Heterogeneity in the expression of fibroblast growth factor receptors during limb regeneration in newts (Notophthalmus viridescens)

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.

New insight into functional limb regeneration: A to Z approaches

Limb/digit amputation is a common event in humans caused by trauma, medical illness, or surgery. Although the loss of a digit is not lethal, it affects quality of life and imposes high costs on amputees. In recent years, the increasing interest in limb regeneration has led to enhanced scientific knowledge. However, the limited ability to develop functional limb regeneration in the clinical setting suggests that a challenging issue remains in limb regeneration. Recently, the emergence of regenerative engineering is a promising field to address this challenge and close the gap between science and clinical applications. Cell signalling and molecular mechanisms involved in the limb regeneration process have been extensively studied; however, there is still insufficient data on cell therapy and tissue engineering for limb regeneration. In this review, we intend to focus on therapeutic approaches for limb regeneration that are closely related to gene, immune, and stem cell therapies, as well as tissue engineering approaches that take into consideration the peculiar developmental properties of the limbs. In addition, we attempt to identify the challenges of these strategies for limb regeneration studies in terms of clinical settings and as a road map to accomplish the goal of functional human limb regeneration.

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.

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?

Immunolocalization of FGF8/10 in the Apical Epidermal Peg and Blastema of the regenerating tail in lizard marks this apical growing area

Previous studies have shown that Fibroblast Growth Factors are present in the regenerating tail tissues of lizards where they may stimulate the process of regeneration. The present study is focused on the immunolocalization of FGF8 and FGF10 in the regenerating lizard tail, two signaling proteins of the apical epidermal cup/ridge and mesenchymal blastema sustaining tail and limb regeneration in amphibians and the development of the tail and limbs in vertebrate embryos. Main immunoreactive protein bands at 15-18kDa for FGF8/10 are detected in the regenerating epidermis and only a band at 30 or 35kDa in the underlying connective tissues. FGF8 appears particularly localized in cells and nuclei of the apical epidermal peg and of the ependymal ampulla present at the tip of the regenerating tail. FGF10 is also immuno-localized in the apical epidermis but is particularly intensely localized in the mesenchyme of the apical blastema. In accordance with previous studies, the present observations supports the hypothesis that the apical epidermal peg and the ependymal tube with the few regenerated neurons present within it, release FGF8/10 that may contribute to maintenance of cell proliferation in the apical front of the mesenchyme for the growth of the regenerating tail.

Immunolocalization of FGF7 (KGF) in the regenerating tail of lizard suggests it is involved in the differentiation of the epidermis

Previous studies showed that Fibroblast Growth Factors (FGF) 1 and 2 are localized in the tissues of the regenerating tail in lizards. In the present immunofluorescence and immunoblotting study we have specifically analyzed the presence and distribution of FGF7 (keratinocyte growth factor) in the regenerating tissues of the tail. FGF7 immunoreactivity is mainly detected in the regenerating epidermis and in sparse fibroblasts of the underlying dermis of the regenerating scales while it is weaker in the other tissues such as the apical ependymal cells and early regenerating muscles. Immunolabeled mesenchymal fibroblasts are frequently present under the epidermis of the forming outer scale surface, a localization that might be connected to beta-cell differentiation in this region of the scale. FGF7 immunolabeling is also seen in differentiating beta-keratinocytes of the beta-layer in the regenerating scales. The present immunofluorescent observations suggest that FGF7 is specifically utilized as a paracrine factor during the process of differentiation of the epidermal layers in the regenerating scales and in particular for beta-cells differentiation.

Growth and differentiation of a long bone in limb development, repair and regeneration

Repair from traumatic bone fracture is a complex process that includes mechanisms of bone development and bone homeostasis. Thus, elucidation of the cellular/molecular basis of bone formation in skeletal development would provide valuable information on fracture repair and would lead to successful skeletal regeneration after limb amputation, which never occurs in mammals. Elucidation of the basis of epimorphic limb regeneration in amphibians would also provide insights into skeletal regeneration in mammals, since the epimorphic regeneration enables an amputated limb to re-develop the three-dimensional structure of bones. In the processes of bone development, repair and regeneration, growth of the bone is achieved through several events including not only cell proliferation but also aggregation of mesenchymal cells, enlargement of cells, deposition and accumulation of extracellular matrix, and bone remodeling.

Developmental coupling of larval and adult stages in a complex life cycle: insights from limb regeneration in the flour beetle, Tribolium castaneum

Background

A complex life cycle, such as complete metamorphosis, is a key innovation that can promote diversification of species. The evolution of a morphologically distinct larval stage is thought to have enabled insects to occupy broader ecological niches and become the most diverse metazoan taxon, yet the extent to which larval and adult morphologies can evolve independently remains unknown. Perturbation of larval limb regeneration allows us to generate larval legs and antennae with altered limb morphologies, which may be used to explore the developmental continuity that might exist between larval and adult appendages. In this study, we determined the roles of several appendage patterning transcription factors, abrupt (ab), dachshund (dac), Distal-less (Dll), and spineless (ss), in the red flour beetle, Tribolium castaneum, during larval appendage regeneration. The functions of these genes in regenerating and non-regenerating limbs were compared using RNA interference.

Results

During limb regeneration, dac and ss were necessary to re-pattern the same larval structures as those patterned during embryogenesis. Removal of these two genes led to larval appendage patterning defects that were carried over to the adult legs. Surprisingly, even though maternal knockdown of ab had minimal effects on limb allocation and patterning in the embryo, it was necessary for blastema growth, an earlier phase of regeneration. Finally, knockdown of Dll prevented the blastema-like bumps from re-differentiating into appendages.

