors, have been used in conjunction with assays for cell adhesivity to show that positional identity of blastema cells is encoded in the cell surface. These molecules are involved in the cell-cell signaling network that re-establishes the original structural pattern of the limb. Other systems of interest that regenerate by histolysis and dedifferentiation of pigmented epithelial cells are the neural retina and lens. Members of the FGF family are also important to the regeneration of these structures. The mechanism of amphibian regeneration by dedifferentiation is of importance to the development of a regenerative medicine, since understanding this mechanism may offer insights into how we might chemically induce the regeneration of mammalian tissues.

Spallanzani's mouse: a model of restoration and regeneration

The ability to regenerate is thought to be a lost phenotype in mammals, though there are certainly sporadic examples of mammalian regeneration. Our laboratory has identified a strain of mouse, the MRL mouse, which has a unique capacity to heal complex tissue in an epimorphic fashion, i.e., to restore a damaged limb or organ to its normal structure and function. Initial studies using through-and-through ear punches showed rapid full closure of the ear holes with cartilage growth, new hair follicles, and normal tissue architecture reminiscent of regeneration seen in amphibians as opposed to the scarring usually seen in mammals. Since the ear hole closure phenotype is a quantitative trait, this has been used to show-through extensive breeding and backcrossing--that the trait is heritable. Such analysis reveals that there is a complex genetic basis for this trait with multiple loci. One of the major phenotypes of the MRL mouse is a potent remodeling response with the absence or a reduced level of scarring. MRL healing is associated with the upregulation of the metalloproteinases MMP-2 and MMP-9 and the downregulation of their inhibitors TIMP-2 and TIMP-3, both present in inflammatory cells such as neutrophils and macrophages. This model has more recently been extended to the heart. In this case, a cryoinjury to the right ventricle leads to near complete scarless healing in the MRL mouse whereas scarring is seen in the control mouse. In the MRL heart, bromodeoxyuridine uptake by cardiomyocytes filling the wound site can be seen 60 days after injury. This does not occur in the control mouse. Function in the MRL heart, as measured by echocardiography, returns to normal.

Intercalary and supernumerary regeneration in the limbs of the frog, Xenopus laevis

Anuran amphibians, such as Xenopus laevis, can regenerate their limbs only when they are young tadpoles, whereas urodele amphibians have a regenerative ability throughout their lives. It is still unclear whether anuran and urodele use the same mechanism during regeneration. In the present study, we analyzed intercalary and supernumerary regeneration in Xenopus. In contrast to urodele blastema that induces intercalary regeneration along the proximodistal (PD) axis, intercalation did not occur in the Xenopus limb bud when the presumptive zeugopodium (fibula and tibia) was removed. However, when the limb bud tip (presumptive autopodium) was transplanted to the presumptive stylopodium (femur) with a 180-degree rotation at stage 52, the complete zeugopodium was regenerated. These results were similar to the results of urodele mature limbs, suggesting that Xenopus limb buds are equivalent to the urodele mature limbs but not to the urodele blastemas. We hypothesized that the ability for intercalation depends on the expression pattern of fibroblast growth factor (fgf)-8, because the expression of fgf-8 in the urodele spreads over the whole blastema and is close enough to activate the growth of the stump. To test this hypothesis, an FGF-8-soaked bead was implanted at the boundary between the stump and tip of a Xenopus limb bud. Intercalary regeneration was induced at stages 52 and 53. These results suggest that the Xenopus limb bud possesses the potential for intercalation, but endogenous FGF-8 in the apical ectodermal ridge (AER) does not induce intercalation to the stump because of the long distance between the AER and stump.

