Ray-interray interactions during fin regeneration of Danio rerio

Growth patterns of caudal fin rays are informed by both external signals from the regenerating organ and remembered identity autonomous to the local tissue

Regenerating tissues must remember or interpret their spatial position, using this information to restore original size and patterning. The external skeleton of the zebrafish caudal fin is composed of 18 rays; after any portion of the fin is amputated, position-dependent regenerative growth restores each ray to its original length. We tested for transcriptional differences during regeneration of proximal versus distal tissues and identified 489 genes that differed in proximodistal expression. Thyroid hormone directs multiple aspects of ray patterning along the proximodistal axis, and we identified 364 transcripts showing a proximodistal expression pattern that was dependent on thyroid hormone context. To test what aspects of ray positional identity are directed by extrinsic environental cues versus remembered identity autonomous to the tissue, we transplanted distal portions of rays to proximal environments and evaluated regeneration within the new location. Native regenerating proximal tissue showed robust expression of scpp7, a transcript with thyroid-regulated proximal enrichment; in contrast, regenerating rays originating from transplanted distal tissue showed reduced (distal-like) expression during outgrowth. These distal-to-proximal transplants regenerated far beyond the length of the graft itself, indicating that cues from the proximal environment promoted additional growth. Nonetheless, these transplants initiated regeneration at a much slower rate compared to controls, suggesting memory of distal identity was retained by the transplanted tissue. This early growth retardation caused rays that originated from transplants to grow noticeably shorter than neighboring native rays. While several aspects of fin ray morphology (bifurcation, segment length) were found to be determined by the environment, we found that both regeneration speed and ray length are remembered autonomously by tissues, and that persist through multiple rounds of amputation and regeneration.

Growth patterns of caudal fin rays are informed by both external signals from the regenerating organ and remembered identity autonomous to the local tissue

Regenerating tissues must remember or interpret their spatial position, using this information to restore original size and patterning. The external skeleton of the zebrafish caudal fin is composed of 18 rays; after any portion of the fin is amputated, position-dependent regenerative growth restores each ray to its original length. We tested for transcriptional differences during regeneration of proximal versus distal tissues and identified 489 genes that differed in proximodistal expression. Thyroid hormone directs multiple aspects of ray patterning along the proximodistal axis, and we identified 364 transcripts showing a proximodistal expression pattern that was dependent on thyroid hormone context. To test what aspects of ray positional identity are directed by extrinsic cues versus remembered identity autonomous to the tissue itself, we transplanted distal portions of rays to proximal environments and evaluated regeneration within the new location. While neighboring proximal tissue showed robust expression of scpp7, a transcript with thyroid-regulated proximal enrichment, regenerating rays originating from transplanted distal tissue showed reduced (distal-like) expression during outgrowth. These distal-to-proximal transplants regenerated far beyond the length of the graft itself, indicating that cues from the proximal environment promoted additional growth. Nonetheless, these transplants initially regenerated at a much slower rate compared to controls, suggesting memory of distal identity was retained by the transplanted tissue. This early growth retardation caused rays that originated from transplants to become noticeably shorter than their native neighboring rays. While several aspects of fin ray morphology (bifurcation, segment length) were found to be determined by the environment, regeneration speed and ray length are remembered autonomously by tissues, persisting across multiple rounds of amputation and regeneration.

Evolution of median fin patterning and modularity in living and fossil osteichthyans

Morphological and developmental similarities, and interactions among developing structures are interpreted as evidences of modularity. Such similarities exist between the dorsal and anal fins of living actinopterygians, on the anteroposterior axis: (1) both fins differentiate in the same direction [dorsal and anal fin patterning module (DAFPM)], and (2) radials and lepidotrichia differentiate in the same direction [endoskeleton and exoskeleton module (EEM)]. To infer the evolution of these common developmental patternings among osteichthyans, we address (1) the complete description and quantification of the DAFPM and EEM in a living actinopterygian (the rainbow trout Oncorhynchus mykiss) and (2) the presence of these modules in fossil osteichthyans (coelacanths, lungfishes, porolepiforms and 'osteolepiforms'). In Oncorhynchus, sequences of skeletal elements are determined based on (1) apparition (radials and lepidotrichia), (2) chondrification (radials), (3) ossification (radials and lepidotrichia), and (4) segmentation plus bifurcation (lepidotrichia). Correlations are then explored between sequences. In fossil osteichthyans, sequences are determined based on (1) ossification (radials and lepidotrichia), (2) segmentation, and (3) bifurcation of lepidotrichia. Segmentation and bifurcation patterns were found crucial for comparisons between extant and extinct osteichthyan taxa. Our data suggest that the EEM is plesiomorphic at least for actinopterygians, and the DAFPM is plesiomorphic for osteichthyans, with homoplastic dissociation. Finally, recurrent patterns suggest the presence of a Lepidotrichia Patterning Module (LPM).

Regeneration of the dermal skeleton and wound epidermis formation depend on BMP signaling in the caudal fin of platyfish

Fin regeneration has been extensively studied in zebrafish, a genetic model organism. Little is known about regulators of this process in distant fish taxa, such as the Poeciliidae family, represented by the platyfish. Here, we used this species to investigate the plasticity of ray branching morphogenesis following either straight amputation or excision of ray triplets. This approach revealed that ray branching can be conditionally shifted to a more distal position, suggesting non-autonomous regulation of bone patterning. To gain molecular insights into regeneration of fin-specific dermal skeleton elements, actinotrichia and lepidotrichia, we localized expression of the actinodin genes and bmp2 in the regenerative outgrowth. Blocking of the BMP type-I receptor suppressed phospho-Smad1/5 immunoreactivity, and impaired fin regeneration after blastema formation. The resulting phenotype was characterized by the absence of bone and actinotrichia restoration. In addition, the wound epidermis displayed extensive thickening. This malformation was associated with expanded Tp63 expression from the basal epithelium towards more superficial layers, suggesting abnormal tissue differentiation. Our data add to the increasing evidence for the integrative role of BMP signaling in epidermal and skeletal tissue formation during fin regeneration. This expands our understanding of common mechanisms guiding appendage restoration in diverse clades of teleosts.