Conclusions

Our results suggest that, similar to vertebrates, the re-patterning phase of Tribolium larval limb regeneration relies on the same genes that are used during embryonic limb patterning. Thus, the re-patterning phase of regeneration is likely to be regulated by taxon-specific patterning mechanisms. Furthermore, Ab and Dll appear to play important roles during blastema proliferation and re-differentiation, respectively. Finally, our results show that continuity exists between larval and adult limb patterning, and that larval and adult leg morphologies may be developmentally coupled. Thus, the evolution of imaginal discs may have been a key step towards completely removing any developmental constraints that existed between larval and adult phenotypes.

De novo transcriptome sequencing of axolotl blastema for identification of differentially expressed genes during limb regeneration

Background

Salamanders are unique among vertebrates in their ability to completely regenerate amputated limbs through the mediation of blastema cells located at the stump ends. This regeneration is nerve-dependent because blastema formation and regeneration does not occur after limb denervation. To obtain the genomic information of blastema tissues, de novo transcriptomes from both blastema tissues and denervated stump ends of Ambystoma mexicanum (axolotls) 14 days post-amputation were sequenced and compared using Solexa DNA sequencing.

Results

The sequencing done for this study produced 40,688,892 reads that were assembled into 307,345 transcribed sequences. The N50 of transcribed sequence length was 562 bases. A similarity search with known proteins identified 39,200 different genes to be expressed during limb regeneration with a cut-off E-value exceeding 10^-5. We annotated assembled sequences by using gene descriptions, gene ontology, and clusters of orthologous group terms. Targeted searches using these annotations showed that the majority of the genes were in the categories of essential metabolic pathways, transcription factors and conserved signaling pathways, and novel candidate genes for regenerative processes. We discovered and confirmed numerous sequences of the candidate genes by using quantitative polymerase chain reaction and in situ hybridization.

Conclusion

The results of this study demonstrate that de novo transcriptome sequencing allows gene expression analysis in a species lacking genome information and provides the most comprehensive mRNA sequence resources for axolotls. The characterization of the axolotl transcriptome can help elucidate the molecular mechanisms underlying blastema formation during limb regeneration.

Nerve independent limb induction in axolotls

Urodele amphibians can regenerate their limbs. During limb regeneration, dermal fibroblasts are transformed into undifferentiated cells called blastema cells. These dermis-blastema cells show multipotency. Such so-called endogenous reprogramming of cell differentiation is one of the main targets of amphibian limb regeneration studies. It is well recognized that nerve presence controls the initiation of limb regeneration. Accordingly, nerve factors have been sought in amphibian limb regeneration. To investigate it, a relatively new study system called the accessory limb model (ALM) was developed. Using ALM, two signaling cascades (Fgf and Gdf5 signaling) came under focus. In the present study, Growth and differentiation factor-5 (Gdf5) application to wounded skin initiated limb regeneration responses and resulted in induction of a blastema-like structure in the absence of a nerve. However, the Gdf5-induced structure showed defects as a regeneration blastema, such as absence of detectable Prrx1 expression by in situ hybridization. The defects could be remedied by additional Fibroblasts growth factor (Fgf) inputs. These two inputs (Gdf5 and Fgfs) were sufficient to substitute for the nerve functions in the induction of limb regeneration. Indeed, Fgf2, Fgf8, and Gdf5 applications with the contralateral skin graft resulted in limb formation without nerve supply. Furthermore, acquisition of cartilage differentiation potential of dermal fibroblasts was tested in an in vivo and in vitro combination assay. Dermal fibroblasts cultured with Gdf5 were difficult to participate in cartilage formation when the cultured cells were grafted into cartilage forming region. In contrast, dermal fibroblasts cultured with Fgf2 and Fgf8 became easier to participate into cartilage formation in the same procedure. These results contribute to our understanding of molecular mechanisms of the early phase of amphibian limb regeneration.

Multi-tissue microarray analysis identifies a molecular signature of regeneration

The inability to functionally repair tissues that are lost as a consequence of disease or injury remains a significant challenge for regenerative medicine. The molecular and cellular processes involved in complete restoration of tissue architecture and function are expected to be complex and remain largely unknown. Unlike humans, certain salamanders can completely regenerate injured tissues and lost appendages without scar formation. A parsimonious hypothesis would predict that all of these regenerative activities are regulated, at least in part, by a common set of genes. To test this hypothesis and identify genes that might control conserved regenerative processes, we performed a comprehensive microarray analysis of the early regenerative response in five regeneration-competent tissues from the newt Notophthalmus viridescens. Consistent with this hypothesis, we established a molecular signature for regeneration that consists of common genes or gene family members that exhibit dynamic differential regulation during regeneration in multiple tissue types. These genes include members of the matrix metalloproteinase family and its regulators, extracellular matrix components, genes involved in controlling cytoskeleton dynamics, and a variety of immune response factors. Gene Ontology term enrichment analysis validated and supported their functional activities in conserved regenerative processes. Surprisingly, dendrogram clustering and RadViz classification also revealed that each regenerative tissue had its own unique temporal expression profile, pointing to an inherent tissue-specific regenerative gene program. These new findings demand a reconsideration of how we conceptualize regenerative processes and how we devise new strategies for regenerative medicine.