Matrix metalloproteinase activity correlates with blastema formation in the regenerating MRL mouse ear hole model

The MRL mouse was proposed as a model of mammalian regeneration because it can close ear holes completely with the restoration of normal tissue. This regeneration process involves the formation of a blastema during healing, the re-appearance of cartilage and hair follicles, and healing without scarring. Such a process requires extensive tissue remodeling. To characterize differences in ear wounding responses between regenerating and nonregenerating mice, we examined and compared the extracellular matrix remodeling and the matrix metalloproteinase (MMP) and tissue inhibitor of metalloproteinase (TIMP) response in the MRL and C57BL/6 mouse strains after injury. We found a correlation between the MRL's ability to break down the basement membrane, form a blastema, and close ear hole wounds and an inflammatory response with neutrophils and macrophages seen in the ear after injury. These cells were positive for MMP-2 and MMP-9 as well as TIMP-2 and TIMP-3. Clear differences between the MRL and B6 response to injury were seen that could explain the differences in healing and blastema formation in the MRL and lack of it in the B6 mice. This finding was further supported by enzyme activity as determined by gelatin zymography.

Regeneration of the urodele limb: a review

Urodele amphibians have been widely used for studies of limb regeneration. In this article, we review studies on blastema cell proliferation and propose a model of blastemal self-organization and patterning. The model is based on local cell interactions that intercalate positional identities within circumferential and proximodistal boundaries that outline the regenerate. The positional identities created by the intercalation process appear to be reflected in the molecular composition of the cell surface. Transcription factors and signaling molecules involved in patterning are discussed within the context of the boundary/intercalation model.

Unique expression patterns of matrix metalloproteinases in regenerating newt limbs

The process of regeneration of urodele limbs includes a drastic remodeling of extracellular matrices (ECMs) that is induced by matrix metalloproteinases (MMPs) and is thought to be one of the triggers of the regeneration. We studied this remodeling in limbs of Japanese newt, Cynops pyrrhogaster, by using five genes of newt MMPs (nMMPs) as probes: nMMP9, nMMP3/10-a, nMMP3/10-b, and nMMP13 that had been characterized previously, and nMMPe that was newly cloned in the present study. nMMPe was 502 amino acid residues long and showed a low homology to other known vertebrate MMPs. Reverse transcriptase-polymerase chain reactions analysis localized the transcript of nMMPe in the apical epidermal cap (AEC) and the non-blastemal wound epidermis but not in the blastemal mesenchyme or the normal epidermis. Northern blot analysis localized the transcripts of nMMP9, nMMP3/10-a, and nMMP13 in the bone of regenerating limbs, whereas those of nMMP3/10-b in AEC. mRNA in situ hybridization experiments identified the nMMP-expressing cells. nMMP9 gene was strongly expressed in chondrocytes of the cartilage of epiphysis. Of interest, basal cells of AEC, but not those of the normal skin, expressed nMMP3/10-b intensely. Immunohistochemical analysis showed that the nMMP9 proteins synthesized by chondrocytes were secreted and distributed widely in the basement membrane of bone and ECMs of the amputation plane. These nMMPs characterized in the present study might cooperatively work to remodel ECMs of regenerating limbs.

Effects of locomotor training on hindlimb regeneration in the urodele amphibian Pleurodeles waltlii

1. The effects of locomotor training on hindlimb regeneration were studied in the urodele amphibian Pleurodeles waltlii. 2. After amputation of one hindlimb at mid-femur, adult animals were subjected to regular training sessions (1 h daily, 5 days a week, over 8 months) of terrestrial stepping. 3. Eight months post-amputation, trained animals exhibited regenerated limbs of reduced size as compared to animals kept in their aquaria. Histological data showed an abnormal regeneration of both the femur and distal structures (e.g. digit muscles, metatarsi and phalanges) while medial structures (e.g. tibia and fibula) were totally re-formed. The study of the electromyographical activity in regenerated limbs during stepping and that of their reflex responsiveness to electrical stimulation showed that both motor and sensory innervations were functional in the limb stump of trained animals. 4. The regenerative capacity of the abnormal stumps was preserved since following a second amputation a quite normal hindlimb was regenerated in 3 months, provided the re-amputated animals were not trained to terrestrial stepping. 5. The stress due to handling, change in locomotor medium (aquatic vs. terrestrial) and the friction of the wound epidermis with the ground were not involved in the disruption of limb regeneration. 6. The locomotor pattern, the reflex responsiveness and the muscle fibre composition were similar in supernumerary forelimbs grafted on the back and in normal forelimbs. However, the supernumerary forelimbs regenerated normally even in animals subjected to locomotor training while the hindlimb did not. It is concluded that the disrupting effects of locomotor training on limb regeneration were localized to the the limb directly involved in locomotion. 7. The mechanisms underlying abnormal limb regeneration in animals subjected to locomotor training are discussed.