Hippo-Yap/Taz signalling in zebrafish regeneration

The extent of tissue regeneration varies widely between species. Mammals have a limited regenerative capacity whilst lower vertebrates such as the zebrafish (Danio rerio), a freshwater teleost, can robustly regenerate a range of tissues, including the spinal cord, heart, and fin. The molecular and cellular basis of this altered response is one of intense investigation. In this review, we summarise the current understanding of the association between zebrafish regeneration and Hippo pathway function, a phosphorylation cascade that regulates cell proliferation, mechanotransduction, stem cell fate, and tumorigenesis, amongst others. We also compare this function to Hippo pathway activity in the regenerative response of other species. We find that the Hippo pathway effectors Yap/Taz facilitate zebrafish regeneration and that this appears to be latent in mammals, suggesting that therapeutically promoting precise and temporal YAP/TAZ signalling in humans may enhance regeneration and hence reduce morbidity.

Evolved Bmp6 enhancer alleles drive spatial shifts in gene expression during tooth development in sticklebacks

Mutations in enhancers have been shown to often underlie natural variation but the evolved differences in enhancer activity can be difficult to identify in vivo. Threespine sticklebacks (Gasterosteus aculeatus) are a robust system for studying enhancer evolution due to abundant natural genetic variation, a diversity of evolved phenotypes between ancestral marine and derived freshwater forms, and the tractability of transgenic techniques. Previous work identified a series of polymorphisms within an intronic enhancer of the Bone morphogenetic protein 6 (Bmp6) gene that are associated with evolved tooth gain, a derived increase in freshwater tooth number that arises late in development. Here, we use a bicistronic reporter construct containing a genetic insulator and a pair of reciprocal two-color transgenic reporter lines to compare enhancer activity of marine and freshwater alleles of this enhancer. In older fish, the two alleles drive partially overlapping expression in both mesenchyme and epithelium of developing teeth, but the freshwater enhancer drives a reduced mesenchymal domain and a larger epithelial domain relative to the marine enhancer. In younger fish, these spatial shifts in enhancer activity are less pronounced. Comparing Bmp6 expression by in situ hybridization in developing teeth of marine and freshwater fish reveals similar evolved spatial shifts in gene expression. Together, these data support a model in which the polymorphisms within this enhancer underlie evolved tooth gain by shifting the spatial expression of Bmp6 during tooth development, and provide a general strategy to identify spatial differences in enhancer activity in vivo.

Regenerative Polarity of the Fin Ray in Zebrafish Caudal Fin and Related Tissue Formation on the Cut Surface

Zebrafish caudal fin rays are used as a model system for regeneration because of their high regenerative ability, but studies on the regeneration polarity of the fin ray are limited. To investigate this regeneration polarity, we made a hole to excise part of the fin ray and analyzed the regeneration process. We confirmed that the fin rays always regenerated from the proximal margin toward the distal margin, as previously reported; however, regeneration-related genes were expressed at both the proximal and distal edges of the hole in the early stage of regeneration, suggesting that the regenerative response also occurs at the distal edge. One difference between the proximal and distal margins is a sheet-like tissue that is formed on the apical side of the regenerated tissue at the proximal margin. This sheet-like tissue was not observed at the distal edge. To investigate whether the distal margin was also capable of forming this sheet-like tissue and subsequent regeneration, we kept the distal margin separated from the proximal margin by manipulation. Consequently, the sheet-like tissue was formed at the distal margin and regeneration of the fin ray was also induced. The regenerated fin rays from the distal margin protruded laterally from the caudal fin and then bent distally, and their ends showed the same characteristics as those of the normal fin rays. These results suggest that fin rays have an ability to regenerate in both directions; however, under normal conditions, regeneration is restricted to the proximal margin because the sheet-like tissue is preferentially formed on the apical side of the regenerating tissue from the proximal margin.

Hydrodynamic stress and phenotypic plasticity of the zebrafish regenerating fin

Understanding how extrinsic factors modulate genetically encoded information to produce a specific phenotype is of prime scientific interest. In particular, the feedback mechanism between abiotic forces and locomotory organs during morphogenesis to achieve efficient movement is a highly relevant example of such modulation. The study of this developmental process can provide unique insights on the transduction of cues at the interface between physics and biology. Here, we take advantage of the natural ability of adult zebrafish to regenerate their amputated fins to assess its morphogenic plasticity upon external modulations. Using a variety of surgical and chemical treatments, we could induce phenotypic responses to the structure of the fin. Through the ablation of specific rays in regenerating caudal fins, we generated artificially narrowed appendages in which the fin cleft depth and the positioning of rays bifurcations were perturbed compared with normal regenerates. To dissect the role of mechanotransduction in this process, we investigated the patterns of hydrodynamic forces acting on the surface of a zebrafish fin during regeneration by using particle tracking velocimetry on a range of biomimetic hydrofoils. This experimental approach enabled us to quantitatively compare hydrodynamic stress distributions over flapping fins of varying sizes and shapes. As a result, viscous shear stress acting on the distal margin of regenerating fins and the resulting internal tension are proposed as suitable signals for guiding the regulation of ray growth dynamics and branching pattern. Our findings suggest that mechanical forces are involved in the fine-tuning of the locomotory organ during fin morphogenesis.