Matrix metalloproteinases (MMPs) are required for wound closure and healing during larval leg regeneration in the flour beetle, Tribolium castaneum

Regenerative abilities are found ubiquitously among many metazoan taxa. To compare mechanisms underlying the initial stages of limb regeneration between insects and vertebrates, the roles of matrix metalloproteinases (MMPs) and fibroblast growth factor (FGF) signaling were investigated in the red flour beetle, Tribolium castaneum. RNA interference-mediated knockdown of MMP2 expression delayed wound healing and subsequent leg regeneration. Additionally, pairwise knockdown of MMP1/2 and MMP2/3, but not MMP1/3, resulted in inhibition of wound closure. Wound healing on the dorsal epidermis after injury was also delayed when MMPs were silenced. Our findings show that functionally redundant MMPs play key roles during limb regeneration and wound healing in Tribolium. This MMP-mediated wound healing is necessary for the subsequent formation of a blastema. In contrast, silencing of FGF receptor did not interfere with the initial stages of leg regeneration despite the alterations in tanning of the cuticle. Thus, insects and vertebrates appear to employ similar developmental processes for the initial stages of wound closure during limb regeneration, while the role of FGF in limb regeneration appears to be unique to vertebrates.

Regeneration: the ultimate example of wound healing

The outcome of wound repair in mammals is often characterized by fibrotic scaring. Vertebrates such as zebrafish, frogs, and salamanders not only heal scarlessly, but also can regenerate lost appendages. Decades of study on the process of animal regeneration has produced key insights into the mechanisms of how complex tissue is restored. By examining our current knowledge of regeneration, we can draw parallels with mammalian wound healing to identify the molecular determinants that produce such differing outcomes.

Spatiotemporal regulation of keratin 5 and 17 in the axolotl limb

BACKGROUND

Amphibians have greater regeneration capability than higher vertebrates. They can regenerate their limbs after an amputation. As a limb is regenerated, a regeneration-specific epithelium called the apical epithelial cap (AEC) is induced. The AEC is an essential structure for limb regeneration. Despite the importance of the AEC, molecular marker genes have not been well studied at the molecular level.

RESULTS

In the present study, keratin5 (KRT5) and KRT17 were investigated in an axolotl-regenerating limb. KRT5 and KRT17 were expressed in a regenerating limb but down-regulated in a differentiating limb. KRT5 showed characteristic regulation in a regenerating blastema. KRT5 was suppressed in the basal layer of the AEC. This KRT5 suppression was correlated to the blastema differentiation and nerve presence. Simple skin wounding could also upregulate both KRT5 and KRT17 gene expression. But these genes were suppressed within a shorter time than in limb regeneration.

CONCLUSIONS

The KRT5 and KRT17 gene profile can be a useful marker gene to investigate AEC in limb regeneration.

FGF signaling regulates rod photoreceptor cell maintenance and regeneration in zebrafish

Fgf signaling is required for many biological processes involving the regulation of cell proliferation and maintenance, including embryonic patterning, tissue homeostasis, wound healing, and cancer progression. Although the function of Fgf signaling is suggested in several different regeneration models, including appendage regeneration in amphibians and fin and heart regeneration in zebrafish, it has not yet been studied during zebrafish photoreceptor cell regeneration. Here we demonstrate that intravitreal injections of FGF-2 induced rod precursor cell proliferation and photoreceptor cell neuroprotection during intense light damage. Using the dominant-negative Tg(hsp70:dn-fgfr1) transgenic line, we found that Fgf signaling was required for homeostasis of rod, but not cone, photoreceptors. Even though fgfr1 is expressed in both rod and cone photoreceptors, we found that Fgf signaling differentially affected the regeneration of cone and rod photoreceptors in the light-damaged retina, with the dominant-negative hsp70:dn-fgfr1 transgene significantly repressing rod photoreceptor regeneration without affecting cone photoreceptors. These data suggest that rod photoreceptor homeostasis and regeneration is Fgf-dependent and that rod and cone photoreceptors in adult zebrafish are regulated by different signaling pathways.

Blastema induction in aneurogenic state and Prrx-1 regulation by MMPs and FGFs in Ambystoma mexicanum limb regeneration

Urodele amphibians can regenerate amputated limbs. It has been considered that differentiated dermal tissues generate multipotent and undifferentiated cells called blastema cells during limb regeneration. In early phases of limb regeneration, blastema cells are induced by nerves and the apical epithelial cap (AEC). We had previously investigated the role of neurotrophic factors in blastema or blastema-like formation consisting of Prrx-1 positive cells. A new system suitable for investigating early phases of limb regeneration, called the accessory limb model (ALM), was recently developed. In this study, we performed a comparative transcriptome analysis between a blastema and wound using ALM. Matrix metalloproteinase (MMP) and fibroblast growth factor (FGF) signaling components were observed to be predominantly expressed in ALM blastema cells. Furthermore, we found that MMP activity induced a blastema marker gene, Prrx-1, in vitro, and FGF signaling pathways worked in coordination to maintain Prrx-1 expression and ALM blastema formation. Furthermore, we demonstrated that these two activities were sufficient to induce an ALM blastema in the absence of a nerve in vivo.