Apical epithelial cap morphology and fibronectin gene expression in regenerating axolotl limbs

Urodele amphibians (salamanders) are unique among adult vertebrates in their ability to regenerate limbs. The regenerated structure is often indistinguishable from the developmentally produced original. Thus, the two processes by which the limb is produced - development and regeneration - are likely to use many conserved biochemical and developmental pathways. Some of these limb features are also likely to be conserved across vertebrate families. The apical ectodermal ridge (AER) of the developing amniote limb and the larger apical epithelial cap (AEC) of the regenerating urodele limb are both found at the limb's distalmost tip and have been suggested to be functionally similar even though their morphology is quite different. Both structures are necessary for limb outgrowth. However, the AEC is uniformly smooth and thickly covers the entire limb-tip, unlike the AER, which is a protruding ridge covering only the dorsoventral boundary. Previous data from our laboratory suggest the multilayered AEC may be subdivided into separate functional compartments. We used hematoxylin and eosin (H+E) staining as well as in situ hybridization to examine the basal layer of the AEC, the layer that lies immediately over the distal limb mesenchyme. In late-stage regenerates, this basal layer expresses fibronectin (FN) message very strongly in a stripe of cells along the dorso-ventral boundary. H+E staining also reveals the unique shape of basal cells in this area. The stripe of cells in the basal AEC also contains the notch/groove structure previously seen in avian and reptilian AERs. In addition, AEC expression of FN message in the cells around the groove correlates with previous amniote AER localization of FN protein inside the groove. The structural and biochemical analyses presented here suggest that there is a specialized ridge-like compartment in the basal AEC in late-stage regenerates. The data also suggest that this compartment may be homologous to the AER of the developing amniote limb. Thus, the external differences between amniote limb development and urodele limb regeneration may be outweighed by internal similarities, which enable both processes to produce morphologically complete limbs. In addition, we propose that this basal layer of the AEC is uniquely responsible for AEC functions in regeneration, such as secreting molecules to promote mesenchymal cell cycling and dictating the direction of limb outgrowth. Finally, we include here a clarification of existing nomenclature to facilitate further discussion of the AEC and its basal layer.

Expression of genes of type I and type II collagen in the formation and development of the blastema of regenerating newt limb

We cloned cDNAs of alpha1(I) and alpha1(II) collagen, and studied their expression profiles in regenerating limbs of newts, Cynops pyrrhogaster. The expression of the alpha1(I) gene was markedly up-regulated at the early bud stage of the blastema. In situ hybridization experiments revealed that the alpha1(I) gene was expressed in not only mesenchymal cells of the blastema, but also the basal cells of the wound epidermis at the wound healing stage when the epidermal basement membrane was absent. This unique expression continued until 21 days (late bud stage), while the basement membrane began to form at 14 days. These results indicate biochemical differences between the wound and normal epidermis, and suggest the direct involvement of the former in the synthesis of blastemal matrices of type I collagen. Actually, immunohistochemistry revealed that type I collagen began to be deposited beneath the wound epidermis at 8 days, and accumulated there and around blastemal mesenchymal cells at 14 to 21 days. Undifferentiated mesenchymal cells associated with the amputated muscle fibers actively expressed the alpha1(I) gene. Mesenchymal cells in the central region of blastemas deposited type I collagen fibers around them. Concomitantly with the appearance of prechondrocytes, the alpha1(II) collagen gene became activated. The present study clearly shows that the expression of the genes of both type I and type II collagen in blastemal cells is temporally and regionally well-regulated in a cooperative manner. Dev Dyn 1999;216:59-71.

Perspective: a suggested role for basement membrane structures during newt limb regeneration

BACKGROUND

Interactions between epithelium and mesenchyme, which occur across a basement membrane (BM) zone, are essential to generate a growth bud, or blastema, from which a new limb regenerates. An intact BM at that interface is believed to inhibit regeneration, but that mechanism of inhibition is not understood.