Through veiled mirrors: Fish fins giving insight into size regulation

Faithful establishment and maintenance of proportion is seen across biological systems and provides a glimpse at fundamental rules of scaling that underlie development and evolution. Dysregulation of proportion is observed in a range of human diseases and growth disorders, indicating that proper scaling is an essential component of normal anatomy and physiology. However, when viewed through an evolutionary lens, shifts in the regulation of relative proportion are one of the most striking sources of morphological diversity among organisms. To date, the mechanisms via which relative proportion is specified and maintained remain unclear. Through the application of powerful experimental, genetic and molecular approaches, the teleost fin has provided an effective model to investigate the regulation of scaling, size, and relative growth in vertebrate organisms. This article is categorized under: Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing Adult Stem Cells, Tissue Renewal, and Regeneration > Regeneration Comparative Development and Evolution > Regulation of Organ Diversity.

Regrowth of zebrafish caudal fin regeneration is determined by the amputated length

Fish have a high ability to regenerate fins, including the caudal fin. After caudal fin amputation, original bi-lobed morphology is reconstructed during its rapid regrowth. It is still controversial whether positional memory in the blastema cells regulates reconstruction of fin morphology as in amphibian limb regeneration, in which limb blastema cells located at the same proximal-distal level have the same positional identity. We investigated growth period and growth rate in zebrafish caudal fin regeneration. We found that both the growth period and growth rate differed for fin rays that were amputated at the same proximal-distal level, indicating that it takes different periods of time for fin rays to restore their original lengths after straight amputation. We also show that more proximal amputation takes longer period to reconstruct the original morphology/size than more distal amputation. Statistical analysis suggested that both the growth period/rate are determined by amputated length (depth) regardless of the fin ray identity along dorsal-ventral axis. In addition, we suggest the possibility that the structural/physical condition such as width of the fin ray at the amputation site (niche at the stump) may determine the growth period/rate.

The Glycogen Synthase Kinase-3β Inhibitor LSN 2105786 Promotes Zebrafish Fin Regeneration

The Wnt pathway has been shown to regulate bone homeostasis and to influence some bone disease states. We utilized a zebrafish model system to study the effects of a synthetic, orally bioavailable glycogen synthase kinase-3β (GSK3β) inhibitor LSN 2105786, which activates Wnt signaling during bone healing and embryogenesis. GSK3β inhibitor treatment was used to phenocopy GSK3β morpholino oligonucleotide (MO) knockdown in zebrafish embryos. Human and zebrafish synthetic mRNA injection were similarly effective at rescue of GSK3β MO knockdown. During caudal fin regeneration, bony rays are the first structure to differentiate in zebrafish fins, providing a useful model to study bone healing. Caudal fin regeneration experiments were conducted using various concentrations of a GSK3β inhibitor, examining duration and concentration dependence on regenerative outgrowth. Experiments revealed continuous low concentration (4-5 nM) treatment to be more effective at increasing regeneration than intermittent dosing. Higher concentrations inhibited fin growth, perhaps by excessive stimulation of differentiation programs. Increased Wnt responsive gene expression and differentiation were observed in response to GSK3b inhibitor treatment. Activating Wnt signaling also increased cell proliferation and osteoblast differentiation in fin regenerates. Together, these data indicate that bone healing in zebrafish fin regeneration was improved by activating Wnt signaling using GSK3b inhibitor treatment. In addition, caudal fin regeneration is useful to evaluate dose-dependent pharmacological efficacy in bone healing, various dosing regimens and possible toxicological effects of compounds.

Robust and local positional information within a fin ray directs fin length during zebrafish regeneration

It has been proposed that cells are regulated to form specific morphologies and sizes according to positional information. However, the entity and nature of positional information have not been fully understood yet. The zebrafish caudal fin has a characteristic V-shape; dorsal and ventral fin rays are longer than the central ones. This fin shape regenerates irrespective of the sites or shape of fin amputation. It is thought that reformation of tissue occurs according to positional information. In this study, we developed a novel transplantation procedure for grafting a whole fin ray to an ectopic position and examined whether the information that specifies fin length exists within each fin ray. Intriguingly, when long and short fin rays were swapped, they regenerated to form longer or shorter fin rays than the adjacent host fin rays, respectively. Further, the abnormal fin ray lengths were maintained for a long time, more than 5 months, and after further re-amputation. In contrast to intra-fin grafting, when fin ray grafting was performed between fish, cells in the grafts disappeared due to immune rejection, and the grafted fin rays adapted to the host position to form a normal fin. Together, our data suggest that the information that directs fin length does exist in cells within a single fin ray and that it has a robust property-it is stable for a long time and is hard to rewrite. Our study highlighted a novel positional information mechanism for directing regenerating fin length.

Widening control of fin inter-rays in zebrafish and inferences about actinopterygian fins

The amputation of a teleost fin rapidly triggers an intricate maze of hierarchically regulated signalling processes which ultimately reconstruct the diverse tissues of the appendage. Whereas the generation of the fin pattern along the proximodistal axis brings with it several well-known developmental regulators, the mechanisms by which the fin widens along its dorsoventral axis remain poorly understood. Utilizing the zebrafish as an experimental model of fin regeneration and studying more than 1000 actinopterygian species, we hypothesized a connection between specific inter-ray regulatory mechanisms and the morphological variability of inter-ray membranes found in nature. To tackle these issues, both cellular and molecular approaches have been adopted and our results suggest the existence of two distinguishable inter-ray areas in the zebrafish caudal fin, a marginal and a central region. The present work associates the activity of the cell membrane potassium channel kcnk5b, the fibroblast growth factor receptor 1 and the sonic hedgehog pathway to the control of several cell functions involved in inter-ray wound healing or dorsoventral regeneration of the zebrafish caudal fin. This ray-dependent regulation controls cell migration, cell-type patterning and gene expression. The possibility that modifications of these mechanisms are responsible for phenotypic variations found in euteleostean species, is discussed.