Fgfr1 signalling in the development of a sexually selected trait in vertebrates, the sword of swordtail fish

Background

One of Darwin's chosen examples for his idea of sexual selection through female choice was the "sword", a colourful extension of the caudal fin of male swordtails of the genus Xiphophorus. Platyfish, also members of the genus Xiphophorus, are thought to have arisen from within the swordtails, but have secondarily lost the ability to develop a sword. The sustained increase of testosterone during sexual maturation initiates sword development in male swordtails. Addition of testosterone also induces sword-like fin extensions in some platyfish species, suggesting that the genetic interactions required for sword development may be dormant, rather than lost, within platyfish. Despite considerable interest in the evolution of the sword from a behavioural or evolutionary point of view, little is known about the developmental changes that resulted in the gain and secondary loss of the sword. Up-regulation of msxC had been shown to characterize the development of both swords and the gonopodium, a modified anal fin that serves as an intromittent organ, and prompted investigations of the regulatory mechanisms that control msxC and sword growth.

Results

By comparing both development and regeneration of caudal fins in swordtails and platyfish, we show that fgfr1 is strongly up-regulated in developing and regenerating sword and gonopodial rays. Characterization of the fin overgrowth mutant brushtail in a platyfish background confirmed that fin regeneration rates are correlated with the expression levels of fgfr1 and msxC. Moreover, brushtail re-awakens the dormant mechanisms of sword development in platyfish and activates fgfr1/msxC-signalling. Although both genes are co-expressed in scleroblasts, expression of msxC in the distal blastema may be independent of fgfr1. Known regulators of Fgf-signalling in teleost fins, fgf20a and fgf24, are transiently expressed only during regeneration and thus not likely to be required in developing swords.

Conclusion

Our data suggest that Fgf-signalling is involved upstream of msxC in the development of the sword and gonopodium in male swordtails. Activation of a gene regulatory network that includes fgfr1 and msxC is positively correlated with fin ray growth rates and can be re-activated in platyfish to form small sword-like fin extensions. These findings point towards a disruption between the fgfr1/msxC network and its regulation by testosterone as a likely developmental cause for sword-loss in platyfish.

Bridging the regeneration gap: genetic insights from diverse animal models

Significant progress has recently been made in our understanding of animal regenerative biology, spurred on by the use of a wider range of model organisms and an increasing ability to use genetic tools in traditional models of regeneration. This progress has begun to delineate differences and similarities in the regenerative capabilities and mechanisms among diverse animal species, and to address some of the key questions about the molecular and cell biology of regeneration. Our expanding knowledge in these areas not only provides insights into animal biology in general, but also has important implications for regenerative medicine and stem-cell biology.

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.

Expression of FGF2 in the limb blastema of two Salamandridae correlates with their regenerative capability

Limb regenerative potential in urodeles seems to vary among different species. We observed that Triturus vulgaris meridionalis regenerate their limbs significantly faster than T. carnifex, where a long gap between the time of amputation and blastema formation occurs, and tried to identify cellular and molecular events that may underlie these differences in regenerative capability. Whereas wound healing is comparable in the two species, formation of an apical epidermal cap (AEC), which is required for blastema outgrowth, is delayed for approximately three weeks in T. carnifex. Furthermore, fewer nerve fibres are present distally early after amputation, consistent with the late onset of blastemal cell proliferation observed in T. carnifex. We investigated whether different expression of putative blastema mitogens, such as FGF1 and FGF2, in these species may underlie differences in the progression of regeneration. We found that whereas FGF1 is detected in the epidermis throughout the regenerative process, FGF2 onset of expression in the wound epidermis of both species coincides with AEC formation and initiation of blastemal cell proliferation, which is delayed in T. carnifex, and declines thereafter. In vitro studies showed that FGF2 activates MCM3, a factor essential for DNA replication licensing activity, and can be produced by blastemal cells themselves, indicating an autocrine action. These results suggest that FGF2 plays a key role in the initiation of blastema growth.

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.

Identification of genes induced in regenerating Xenopus tadpole tails by using the differential display method

To identify candidate gene(s) involved in the tail regeneration of Xenopus laevis tadpoles, we used the differential display method to isolate four genes (clones 1, 2, 13a, and 13b) whose expression is induced in regenerating tadpole tails. Among them, clones 13a and 13b were found to encode the Xenopus homologues of the alpha1 chain of type XVIII collagen and neuronal pentraxin I, respectively. Expression of clone 2 and neuronal pentraxin I genes increased dramatically in the blastema 3 days after amputation, whereas that for the clone 1 and type XVIII collagen genes was induced gradually after amputation. In situ hybridization revealed that the neuronal pentraxin I gene is expressed specifically in the regenerating tail epidermis but not in the normal tail epidermis or the most distal margin of the tail blastema, suggesting that it has a tissue-inductive role in tail regeneration. Expression of the four genes was induced in the limb and in the tail blastema, suggesting that they are involved in the regeneration of both organs. Finally, expression of clone 2 and neuronal pentraxin I genes was scarce during embryonic stages in comparison to the tail blastema, suggesting that their main functions are in organ regeneration. Our results demonstrate unique features of spatial and temporal gene expression patterns during Xenopus tadpole tail regeneration.