METHODS

Interference contrast microscopy and antibodies to laminin have been used to describe reformation of the BM and the basal lamina (BL) and their relationships to wound epithelium and mesenchyme in successive stages of blastema formation.

RESULTS

The BL is initially absent from the amputation surface and is reestablished to continuity by the late bud stage of regeneration. It forms generally from base to apex, precedes reticular lamina (RL) formation, and is absent beneath most of the wound epithelium. Our inability to correlate mesenchymal cell accumulation exclusively with the area lacking BL apically and postaxially prompted rethinking of the significance of the BL.

CONCLUSIONS

Consistent with these and other observations, we suggest that the BL, when it forms during blastema formation, appears to function as in other developing systems to stabilize the phenotype of adjacent cells. Thus, epithelium becomes epidermis and adjacent mesenchyme synthesizes RL and becomes dermis. Accordingly, the feature that distinguishes regenerating from nonregenerating appendages is the ability of regenerating appendages to delay BL closure until after a critical mass of mesenchymal cells has accumulated.

Re-programming of expression of the KGFR and bek variants of fibroblast growth factor receptor 2 during limb regeneration in newts (Notophthalmus viridescens)

We have previously shown, by in situ hybridization, that fibroblast growth factor receptor 2 (FGFR2) is present in the basal layer of wound epithelium during limb regeneration in newts (Notophthalmus viridescens). In contrast, FGFR1 expression is observed throughout the blastema mesenchyme but is distinctly absent from the wound epithelium (Poulin et al. [1993] Development 119:353-361). Sequence analysis revealed that we have isolated both the KGFR and bek variants of FGFR2. These two variants differ only in the second half of the last of their three (or two) Ig-like domains. In this report, we show the expression patterns of FGFR2 variants during limb regeneration by in situ hybridization. During the pre-blastema stages of regeneration, FGFR2 expression was observed in the basal layer of the wound epithelium and in the cells of the periosteum. The wound epithelial hybridization was observed when the KGFR-specific probe was used while the bek-specific probe hybridized to mRNA in the cells of the periosteum. As regeneration progresses to the blastema stages, KGFR expression continued to be observed in the basal layer of the wound epithelium with additional hybridization seen in the blastema mesenchyme closely associated with the bisected bones. The bek-specific hybridization pattern observed at this stage corresponds specifically to the mesenchymal hybridization. In the differentiation stages of regeneration, the mesenchymal expression of FGFR2 becomes restricted to the cells of the condensing cartilage and later to the perichondrium. Interestingly, there appears to be a dorsoventral gradient of the expression of both KGFR and bek variants of FGFR2, which are opposite each other at the later stages of regeneration. Thus, re-programming of expression of the two FGFR2 variants is required during the initial wound closure of limb regeneration. Remarkably, the expression patterns of KGFR and bek mimic those observed in the mouse limb bud during early embryonic development (Orr-Urtreger et al. [1993] Dev. Biol. 18:475-486). Moreover, our results suggest that the two FGFR2 variants have distinct roles in limb regeneration. Further investigation regarding the potential sources of the FGF ligands will help establish the roles that FGFs and FGFRs play in limb regeneration.

Insulin-like growth factor I and II and insulin-like growth factor binding protein-2 RNAs are expressed in adjacent tissues within rat embryonic and fetal limbs