Shh promotes direct interactions between epidermal cells and osteoblast progenitors to shape regenerated zebrafish bone

Zebrafish innately regenerate amputated fins by mechanisms that expand and precisely position injury-induced progenitor cells to re-form tissue of the original size and pattern. For example, cell signaling networks direct osteoblast progenitors (pObs) to rebuild thin cylindrical bony rays with a stereotypical branched morphology. Hedgehog/Smoothened (Hh/Smo) signaling has been variably proposed to stimulate overall fin regenerative outgrowth or promote ray branching. Using a photoconvertible patched2 reporter, we resolve active Hh/Smo output to a narrow distal regenerate zone comprising pObs and adjacent motile basal epidermal cells. This Hh/Smo activity is driven by epidermal Sonic hedgehog a (Shha) rather than Ob-derived Indian hedgehog a (Ihha), which nevertheless functions atypically to support bone maturation. Using BMS-833923, a uniquely effective Smo inhibitor, and high-resolution imaging, we show that Shha/Smo is functionally dedicated to ray branching during fin regeneration. Hh/Smo activation enables transiently divided clusters of Shha-expressing epidermis to escort pObs into similarly split groups. This co-movement likely depends on epidermal cellular protrusions that directly contact pObs only where an otherwise occluding basement membrane remains incompletely assembled. Progressively separated pObs pools then continue regenerating independently to collectively re-form a now branched skeletal structure.

Functional bone histology of zebrafish reveals two types of endochondral ossification, different types of osteoblast clusters and a new bone type

The zebrafish is as an important vertebrate animal model system for studying developmental processes, gene functions and signalling pathways. It is also used as a model system for the understanding of human developmental diseases including those related to the skeleton. However, surprisingly little is known about normal zebrafish skeletogenesis and osteogenesis. As in most vertebrates, it is commonly known that the bones of adult zebrafish are cellular unlike that of some other teleosts. After careful histological analyses of each zebrafish adult bone, we identified several acellular bones, with no entrapped osteocytes in addition to several cellular bones. We show that both cellular and acellular bones can even occur within the same skeletal element and transitions between these two cell types can be found. Furthermore, we describe two types of osteoblast clusters during skeletogenesis and two different types of endochondral ossification. The epiphyseal plate, for example, lacks a zone of calcification and a degradation zone with osteoblasts. A new bone type that we term tubular bone was also identified. This bone is completely filled with adipose tissue, unlike spongy bones. This study provides important insight on how osteogenesis takes place in zebrafish, and especially on the transition from cellular to acellular bones. Overall, this study leads to a deeper understanding of the functional histological composition of adult zebrafish bones.

Quantitative trait locus analysis of body shape divergence in nine-spined sticklebacks based on high-density SNP-panel

Heritable phenotypic differences between populations, caused by the selective effects of distinct environmental conditions, are of commonplace occurrence in nature. However, the actual genomic targets of this kind of selection are still poorly understood. We conducted a quantitative trait locus (QTL) mapping study to identify genomic regions responsible for morphometric differentiation between genetically and phenotypically divergent marine and freshwater nine-spined stickleback (Pungitius pungitius) populations. Using a dense panel of SNP-markers obtained by restriction site associated DNA sequencing of an F2 recombinant cross, we found 22 QTL that explained 3.5-12.9% of phenotypic variance in the traits under investigation. We detected one fairly large-effect (PVE = 9.6%) QTL for caudal peduncle length-a trait with a well-established adaptive function showing clear differentiation among marine and freshwater populations. We also identified two large-effect QTL for lateral plate numbers, which are different from the lateral plate QTL reported in earlier studies of this and related species. Hence, apart from identifying several large-effect QTL in shape traits showing adaptive differentiation in response to different environmental conditions, the results suggest intra- and interspecific heterogeneity in the genomic basis of lateral plate number variation.

Human regeneration: An achievable goal or a dream?

The main objective of regenerative medicine is to replenish cells or tissues or even to restore different body parts that are lost or damaged due to disease, injury and aging. Several avenues have been explored over many decades to address the fascinating problem of regeneration at the cell, tissue and organ levels. Here we discuss some of the primary approaches adopted by researchers in the context of enhancing the regenerating ability of mammals. Natural regeneration can occur in different animal species, and the underlying mechanism is highly relevant to regenerative medicine-based intervention. Significant progress has been achieved in understanding the endogenous regeneration in urodeles and fishes with the hope that they could help to reach our goal of designing future strategies for human regeneration.

Genetic Architecture of the Variation in Male-Specific Ossified Processes on the Anal Fins of Japanese Medaka

Traits involved in reproduction evolve rapidly and show great diversity among closely related species. However, the genetic mechanisms that underlie the diversification of courtship traits are mostly unknown. Japanese medaka fishes (Oryzias latipes) use anal fins to attract females and to grasp females during courtship; the males have longer anal fins with male-specific ossified papillary processes on the fin rays. However, anal fin morphology varies between populations: the southern populations tend to have longer anal fins and more processes than the northern populations. In the present study, we conducted quantitative trait locus (QTL) mapping to investigate the genetic architecture underlying the variation in the number of papillary processes of Japanese medaka fish and compared the QTL with previously identified QTL controlling anal fin length. First, we found that only a few QTL were shared between anal fin length and papillary process number. Second, we found that the numbers of papillary processes on different fin rays often were controlled by different QTL. Finally, we produced another independent cross and found that some QTL were repeatable between the two crosses, whereas others were specific to only one cross. These results suggest that variation in the number of papillary processes is polygenic and controlled by QTL that are distinct from those controlling anal fin length. Thus, different courtship traits in Japanese medaka share a small number of QTL and have the potential for independent evolution.