Differential regulation of fibroblast growth factor receptors in the regenerating amphibian spinal cord in vivo

Unlike mammals, adult urodele amphibians can regenerate their spinal cord and associated ganglia, but the molecular mechanisms controlling regeneration are not fully understood. We have recently shown that expression of FGF2, a member of the fibroblast growth factor family, is induced in the progenitor cells of the regenerating spinal cord and appears to play a role in their proliferation and possibly in their differentiation. In order to investigate which receptor(s) may mediate FGF2 signaling and their role in regeneration, we have studied expression of the four fibroblast growth factor receptors, FGFR1, FGFR2, FGFR3 and FGFR4, and of the spliced variants, sFGFR and KGFR, in the regenerating spinal cord of the adult urodele, Pleurodeles waltl, following tail amputation. We show that all FGFRs are expressed in normal and regenerating spinal cord, with the exception of the spliced variants that are expressed only in non-neural tissues of the tail. FGFR1 and 4 show the more interesting spatio-temporal patterns of expression. They are not detectable in the ependymal cells of normal cords, from which neural progenitors for regeneration are believed to originate, though they are expressed in some mature neurons. During regeneration, significant up-regulation of FGFR1 precedes that of FGFR4 in the ependymal tube from which the new cord will form. FGFR4 is highly expressed in these cells at later stages of regeneration, when neuronal differentiation is becoming apparent, and like FGFR1 is also expressed in some newborn neurons. In addition to the known form of FGFR1, the antibody against this receptor reacts also with a non-phosphorylated protein that appears to be present only during regeneration, and might represent a yet undescribed variant of the receptor. Altogether this study shows that fibroblast growth factor signaling is finely modulated during tail and spinal cord regeneration, and points to FGFR1 and FGFR4 as key players in this process, suggesting that FGFR1 is primarily associated with proliferation of progenitor cells and FGFR4 with early stages of neuronal differentiation.

Expression and biological effect of urodele fibroblast growth factor 1: relationship to limb regeneration

Fibroblast growth factors (FGFs) have been previously implicated in urodele limb regeneration. Here, we examined expression of FGF-1 by blastema cells and neurons and investigated its involvement in wound epithelial formation and function and in the trophic effect of nerves. Neurons innervating the limb and blastema cells in vivo and in vitro expressed the FGF-1 gene. The peptide was present in blastemas in vivo. Wound epithelium thickened when recombinant newt FGF-1 was provided on heparin-coated beads, demonstrating that the FGF-1 was biologically active and that the wound epithelium is a possible target tissue of FGF. FGF-1 did not stimulate accessory limb formation. FGF-1 was as effective as 10% fetal bovine serum in maintaining proliferative activity of blastema cells in vitro but was unable to maintain growth of denervated, nerve-dependent stage blastemas when provided on beads or by injection. FGF-1 had a strong stimulating effect on blastema cell accumulation and proliferation of limbs inserted into the body cavity that were devoid of an apical epithelial cap (AEC). These results show that FGF-1 can signal wound epithelium cap formation and/or function and can stimulate mesenchyme accumulation/proliferation in the absence of the AEC but that FGF-1 is not directly involved in the neural effect on blastema growth.

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.

Nerve-independence of limb regeneration in larval Xenopus laevis is correlated to the level of fgf-2 mRNA expression in limb tissues

In both larval and adult urodele amphibians, limb blastema formation requires the presence of an adequate nerve supply. In previous research, we demonstrated that the hindlimb of early Xenopus laevis larvae formed a regeneration blastema even when denervated, while the denervated limb of late larvae did not. We hypothesized that the nerve-independence was due to the autonomous synthesis of a mitogenic neurotrophic-like factor by undifferentiated limb bud cells. In this paper, we demonstrate that fgf-2 mRNA is present in larval limb tissues and that its level is correlated to the extent of mesenchymal cells populating the limb: in early limbs, fgf-2 mRNA is present at high levels all over the limb, while, in late limbs, the fgf-2 expression is low and detectable only in the distal autopodium. After denervation, fgf-2 mRNA synthesis increases in amputated early limbs but not in amputated late limbs. The implantation of anti-FGF-2 beads into amputated early limbs hardly lowers the mitotic activity of blastema cells. However, FGF-2 beads implanted into the blastema of late limbs prevent the denervation-induced inhibition of mitosis and oppose blastema regression. Our data indicate that FGF-2 is a good candidate for the endogenous mitogenic factor responsible for blastema formation and growth in amputated and denervated early limbs. However, in amputated late limbs, the very limited fgf-2 expression is not sufficient to promote blastema formation in the absence of nerves.

Expression patterns of Fgf-8 during development and limb regeneration of the axolotl

Fgf-8 is one of the key signaling molecules implicated in the initiation, outgrowth, and patterning of vertebrate limbs. However, it is not clear whether FGF-8 plays similar role in development and regeneration of urodele limbs. We isolated a Fgf-8 cDNA from the Mexican axolotl (Ambystoma mexicanum) through the screening of an embryo cDNA library. The cloned 1.26-kb cDNA contained an open reading frame encoding 212 amino acid residues with 84%, 86%, and 80% amino acid identities to those of Xenopus, chick, and mouse, respectively. By using the above clone as a probe, we examined the temporal and spatial expression patterns of Fgf-8 in developing embryos and in regenerating larval limbs. In developing embryos, Fgf-8 was expressed in the neural fold, midbrain-hindbrain junction, tail and limb buds, pharyngeal clefts, and primordia of maxilla and mandible. In the developing axolotl limb, Fgf-8 began to be expressed in the prospective forelimb region at pre-limb-bud and limb bud stages. Interestingly, strong expression was detected in the mesenchymal tissue of the limb bud before digit forming stages. In the regenerating limb, Fgf-8 expression was noted in the basal layer of the apical epithelial cap (AEC) and the underlying thin layer of mesenchymal tissue during blastema formation stages. These data suggest that Fgf-8 is involved in the organogenesis of various craniofacial structures, the initiation and outgrowth of limb development, and the blastema formation and outgrowth of regenerating limbs. In the developing limb of axolotl, unlike in Xenopus or in amniotes such as chick and mouse, the Fgf-8 expression domain was localized mainly in the mesenchyme rather than epidermis. The unique expression pattern of Fgf-8 in axolotl suggests that the regulatory mechanism of Fgf-8 expression is different between urodeles and other higher species. The expression of Fgf-8 in the deep layer of the AEC and the thin layer of underlying mesenchymal tissue in the regenerating limbs support the previous notion that the amphibian AEC is a functional equivalent of the AER in amniotes.