Since the rapid proliferation of cells in a directed manner is a necessary component of limb formation, the distribution of locally produced mitogenic molecules within the developing limb is of considerable interest. We have used in situ hybridization to localize transcripts for both insulin-like growth factor binding protein-2 (IGFBP-2) and its ligands, the insulin-like growth factors I and II (IGF-I and IGF-II), within limb buds of rat embryos 10-16 days after conception (equivalent to stages 1-12 of mouse limb morphogenesis, Wanek et al, 1989. J. Exp. Zool. 249, 41-49). The mRNA for IGFBP-2 is very abundant in an anterior-posterior strip of ectoderm along the distal edge of the limb bud (the progenitor of the apical ectodermal ridge or AER) from as early as limb stage 1 (Embryonic Day 10) and is much less abundant in the rest of the limb ectoderm. A high level of IGFBP-2 expression continues to characterize the AER following its definitive appearance (stage 3) and throughout its existence (until stage 7). This is a period of rapid outgrowth during which the rate of mesodermal cell division is highest in cells nearest to the AER. The AER is known to have mitogenic activity in vitro and to direct limb outgrowth in vivo, but, until recently, few putative molecular correlates of these activities have been detected. The transcripts for IGF-I and IGF-II are also present at high abundance in developing limbs, especially in mesodermally derived cells. IGF-I mRNA is abundant in presumptive limb mesoderm from the beginning of limb outgrowth (just before stage 1), but is very low or undetectable in much of the rest of the embryo, while IGF-II mRNA becomes very abundant in limb mesoderm at stage 2. The distribution in limbs of both IGF-I and IGF-II mRNA changes dramatically during outgrowth and differentiation, so that their expression characterizes complementary populations of cells by stage 11. Taken together, these data suggest that IGFs and the IGF binding proteins, which may modulate IGF action, contribute to limb outgrowth and patterning.

Nerve-independent DNA synthesis and mitosis in regenerating hindlimbs of larval Xenopus laevis

Xenopus laevis larvae at stage 52-53 (according to Nieuwkoop and Faber 1956) were subjected to amputation of both limbs at the thigh level as well as to repeated denervations of the right limb. Results obtained in larvae sacrificed during wound healing (1 after amputation), blastema formation (3 days) and blastema growth (5 and 7 days) showed that denervated right limbs have undergone the same histological modifications observed in innervated left limbs and have formed a regeneration blastema consisting of mesenchymal cells with a pattern of DNA synthesis and mitosis very similar to that in presence of nerves. Also, the patterns of cellular density in regenerating right and left limbs were very similar. On the whole, the data here reported show a highly remarkable degree of nerve-independence for regeneration in hindlimbs of larval Xenopus laevis at stage 52-53 and lend some substance to the hypothesis that, in early limbs, there would exist trophic factors capable of replacing those released by nerves, promoting DNA synthesis and mitosis in blastemal cells.

Acidic fibroblast growth factor is present in regenerating limb blastemas of axolotls and binds specifically to blastema tissues

The growth of regenerating limbs of amphibians depends upon proliferation of the blastema cells that accumulate beneath the epidermal cap. The epidermal cap is known to be mitogenic for the blastema cells. We have extracted a mitogenic activity from both the mesenchymal and epidermal (epidermal cap) components of cone stage blastemas which is retained on heparin-Sepharose and elutes with 1.15 M NaCl. This fraction stimulates neurite outgrowth of PC12 cells and [3H]thymidine incorporation into CCL 39 cells and is potentiated by heparin. The 2 M fraction was inactive. The heparin-Sepharose-purified growth factor cross-reacts with bovine acidic FGF polyclonal antibodies and shows a Mr of 16,000 on Western blots. Blastema membranes contain specific high affinity binding sites (Kd = 25 pM; capacity = 30 fmole/mg protein) and low affinity binding sites (Kd = 18 nM; capacity = 30 pmole/mg protein) for aFGF as revealed by Scatchard analysis. 125I-aFGF which is bound specifically by both the epidermal cap and mesenchyme of blastema frozen sections is displaced by an excess of unlabeled factor and inhibited by heparin. Heparinase treatment and 2 M NaCl washing which decreased the binding was fourfold more efficient for epidermal cap than for mesenchyme suggesting the presence of high affinity receptors in the latter tissue. The presence of aFGF (or a closely related molecule) in blastemas is consistent with our earlier results that showed stimulation of proliferation of cultured blastema cells by acidic or basic FGF or heparin alone. These results suggest the possibility that aFGF is stored in the epidermal cap during limb regeneration and that it stimulates the proliferation of the underlaying mesenchyme.