Osteogenic programs during zebrafish fin regeneration

Recent advances in genomic, screening and imaging technologies have provided new opportunities to examine the molecular and cellular landscape underlying human physiology and disease. In the context of skeletal research, technologies for systems genetics, high-throughput screening and high-content imaging can aid an unbiased approach when searching for new biological, pathological or therapeutic pathways. However, these approaches necessitate the use of specialized model systems that rapidly produce a phenotype, are easy to manipulate, and amenable to optical study, all while representing mammalian bone physiologies at the molecular and cellular levels. The emerging use of zebrafish (Danio rerio) for modeling human disease highlights its potential to accelerate therapeutic and pathway discovery in the mammalian skeleton. In this review, we consider the potential value of zebrafish fin ray regeneration (a rapid, genetically tractable and optically transparent model of intramembranous ossification) as a translational model for such studies.

Sexual selection drives the evolution of limb regeneration in Harmonia axyridis (Coleoptera: Coccinellidae)

When Harmonia axyridis larvae were subjected to amputation of a foreleg in the fourth instar, 83% survived and, of these, 75% regenerated the leg during pupation. Regenerators pupated at heavier weights than controls (unoperated) or non-regenerators, and spent longer in pupation. Regenerated males were preferred by females in choice tests and produced more viable progeny than control males. Unregenerated males were less preferred by females, copulated for shorter periods than control males, and reduced female fecundity. Amputation diminished beneficial paternal effects, whether males regenerated or not, resulting in progeny with slower development and smaller adult body mass relative to control paternity. Progeny of unregenerated males had lower survival and body mass, whether male or female, confirming that regeneration was an honest signal of mate quality. When offspring had a foreleg amputated, a regenerated paternity yielded higher survival than control paternity, but similar rates of regeneration, whereas an unregenerated paternity yielded lower rates of survival and leg regeneration than control paternity. Regenerating beetles were twice as likely to be melanic as non-regenerating or control beetles, suggesting pleiotropic effects of melanism on processes involved in regeneration. This is the first report of complete limb regeneration by a holometabolous insect in the pupal stage, and the first example of sexual selection for regenerative capacity.

Holmgren's principle of delamination during fin skeletogenesis

During fin morphogenesis, several mesenchyme condensations occur to give rise to the dermal skeleton. Although each of them seems to create distinctive and unique structures, they all follow the premises of the same morphogenetic principle. Holmgren's principle of delamination was first proposed to describe the morphogenesis of skeletal elements of the cranium, but Jarvik extended it to the development of the fin exoskeleton. Since then, some cellular or molecular explanations, such as the "flypaper" model (Thorogood et al.), or the evolutionary description by Moss, have tried to clarify this topic. In this article, we review new data from zebrafish studies to meet these criteria described by Holmgren and other authors. The variety of cell lineages involved in these skeletogenic condensations sheds light on an open discussion of the contributions of mesoderm- versus neural crest-derived cell lineages to the development of the head and trunk skeleton. Moreover, we discuss emerging molecular studies that are disclosing conserved regulatory mechanisms for dermal skeletogenesis and similarities during fin development and regeneration, which may have important implications in the potential use of the zebrafish fin as a model for regenerative medicine.

Bmp7 and Lef1 are the downstream effectors of androgen signaling in androgen-induced sex characteristics development in medaka

Androgens play key roles in the morphological specification of male type sex attractive and reproductive organs, whereas little is known about the developmental mechanisms of such secondary sex characters. Medaka offers a clue about sexual differentiation. They show a prominent masculine sexual character for appendage development, the formation of papillary processes in the anal fin, which has been induced in females by exogenous androgen exposure. This current study shows that the development of papillary processes is promoted by androgen-dependent augmentation of bone morphogenic protein 7 (Bmp7) and lymphoid enhancer-binding factor-1 (Lef1). Androgen receptor (AR) subtypes, ARα and ARβ, are expressed in the distal region of outgrowing bone nodules of developing papillary processes. Development of papillary processes concomitant with the induction of Bmp7 and Lef1 in the distal bone nodules by exposure to methyltestosterone was significantly suppressed by an antiandrogen, flutamide, in female medaka. When Bmp signaling was inhibited in methyltestosterone-exposed females by its inhibitor, dorsomorphin, Lef1 expression was suppressed accompanied by reduced proliferation in the distal bone nodules and retarded bone deposition. These observations indicate that androgen-dependent expressions of Bmp7 and Lef1 are required for the bone nodule outgrowth leading to the formation of these secondary sex characteristics in medaka. The formation of androgen-induced papillary processes may provide insights into the mechanisms regulating the specification of sexual features in vertebrates.