Two Nkx-3-related genes are expressed in the adult and regenerating central nervous system of the urodele Pleurodeles waltl

We report the isolation and characterization of two NK-3-related genes (PwNkx-3.2 and PwNkx-3.3) and their expression patterns during embryonic development, in the adult CNS, and during tail regeneration in the urodele Pleurodeles waltl. PwNkx-3.2 is the ortholog of the mouse and Xenopus genes, Bapx 1 and Xbap, but PwNkx-3.3 has no known homologue in any other vertebrate. We demonstrate that PwNkx-3.2 and PwNkx-3.3 exhibit graded axial expression patterns in adult spinal cord. During tail regeneration, the two genes are expressed in the wound epidermis, the regenerating muscle masses, the regenerating neural tube, the spinal ganglia, and the cartilage rod. The spatial distribution of transcripts in the CNS suggests that these genes could participate in maintaining the position information along the anteroposterior axis and may explain the ability of the adult CNS to regenerate. During tail regeneration, both genes could be implicated in the reformation of the axial skeleton.

Roles for Fgf signaling during zebrafish fin regeneration

Following amputation of a urodele limb or teleost fin, the formation of a blastema is a crucial step in facilitating subsequent regeneration. Using the zebrafish caudal fin regeneration model, we have examined the hypothesis that fibroblast growth factors (Fgfs) initiate blastema formation from fin mesenchyme. We find that fibroblast growth factor receptor 1 (fgfr1) is expressed in mesenchymal cells underlying the wound epidermis during blastema formation and in distal blastemal tissue during regenerative outgrowth. fgfr1 transcripts colocalize with those of msxb and msxc, putative markers for undifferentiated, proliferating cells. A zebrafish Fgf member, designated wfgf, is expressed in the regeneration epidermis during outgrowth. Furthermore, we show that a specific inhibitor of Fgfr1 applied immediately following fin amputation blocks blastema formation, without obvious effects on wound healing. This inhibitor blocks the proliferation of blastemal cells and the onset of msx gene transcription. Inhibition of Fgf signaling during ongoing fin regeneration prevents further outgrowth while downregulating the established expression of blastemal msx genes and epidermal sonic hedgehog. Our findings indicate that zebrafish fin blastema formation and regenerative outgrowth require Fgf signaling.

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.

Lens formation by pigmented epithelial cell reaggregate from dorsal iris implanted into limb blastema in the adult newt

In newt lens regeneration, the dorsal iris has lens forming ability and the ventral iris has no such capability, whereas there is no difference in the morphological criteria. To investigate the real aspects of this characteristic lens regeneration in the newt at the cellular level, a useful model system was constructed by transplanting the dorsal and ventral reaggregate derived from singly dissociated pigmented epithelial cells of the iris into the blastema of the forelimb in the newt. The lens was formed from the dorsal reaggregate with high efficiency, but not from the ventral one. No lens formation was observed in the implantation of the reaggregate into the tissue of the intact limbs. In detailed examination of the process of lens formation from the reaggregate, it was shown that tubular formation was the first step in the rearrangement of cells within the reaggregate. This was followed by depigmentation, vesicle formation with active cell growth, and the final step was lens fiber formation by transdifferentiation of epithelial cells composing the lens vesicle. The process was almost the same as in situ lens regeneration except the reconstitution of the two-layered epithelial structure was embodied as flattened tubular formation in the first step. The present study made it possible for the first time to examine lens forming ability in the reaggregate mixed with dorsal and ventral cells, because the formation of a reaggregate was started from singly dissociated cells of the dorsal and ventral cells of the iris. Mixed reaggregate experiments indicated that the existence of the dorsal cells in a cluster within the reaggregate is important in lens formation, and ventral cells showed an inhibitory effect on the formation. The present study demonstrated that the limb system thus constructed was effective for the analysis of lens formation at the cellular level and made it possible to examine the role of dorsal and ventral cells in lens regeneration.

Regulation of lens regeneration by fibroblast growth factor receptor 1

Lens regeneration in vivo is restricted to some urodeles only. After removal of the lens, this remarkable event is initiated from the dorsal iris. The pigmented epithelial cells from the dorsal iris dedifferentiate and subsequently transdifferentiate to form the regenerating lens. This property of the dorsal iris implies specific regulation along the dorsal-ventral axis. To date, no known genes are known to be specifically expressed in the dedifferentiating cells and to be involved in lens regeneration. In this paper, we show that FGFR-1 expression and function is correlated with the process of lens regeneration from the dorsal iris. Following lentectomy, FGFR-1 protein is specifically present in the dedifferentiating pigment epithelial cells in the dorsal iris, but is absent from the ventral iris. Subsequently, FGFR-1 protein is present throughout the process of lens regeneration and fiber differentiation. Furthermore, we show that an FGFR-1-specific inhibitor is able to inhibit the process of transdifferentiation and lens regeneration. In this sense, FGFR-1 can be regarded as the first known lens regeneration-associated factor.