Expression of the 9G1 antigen in the apical cap of axolotl regenerates requires nerves and mesenchyme

Monoclonal antibody 9G1 (mAb 9G1) is reactive to the wound epithelium of axolotl larvae and therefore provided the opportunity to examine the interaction between the wound epithelium, nerves, and blastemal mesenchyme during axolotl limb regeneration. In unamputated limbs, mAb 9G1 is reactive to most or all cells of the dermis, skeletal elements, blood vessels, and nerves, to a few unidentified cells in muscle, and to none in epidermis. During regeneration of axolotl limbs, mAb 9G1 reacts strongly to an intracellular antigen of the blastemal mesenchyme and of the distal-most portion of the wound epithelium, the so-called apical epithelial cap (AEC). Because this thickened wound epithelium of regenerating amphibian limbs has been suggested as functioning in a manner similar to the apical ectodermal ridge (AER) of embryonic limb buds, it was of interest to further examine the reactivity of mAb 9G1 during various stages of regeneration. Whether mAb 9G1 reactivity in the AEC depended on mesenchyme and/or nerves was also tested. Monoclonal antibody 9G1 reactivity appears in the AEC of regenerating limbs prior to outgrowth of the blastema and persists throughout blastemal stages. Apical epithelial cap reactivity to mAb 9G1 is nerve dependent during early stages of blastema development and becomes nerve-independent at later stages. When epithelium-free blastemal mesenchyme is grafted onto injured flank musculature, ectopic limb regeneration occurs and the AEC derived from flank epidermis exhibits mAb 9G1 reactivity. These results show that a mAb 9G1 reactive AEC is characteristic of regenerating limbs and that expression of the 9G1 antigen by the AEC is dependent upon underlying blastemal mesenchyme and nerves.

In vitro control of blastema cell proliferation by extracts from epidermal cap and mesenchyme of regenerating limbs of axolotls

The presence of a mitogenic activity in limb blastemas of axolotls was detected in crude extracts of blastemas at the mid-bud stage. The mitogenicity of the extracts was estimated from the mitotic index of blastema cells grown for 6 days in the presence of limb blastema extracts, with colchicine present for the last 2 days. All the extracts tested (whole blastema, blastemal mesenchyme, epidermal cap) significantly enhanced proliferation of blastema cells. The highest stimulation factors we observed were 7 × with 7 μg protein/ml whole blastema extracts, 5.2 × with 14 μg/ml blastemal mesenchyme extracts, and 11 x with 3.5 μg/ml epidermal cap extracts. Hence the epidermal cap extracts appeared to be the most mitogenic. Extracts from the blastemal mesenchyme, although less mitogenic than the epidermal cap extracts, were more potent than nerve extracts [Albert P, Boilly B (1986) Biol Cell 58:251-262]. These results are discussed with regard to the production of growth factors during limb regeneration.

A search for immunoreactive substance P and other neural peptides in the limb regenerate of the newt Notophthalmus viridescens

Immunochemical studies demonstrate that the undecapeptide substance P (SP) may be detected by radioimmunoassay in newt limb regenerates and that SP is localized in the blastemal epidermis by immunofluorescence and peroxidase-antiperoxidase staining. Immunoreactive SP is predominantly distributed at the periphery of epidermal cells, suggesting the presence of SP binding sites on the cell surface; the basal germinative layer of the epidermis and blastemal mesenchyme cells remain unreactive. The pattern of SP immunoreactivity in the blastema was compared with that of four other tachykinin-family peptides (eledoisin, kassinin, substance K, and neuromedin K) and with three non-tachykinin neural peptides (bombesin, neurotensin, and metenkephalin). With the exception of neurotensin, which showed weak staining in the basal layer but an absence in the peripheral layers of the epidermis, none of the peptides examined exhibited immunoreactivity in the blastema epidermis comparable to that of SP.

Control of blastema cell proliferation by possible interplay of calcium and cyclic nucleotides during newt limb regeneration

The effects of the divalent cation ionophore A23187, papaverine, and chlorpromazine on the mitotic index and cyclic nucleotide levels in newt limb regeneration blastemata (Notophthalmus viridescens) were assessed. The results of the experiments suggest that an intracellular increase in divalent cation (Ca2+) concentration results in elevated cGMP levels, suppressed cAMP levels, and a corresponding increase in blastema cell proliferation. The results also suggest that the converse conditions, namely, calcium efflux or inhibition of calmodulin activation (i.e., inhibition of Ca2+ binding), yields elevated cAMP levels, suppressed cGMP levels, and a corresponding decrease in blastema cell divisions.