The expression of alkaline phosphatase, osteopontin, osteocalcin, and chondroitin sulfate during pectoral fin regeneration in Carassius auratus gibelio: a combined histochemical and immunohistochemical study

Dermal bone is an important component of the teleost fins, and its ability to regenerate after fin amputation appears to be unlimited. The organic bone matrix contain type I collagen fibers, proteoglycans enriched in chondroitin sulfate, and noncollagenous matrix protein such as osteocalcin, osteopontin, and osteonectin. These molecules are synthesized by fin osteoblasts. Inorganic components chiefly consist of calcium and phosphate that form crystals of hydroxyapatite. Fin rays are described as models to study ossification. Due to this, the identification of the components involved in the synthesis of the organic and inorganic components of lepidotrichial bone are of great interest for the analysis of skeletal disorders in fish ossification. The present study investigates expression of alkaline phosphatase, osteopontin, osteocalcin, and chondroitin sulfate during pectoral fin regeneration in Carassius auratus gibelio. Alkaline phosphatase reaction has been found in the epidermis covering the wound, proximal blastema, near the cells that surround newly-formed lepidotrichia matrix and the tips of regenerating fin rays. Osteopontin has been observed throughout the regeneration blastema but excluded from the scleroblasts lining the inner side of the lepidotrichia. Osteocalcin and chondroitin sulfate expression coincides with the onset of mineralization of lepidotrichial matrix, suggesting its involvement in bone mineralization.

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.

An amputation resets positional information to a proximal identity in the regenerating zebrafish caudal fin

Background

Zebrafish has emerged as a powerful model organism to study the process of regeneration. This teleost fish has the ability to regenerate various tissues and organs like the heart, spinal cord, retina and fins. In this study, we took advantage of the existence of an excellent morphological reference in the zebrafish caudal fin, the bony ray bifurcations, as a model to study positional information upon amputation. We investigated the existence of positional information for bifurcation formation by performing repeated amputations at different proximal-distal places along the fin.

Results

We show that, while amputations performed at a long distance from the bifurcation do not change its final proximal-distal position in the regenerated fin, consecutive amputations done at 1 segment proximal to the bifurcation (near the bifurcation) induce a positional reset and progressively shift its position distally. Furthermore, we investigated the potential role of Shh and Fgf signalling pathways in the determination of the bifurcation position and observed that they do not seem to be involved in this process.

Conclusions

Our results reveal that, an amputation near the bifurcation inhibits the formation of the regenerated bifurcation in the pre-amputation position, inducing a distalization of this structure. This shows that the positional memory for bony ray bifurcations depends on the proximal-distal level of the amputation.

Morphogen-based simulation model of ray growth and joint patterning during fin development and regeneration

The fact that some organisms are able to regenerate organs of the correct shape and size following amputation is particularly fascinating, but the mechanism by which this occurs remains poorly understood. The zebrafish (Danio rerio) caudal fin has emerged as a model system for the study of bone development and regeneration. The fin comprises 16 to 18 bony rays, each containing multiple joints along its proximodistal axis that give rise to segments. Experimental observations on fin ray growth, regeneration and joint formation have been described, but no unified theory has yet been put forward to explain how growth and joint patterns are controlled. We present a model for the control of fin ray growth during development and regeneration, integrated with a model for joint pattern formation, which is in agreement with published, as well as new, experimental data. We propose that fin ray growth and joint patterning are coordinated through the interaction of three morphogens. When the model is extended to incorporate multiple rays across the fin, it also accounts for how the caudal fin acquires its shape during development, and regains its correct size and shape following amputation.

Mature and juvenile tissue models of regeneration in small fish species

The multitude of cells constituting organisms are fragile and easily damaged day by day. Therefore, maintenance of tissue morphology and function is fundamental for multicellular organisms to attain long life. For proper maintenance of tissue integrity, organisms must have mechanisms that detect the loss of tissue mass, activate the de novo production of cells, and organize those cells into functional tissues. However, these processes are only poorly understood. Here we give an overview of adult and juvenile tissue regeneration models in small fish species, such as zebrafish and medaka, and highlight recent advances at the molecular level. From these advances, we have come to realize that the epidermal and mesenchymal parts of the regenerating fish fin-that is, the wound epidermis and blastema, respectively-comprise heterogeneous populations of cells with different molecular identities that can be termed "compartments." These compartments and their mutual interactions are thought to play important roles in promoting the proper progression of tissue regeneration. We further describe the current understanding of these compartments and discuss the possible approaches to affording a better understanding of their roles and interactions during regeneration.

Dermoskeleton morphogenesis in zebrafish fins

Zebrafish fins have a proximal skeleton of endochondral bones and a distal skeleton of dermal bones. Recent experimental and genetic studies are discovering mechanisms to control fin skeleton morphogenesis. Whereas the endochondral skeleton has been extensively studied, the formation of the dermal skeleton requires further revision. The shape of the dermal skeleton of the fin is generated in its distal growing margin and along a proximal growing domain. In these positions, dermoskeletal fin morphogenesis can be explained by intertissue interactions and the function of several genetic pathways. These pathways regulate patterning, size, and cell differentiation along three axes. Finally, a common genetic control of late development, regeneration, and tissue homeostasis of the fin dermoskeleton is currently being analyzed. These pathways may be responsible for the similar shape obtained after each morphogenetic process. This provides an interesting conceptual framework for future studies on this topic. Developmental Dynamics 239:2779–2794, 2010. © 2010 Wiley-Liss, Inc.

Comparative aspects of animal regeneration

Most but not all phyla include examples of species that are able to regenerate large sections of the body plan. The mechanisms underlying regeneration on this scale are currently being studied in a variety of contexts in both vertebrates and invertebrates. Regeneration generally involves the formation of a wound epithelium after transection or injury, followed by the generation of regenerative progenitor cells and morphogenesis to give the regenerate. Common mechanisms may exist in relation to each of these aspects. For example, the initial proliferation of progenitor cells often depends on the nerve supply, whereas morphogenesis reflects the generation of positional disparity between adjacent cells-the principle of intercalation. These mechanisms are reviewed here across a range of contexts. We also consider the evolutionary origins of regeneration and how regeneration may relate to both agametic reproduction and to ontogeny.