Gene expression during amphibian limb regeneration

Limb regeneration in adult urodeles is an important phenomenon that poses fundamental questions both in biology and in medicine. In this review, we focus on recent advances in the characterization of the regeneration blastema at cellular and molecular levels and on the current understanding of the molecular basis of limb regeneration and its relationship to development. In particular, we discuss (i) the spatiotemporal distribution of genes and gene products in the mesenchyme and wound epidermis of the regenerating limb, (ii) how growth is controlled in the regeneration blastema, and (iii) molecules that are likely to be involved in patterning the regenerating limb such as homeobox genes and retinoids.

Conservation of fibroblast growth factor function in lens regeneration

In urodele amphibians, lens induction during development and regeneration occurs through different pathways. During development, the lens is induced from the mutual interaction of the ectoderm and the optic vesicle, whereas after lentectomy the lens is regenerated through the transdifferentiation of the iris-pigmented epithelial cells. Given the known role of fibroblast growth factors (FGFs) during lens development, we examined whether or not the expression and the effects of exogenous FGF during urodele lens regeneration were conserved. In this paper, we describe expression of FGF-1 and its receptors, FGFR-2 (KGFR and bek variants) and FGFR-3, in newts during lens regeneration. Expression of these genes was readily observed in the dedifferentiating pigmented epithelial cells, and the levels of expression were high in the lens epithelium and the differentiating fibers and lower in the retina. These patterns of expression implied involvement of FGFs in lens regeneration. To further elucidate this function, we examined the effects of exogenous FGF-1 and FGF-4 during lens regeneration. FGF-1 or FGF-4 treatment in lentectomized eyes resulted in the induction of abnormalities reminiscent to the ones induced during lens development in transgenic mice. Effects included transformation of epithelial cells to fiber cells, double lens regeneration, and lenses with abnormal polarity. These results establish that FGF molecules are key factors in fiber differentiation, polarity, and morphogenesis of the lens during regeneration even though the regenerating lens is induced by a different mechanism than in lens development. In this sense, FGF function in lens regeneration and development should be regarded as conserved. Such conservation should help elucidate the mechanisms of lens regeneration in urodeles and its absence in higher vertebrates.

Hedgehog and patched gene expression in adult ocular tissues

We analysed the expression of members of the hh gene family in adult ocular tissues of newt, frog and mouse by RT-PCR method. Shh displayed restricted expression in the neural retina that was conserved in each species analyzed. X-bhh, X-chh and mouse Ihh were detected in the iris and in the retinal pigment epithelium, while mouse Dhh was detected additionally in the neural retina and faintly in the cornea. We also found that two types of ptc genes, potential hh targets and receptors, were expressed in these tissues, suggesting the presence of active hh signalling there.

Activation of fibroblast growth factor 8 gene expression in human embryonal carcinoma cells

To study the role of fibroblast growth factor 8 (FGF-8) in human development and malignancies, we have isolated and characterized its gene. The gene spans 6.0 kbp and is comprised of five exons. Using reverse transcription-polymerase chain reaction, we were able to show that FGF-8 is expressed in two of the seven human mammary carcinoma cell lines tested and in only one of nine breast tumors. In contrast, both of the two normal breast tissues tested express FGF-8. FGF-8 was previously shown to be present in adult testis and ovary. Surprisingly, only one of the seven testis carcinomas and one of 12 ovary carcinomas express FGF-8, whereas all three kidney carcinomas tested express FGF-8. We further showed that fetal brain and lung express FGF-8, whereas fetal intestine and liver do not. Finally, we showed that a teratocarcinoma cell line, Tera-2, can be induced to express FGF-8 mRNA by fetal bovine serum.

A novel family of T-box genes in urodele amphibian limb development and regeneration: candidate genes involved in vertebrate forelimb/hindlimb patterning

In certain urodeles, a lost appendage, including hand and foot, can be completely replaced through epimorphic regeneration. The regeneration process involves cellular activities similar to those described for embryogenesis. Working on the assumption that the morphological pattern specific for a forelimb or a hindlimb is controlled by different gene activities in the two limbs, we employed a mRNA differential display screen for the detection of candidate limb identity genes. Using this approach, we have isolated a newt gene which in regenerating and developing limbs reveals properties expected of a gene having a role in controlling limb morphology: (1) it is exclusively expressed in the forelimbs, but not hindlimbs, (2) during embryonic development its expression is co-incident with forelimb bud formation, (3) it has an elevated message level throughout the undifferentiated limb bud and the blastema, respectively, and (4) it is expressed only in mesenchymal, but not in epidermal tissues. This novel newt gene shares a conserved DNA-binding domain, the T-box, with putative transcription factors including the Brachyury (T) gene product. In a following PCR-based screen, we used the evolutionarily conserved T-box motif and amplified a family of related genes in the newt; their different expression patterns in normal and regenerating forelimbs, hindlimbs and tail suggest, in general, an important role of T-domain proteins in vertebrate pattern formation.

Reactivation and graded axial expression pattern of Wnt-10a gene during early regeneration stages of adult tail in amphibian urodele Pleurodeles waltl

Adult urodele amphibians such as Pleurodeles waltl are able to regenerate their amputated limbs or tail. The mechanisms implicated in growth control and formation of the blastema are unknown but it has been proposed that regeneration in newts may proceed through reactivation of genes involved in embryonic development. Knowing the role of Wnt genes in the patterning of the primary and secondary axes of the vertebrate embryo, we suspected that some of these genes could be involved in axial pattern during newt tail regeneration. Pwnt-10a gene, cloned from a newt tail regenerate cDNA library, showed an expression pattern compatible with such a role in tail regenerates. Pwnt-10a, which is highly expressed during embryonic development (from gastrula to tailbud-stage) and weakly expressed in the adult tail, is strongly re-expressed during tail regeneration. In the blastemal mesenchyme Pwnt-10a transcripts exhibited a graded distribution along the antero-posterior axis, the mRNA accumulation being maximal in the caudal most part corresponding to the growing zone. These findings strongly support the view that Pwnt-10a may act in cooperation with other factors to control growth and patterning in newt tail regeneration. Until now Wnt-10a was only known to be involved in central nervous system development; our results suggest that this gene may also play a role in other developmental processes.