Association of mesenchyme with attenuated basement membranes during morphogenetic stages of newt limb regeneration

The interface between epithelium and mesenchyme may be involved in inductive interactions which occur during development. This interface within the growth bud, or blastema, of a regenerating limb has been examined to determine whether changes in basement-membrane structures are visible in regions of putative epithelial-mesenchymal inductive interaction. Regenerating forelimbs of adult newts were fixed by perfusion with osmotically balanced aldehydes. Late-bulb to early-digit stage regenerates were collected and processed either for light and transmission electron microscopy or for scanning electron microscopy. Light microscopy confirmed that regions characterized by increased numbers of subepithelial mesenchymal cells were covered by a diffusely stained basement membrane. Transmission electron microscopy of these regions revealed two structural components of the basement membrane. The thin basal lamina was continuous in all regions of all stages examined, but it was attenuated apically in areas of mesenchymal cell accumulation. The thicker underlying reticular lamina was markedly attenuated in these regions near the blastemal apex. Scanning electron microscopy of de-epithelialized blastemas revealed that, apically, the reticular lamina formed only a delicate lacelike network. On the base of the blastema, it formed a dense fibrillar meshwork which was further organized into a geometric pattern in the adjacent stump skin. Cumulatively, these observations suggest that physical contact between epithelial and mesenchymal cells is not essential at these stages, but that regions of putative epithelial-mesenchymal interaction are characterized by a distinctly diminished reticular lamina. Structural changes in basement-membrane components may be related to termination of local inductive events.

Regenerate epithelium and skin glands of the adult newt react to the same monoclonal antibody

A search for specific proteins involved in newt limb regeneration, using monoclonal antibodies against forelimb blastemas, led to the detection of an antigen in the regenerate epithelium. Fluorescent-antibody-labeled cells first appeared just prior to blastema outgrowth. From bud through early digit stages this antibody reacted with nearly all of the regenerate epithelial cells. Other tissues also reacted, including nerve, blood vessels, and gastrointestinal tract. The behavior of the reactive cells in the regenerate epithelium, and their close association with immediately adjacent skin glands, raises several new possibilities for the origin of the regenerate epithelium.

Effect of insulin on cyclic nucleotide levels and promotion of mitosis by insulin and ionophore A23187 in cultured newt blastemata

The levels of cyclic GMP (cGMP) and cyclic AMP (cAMP) were assayed, using radioimmunoassay, in newt blastemata cultured with and without insulin. Our observations show that insulin significantly increases the levels of cGMP over the control values, whereas the levels of cAMP remain unaltered. Our in vitro studies also show that Ca2+-carrying ionophore A23187, albeit capable of promoting blastema cell proliferation, is unable to replace the insulin effect. The possible role of cGMP and Ca2+ as mediators of insulin action in regeneration is discussed.

Pattern-deficient forelimb regeneration in adult bullfrogs

This study was designed to test the ability of adult bullfrogs (Rana catesbeiana) to regenerate forelimbs, both with and without various experimental treatments. Distal humerus-level forelimb amputations provided with additional deviated (sciatic) nerve and/or repeated soft-tissue injury exhibited considerable outgrowth. However, control sham-operated forelimbs also produced regenerates with comparable frequency, size, and morphological complexity. The lengths of the regenerates ranged from 0.4 to 2.6 cm, representing an outgrowth of 10-65% of the portion removed by the distal humerus amputation plane; some regenerates exhibited an external morphology indicative of digitlike structures. Some outgrowths were flexible but only one was capable of independent movement. Victoria Blue staining of whole regenerates revealed a variety of internal cartilage elements. Staining showed a single solid mass of cartilage in some regenerates while others had several individual and variably shaped cartilages projecting distally. Histological analysis also revealed the presence of connective tissue, striated muscle, and abundant nerve fibers in addition to the individual cartilage elements. We have tentatively termed these responses pattern-deficient regeneration.