Identification of novel markers expressed during fin regeneration by microarray analysis in medaka fish

Urodeles and fish have a remarkable ability to regenerate lost body parts, whereas many higher vertebrates, including mammals, retain only a limited capacity. It is known that the formation of specialized cell populations such as the wound epidermis or blastema is crucial for regeneration; however, the molecular basis for their formation has not been elucidated. Recently, approaches using differential display and microarray have been done in zebrafish for searching molecules involved in regeneration. Here, we used the medaka fish, a distantly diverged fish species, for microarray screening of transcripts up-regulated during regeneration. By setting criteria for selecting transcripts that are reliably and reproducibly up-regulated during regeneration, we identified 140 transcripts. Of them, localized in situ expression of 12 transcripts of 22 tested was detected either in differentiating cartilage, basal wound epidermis, or blastema. Our results provide useful molecular markers for dissecting the regeneration process at a fine cellular resolution.

Position dependence of hemiray morphogenesis during tail fin regeneration in Danio rerio

The fins of actinopterygian can regenerate following amputation. Classical papers have shown that the ray, a structural unit of these fins, might regenerate independent of this appendage. Each fin ray is formed by two apposed contralateral hemirays. A hemiray may autonomously regenerate and segmentate in a position-independent manner. This is observed when heterotopically grafted into an interray space, after amputation following extirpation of the contralateral hemiray or when simply ablated. During this process, a proliferating hemiblastema is formed, as shown by bromodeoxyuridine incorporation, from which the complete structure will regenerate. This hemiblastema shows a patterning of gene expression domain similar to half ray blastema. Interactions between contralateral hemiblastema have been studied by recombinant rays composed of hemirays from different origins on the proximo-distal or dorso-ventral axis of the caudal fin. Dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocianine perchlorate labeling of grafted tissues was used as tissular marker. Our results suggest both that there are contralateral interactions between hemiblastema of each ray, and that hemiblastema may vary its morphogenesis, always differentiating as their host region. These non-autonomous, position-dependent interactions control coordinated bifurcations, segment joints and ray length independently. A morphological study of the developing and regenerating fin of another long fin mutant zebrafish suggests that contralateral hemiblastema interactions are perturbed in this mutant.

Cellular and molecular processes of regeneration, with special emphasis on fish fins

The phenomenon of 'epimorphic regeneration', a complete reformation of lost tissues and organs from adult differentiated cells, has been fascinating many biologists for many years. While most vertebrate species including humans do not have a remarkable ability for regeneration, the lower vertebrates such as urodeles and fish have exceptionally high regeneration abilities. In particular, the teleost fish has a high ability to regenerate a variety of tissues and organs including scales, muscles, spinal cord and heart among vertebrate species. Hence, an understanding of the regeneration mechanism in teleosts will provide an essential knowledge base for rational approaches to tissue and organ regeneration in mammals. In the last decade, small teleost fish such as the zebrafish and medaka have emerged as powerful animal models in which a variety of developmental, genetic and molecular approaches are applicable. In addition, rapid progress in the development of genome resources such as expressed sequence tags and genome sequences has accelerated the speed of the molecular analysis of regeneration. This review summarizes the current status of our understanding of the cellular and molecular basis of regeneration, particularly that regarding fish fins.

Inhibition of BMP signaling during zebrafish fin regeneration disrupts fin growth and scleroblast differentiation and function

The zebrafish caudal fin provides a simple model to study molecular mechanisms of dermal bone regeneration. We previously showed that misexpression of Bone morphogenetic protein 2b (Bmp2b) induces ectopic bone formation within the regenerate. Here we show that in addition to bmp2b and bmp4 another family member, bmp6, is involved in fin regeneration. We further investigated the function of BMP signaling by ectopically expressing the BMP signaling inhibitor Chordin which caused: (1) inhibition of regenerate outgrowth due to a decrease of blastema cell proliferation and downregulation of msxb and msxC expression and (2) reduced bone matrix deposition resulting from a defect in the maturation and function of bone-secreting cells. We then identified targets of BMP signaling involved in regeneration of the bone of the fin rays. runx2a/b and their target col10a1 were downregulated following BMP signaling inhibition. Unexpectedly, the sox9a/b transcription factors responsible for chondrocyte differentiation were detected in the non-cartilaginous fin rays, sox9a and sox9b were not only differentially expressed but also differentially regulated since sox9a, but not sox9b, was downregulated in the absence of BMP signaling. Finally, this analysis revealed the surprising finding of the expression, in the fin regenerate, of several factors which are normally the signatures of chondrogenic elements during endochondral bone formation although fin rays form through dermal ossification, without a cartilage intermediate.

Temporal requirement for bone morphogenetic proteins in regeneration of the tail and limb of Xenopus tadpoles

Bone morphogenetic protein (BMP) signalling is necessary for both the development of the tail bud and for tail regeneration in Xenopus laevis tadpoles. Using a stable transgenic line in which expression of the soluble BMP inhibitor noggin is under the control of the temperature inducible hsp70 promoter, we have investigated the timing of the requirement for BMP signalling during tail regeneration. If noggin expression is induced followed by partial amputation of the tail, then wound closure and the formation of the neural ampulla occur normally but outgrowth of the regeneration bud is inhibited. Furthermore, we show that BMP signalling is also necessary for limb bud regeneration, which occurs in Xenopus tadpoles prior to differentiation. When noggin expression is induced, limb bud regeneration fails at an early stage and a stump is formed. The situation appears similar to the tail, with formation of the limb bud blastema occurring but renewed outgrowth inhibited. The transcriptional repressor Msx1, a direct target of BMP signalling with known roles in vertebrate appendage regeneration, fails to be re-expressed in both tail and limb in the presence of noggin. DNA labelling studies show that proliferation in the notochord and spinal cord of the tail, and of the blastema in the limb bud, is significantly inhibited by noggin induction, suggesting that in the context of these regenerating appendages BMP is mainly required, directly or indirectly, as a mitogenic factor.