Amphibian FGF-1 is structurally and functionally similar to but antigenically distinguishable from its mammalian counterpart

Recent studies have shown that fibroblast growth factors (FGF) play an important role in the diverse cellular mechanisms involved with vertebrate development. One system which has received a great deal of attention is the developing limb in part because of the extensive epithelial-mesenchymal interactions that take place during this process. Because it closely parallels the developmental process of the limb and is a model for wound repair, the phenomenon of amphibian limb regeneration has been used to investigate the role of FGF in these processes. We have recently reported on the cloning and functional characterization of an FGF receptor (FGFR) isolated from amphibian regenerative tissue. In this report, we describe the isolation and characterization of an FGF-1 molecule from the newt, Notophthalmus viridescens. Amino acid sequence comparisons indicate that the newt FGF-1 exhibits between 79 to 83% identity with FGF-1 from mammalian and avian species. The full length cDNA of the newt FGF-1 was cloned into a prokaryotic expression vector and purified from E. coli. Although the newt FGF-1 shares a high degree of primary amino acid sequence similarity with other FGF-1 molecules, the recombinant protein was not detected in a Western blot analysis using a polyclonal antibody directed against mammalian FGF-1. Despite the antigenic divergence, the newt FGF-1 was capable of binding to NIH/3T3 and Chinese hamster ovary cells overexpressing mammalian and amphibian FGFRs with dissociation constants comparable to those reported for mammalian FGF-1. Newt FGF-1 could also be cross-linked to receptors on the surface of NIH/3T3 cells. In addition, it elicits a mitogenic response in NIH/3T3 cells indistinguishable from human recombinant FGF-1.

Nerve dependency of regeneration: the role of Distal-less and FGF signaling in amphibian limb regeneration

Dlx-3, a homolog of Drosophila Dll, has been isolated from an axolotl blastema cDNA library, and its expression in developing and regenerating limbs characterized. The normal expression pattern, and the changes that occur during experimental treatments, indicate a correlation between Dlx-3 expression and the establishment of the outgrowth-permitting epidermis. Dlx-3 is expressed at high levels in a distal-to-proximal gradient in the epidermis of developing limb buds, and is upregulated in the apical ectodermal cap (AEC) during limb regeneration. Expression is maximal at the late bud stage of regeneration, coincident with the transition from the early phase of nerve dependency to the later phase of nerve independence. Dlx-3 expression in the epidermis is rapidly downregulated by denervation during the nerve-dependent phase and is unaffected by denervation during the nerve-independent phase. We investigated this relationship between nerves and Dlx-3 expression by implanting FGF-2 beads into regenerates that had been denervated at a nerve-dependent stage. Dlx-3 expression was maintained by FGF-2 after denervation, and regeneration progressed to completion. In addition, we detected FGF-2 protein in the AEC and in nerves, and observed that the level of expression in both tissues decreases dramatically in response to denervation. We conclude that both limb development and regeneration require a permissive epidermis, characterized by Dlx-3 and FGF expression, both of which are maintained by FGF through an autocrine loop. The transformation of the limb epidermis into a functional AEC that produces and responds to FGF autocatalytically, is presumed to be induced by FGF. Since nerves appear to be a source of this priming FGF, it is possible that a member of the FGF family of growth factors is the elusive neurotrophic factor of limb regeneration.

A Distal-less-like gene is induced in the regenerating central nervous system of the urodele Pleurodeles waltl

We report the cloning of a Distal-less-like gene (PwDlx-3) and its pattern of expression during embryonic development and adult tail regeneration in the urodele Pleurodeles waltl. Using RT-PCR and in situ hybridization experiments we determined that, during regeneration, PwDlx-3 is expressed in the epidermis, the cells associated with muscle masses and in the ventrolateral parts of the ependymal tube. PwDlx-3 localization in the muscle masses and in cells of the ependymal tube, which give rise during regeneration to the ventral roots and the spinal ganglia, suggests that this gene might be expressed in cells which have some neural crest cell potentialities. PwDlx-3 is the first homeobox gene shown to be expressed in the regenerating spinal cord but not in the adult one and could thus be involved in the regeneration of the nervous system.

Changes in the expression of fibroblast growth factor receptors mark distinct stages of chondrogenesis in vitro and during chick limb skeletal patterning

Members of the fibroblast growth factor (FGF) family of growth factors are key regulators of limb skeletal patterning and growth. Abnormal expression of FGFs or mutations in their receptors (fgfrs) result in skeletal disorders. Here we show that changes in the expression of fgfrs are intrinsic properties of differentiating cartilage. In mesenchymal micromass cultures differentiating into cartilage, as in ovo, fgfr 1 mRNA was found predominantly in undifferentiated, proliferating mesenchyme, fgfr 2 in precartilage cell aggregates, and fgfr 3 in differentiating cartilage nodules. Thus, our data suggest that switches in the expression of fgfr 1, 2, and 3 mRNAs are associated with phases of cartilage patterning both in vitro and in ovo, and mark distinct stages in the development of the limb skeleton.