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

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

Limb regeneration in higher vertebrates: developing a roadmap

We review what is known about amphibian limb regeneration from the prospective of developing strategies for the induction of regeneration in adult mammals. Prominent in urodele amphibian limb regeneration is the formation of a blastema of undifferentiated cells that goes on to reform the limb. The blastema shares many properties with the developing limb bud; thus, the outgrowth phase of regeneration can be thought of as cells going through development again, i.e., redevelopment. Getting to a redevelopment phase in mammals would be a major breakthrough given our extensive understanding of limb development. The formation of the blastema itself represents a transition phase in which limb cells respond to injury by dedifferentiating to become embryonic limb progenitor cells that can undergo redevelopment. During this phase, rapid wound closure is followed by the dedifferentiation of limb cells to form the blastema. Thus, the regeneration process can be divided into a wound-healing/dedifferentiation phase and a redevelopment phase, and we propose that the interface between the wound-healing response and gaining access to developmentally regulated programs (dedifferentiation) lies at the heart of the regeneration problem in mammals. In urodele amphibians, dedifferentiation can occur in all of the tissues of the limb; however, numerous studies lead us to focus on the epidermis, the dermis, and muscle as key regulators of regeneration. Among higher vertebrates, the digit tip in mammals, including humans, is regeneration-competent and offers a unique mammalian model for regeneration. Recent genetic studies in mice identify the Msx1 gene as playing a critical role in the injury response leading to digit tip regeneration. The results from regeneration studies ranging from amphibians to mammals can be integrated to develop a roadmap for mammalian regeneration that has as its focus understanding the phenomenon of dedifferentiation.

Large-scale analysis of the genes involved in fin regeneration and blastema formation in the medaka, Oryzias latipes

Medaka is an attractive model to study epimorphic regeneration. The fins have remarkable regenerative capacity and are replaced about 14 days after amputation. The formation of blastema, a mass of undifferentiated cells, is essential for regeneration; however, the molecular mechanisms are incompletely defined. To identify the genes required for fin regeneration, especially for blastema formation, we constructed cDNA libraries from fin regenerates at 3 days postamputation and 10 days postamputation. A total of 16,866 expression sequence tags (ESTs) were sequenced and subjected to BLASTX analysis. The result revealed that about 60% of them showed strong matches to previously identified proteins, and major signaling molecules related to development, including FGF, BMP, Wnt, Notch/Delta, and Ephrin/Eph signaling pathways were isolated. To identify novel genes that showed specific expression during fin regeneration, cDNA microarray was generated based on 2900 independent ESTs from each library which had no sequence similarity to known proteins. We obtained 6 candidate genes associated with blastema formation by gene expression pattern screening in competitive hybridization analyses and in situ hybridization. Olrfe16d23 and olrfe14k04 were expressed only in early regenerating stages when blastema formation was induced. The expression of olrf5n23, which encodes a novel signal peptide, was detected in wound epidermis throughout regeneration. Olrfe23l22, olrfe20n22, and olrfe24i02 were expressed notably in the blastema region. Our study has thus identified the gene expression profiles and some novel candidate genes to facilitate elucidation of the molecular mechanisms of fin regeneration.

Differential regulation of msx genes in the development of the gonopodium, an intromittent organ, and of the "sword," a sexually selected trait of swordtail fishes (Xiphophorus)

The possession of a conspicuous extension of colored ventral rays of the caudal fin in male fish of swordtails (genus Xiphophorus) is a prominent example for a trait that evolved by sexual selection. To understand the evolutionary history of this so-called sword molecularly, it is of interest to unravel the developmental pathways responsible for extended growth of sword rays during development of swordtail males. We isolated two msx genes and showed that they are differentially regulated during sword outgrowth. During sword growth in juvenile males, as well as during testosterone-induced sword development and fin ray regeneration in the sword after amputation, expression of msxC is markedly up-regulated in the sword forming fin rays. In contrast, msxE/1 is not differentially expressed in ventral and dorsal male fin rays, suggesting a link between the development of male secondary sexual characters in fins and up-regulation of msxC expression. In addition, we showed that msx gene expression patterns differ significantly between Xiphophorus and zebrafish. We also included in our study the gonopodium, a testosterone-dependent anal fin modification that serves as a fertilization organ in males of live-bearing fishes. Our finding that increased levels of msxC expression are associated with the testosterone-induced outgrowth of the gonopodium might suggest either that at least parts of the signaling pathways that pattern the evolutionary older gonopodium have been coopted to evolve a sexually selected innovation such as the sword or that increased msxC expression may be inherent to the growth process of long fin rays in general.

Old questions, new tools, and some answers to the mystery of fin regeneration

Pluridisciplinary approaches led to the notion that fin regeneration is an intricate phenomenon involving epithelial-mesenchymal and reciprocal exchanges throughout the process as well as interactions between ray and interray tissue. The establishment of a blastema after fin amputation is the first event leading to the reconstruction of the missing part of the fin. Here, we review our knowledge on the origin of the blastema, its formation and growth, and of the mechanisms that control differentiation and patterning of the regenerate. Our current understanding results from studies of fin regeneration performed in various teleost fish over the past century. We also report the recent breakthroughs that have been made in the past decade with the arrival of a new model, the zebrafish, Danio rerio, which now offers the possibility to combine cytologic, molecular, and genetic analyses and open new perspectives in this field.