Limited dedifferentiation provides replacement tissue during zebrafish fin regeneration

Cellular dedifferentiation. Revisiting Betty Hay's legacy

The concept of mature specialized cells and the stability of the differentiated state was fundamentally challenged by Elizabeth Hay's groundbreaking observations on amphibian limb regeneration, published in 1959. Building on previous work by C.S. Thornton, she discovered that muscle cells could dedifferentiate and transform into progenitor cells within the regeneration blastema reshaping our understanding of cell differentiation. This pivotal finding reshaped our understanding of cell differentiation, opening new avenues of research. Though controversial, her findings significantly advanced the fields of cell plasticity and regenerative biology.

Voltage-gated calcium channels generate blastema Ca 2+ fluxes restraining zebrafish fin regenerative outgrowth

Adult zebrafish fins regenerate to their original size regardless of damage extent, providing a tractable model of organ size and scale control. Gain-of-function of voltage-gated K + channels expressed in fibroblast-lineage blastema cells promotes excessive fin outgrowth, leading to a long-finned phenotype. Similarly, inhibition of the Ca 2+ -dependent phosphatase calcineurin during regeneration causes dramatic fin overgrowth. However, Ca 2+ fluxes and their potential origins from dynamic membrane voltages have not been explored or linked to fin size restoration. We used fibroblast-lineage GCaMP imaging of regenerating adult fins to identify widespread, heterogeneous Ca 2+ transients in distal blastema cells. Membrane depolarization of isolated regenerating fin fibroblasts triggered Ca 2+ spikes dependent on voltage-gated Ca 2+ channel activity. Single cell transcriptomics identified the voltage-gated Ca 2+ channels cacna1c (L-type channel), cacna1ba (N-type), and cacna1g (T-type) as candidate mediators of fibroblast-lineage Ca 2+ signaling. Small molecule inhibition revealed L- and/or N-type voltage-gated Ca 2+ channels act during regenerative outgrowth to restore fins to their original scale. Strikingly, cacna1g homozygous mutant zebrafish regenerated extraordinarily long fins due to prolonged outgrowth. The regenerated fins far exceeded their original length but with otherwise normal ray skeletons. Therefore, cacna1g mutants uniquely provide a genetic loss-of-function long-finned model that decouples developmental and regenerative fin outgrowth. Live GCaMP imaging of regenerating fins showed T-type Cacna1g channels enable Ca 2+ dynamics in distal fibroblast-lineage blastemal mesenchyme during the outgrowth phase. We conclude "bioelectricity" for fin size control likely entirely reflects voltage-modulated Ca 2+ dynamics in fibroblast-lineage blastemal cells that specifically and steadily decelerates outgrowth at a rate tuned to restore the original fin size.

Decaying and expanding Erk gradients process memory of skeletal size during zebrafish fin regeneration

Regeneration of an amputated salamander limb or fish fin restores pre-injury size and structure, illustrating the phenomenon of positional memory. Although appreciated for centuries, the identity of position-dependent cues and how they control tissue growth are not resolved. Here, we quantify Erk signaling events in whole populations of osteoblasts during zebrafish fin regeneration. We find that osteoblast Erk activity is dependent on Fgf receptor signaling and organized into millimeter-long gradients that extend from the distal tip to the amputation site. Erk activity scales with the amount of tissue amputated, predicts the likelihood of osteoblast cycling, and predicts the size of regenerated skeletal structures. Mathematical modeling suggests gradients are established by the transient deposition of long-lived ligands that are transported by tissue growth. This concept is supported by the observed scaling of expression of the essential epidermal ligand fgf20a with extents of amputation. Our work provides evidence that localized, scaled expression of pro-regenerative ligands instructs long-range signaling and cycling to control skeletal size in regenerating appendages.

Disruption of BMP and FGF signaling prior to blastema formation causes permanent bending and skeletal malformations in Poecilia latipinna tail fin

Teleost fish, such as Poecilia latipinna, exhibit remarkable regenerative capabilities, making them excellent models for studying tissue regrowth. They regenerate body parts like the tail fin through epimorphic regeneration, involving wound healing, blastema formation (a pool of proliferative cells), and tissue differentiation. Bone Morphogenetic Protein (BMP) and Fibroblast Growth Factor (FGF) signaling pathways play crucial roles in this process, but their specific functions during blastema formation remain unclear. To explore this, BMP and FGF signaling were inhibited using targeted drug treatments prior to blastema formation in amputated tail fins. The treatment group of P. latipinna received drugs at set intervals, and analyses were conducted using skeletal staining, gene expression via quantitative real-time PCR, and protein analysis with Western blotting to assess blastema formation, extracellular matrix (ECM) remodeling, and skeletal patterning. Dual inhibition of BMP and FGF pathways led to significant regenerative defects, including bent blastema and disrupted bone structure, along with downregulation of essential patterning genes like sonic hedgehog (Shh) and bmp2b. Additionally, ECM remodeling and epithelial-to-mesenchymal transition (EMT) were impaired, as shown by reduced matrix metalloproteinases (MMP2 and MMP9), hindering cell migration and blastema stability. Cell proliferation was markedly decreased, as evidenced by reduced proliferating cell nuclear antigen (PCNA) expression and bromodeoxyuridine (BrdU) incorporation, while apoptosis increased, with elevated markers like caspase 3 (casp3) and higher DNA fragmentation. These findings indicate that BMP and FGF signaling are essential for blastema formation and skeletal patterning, with their inhibition causing major regenerative abnormalities.

Early exercise disrupts a pro-repair extracellular matrix program during zebrafish fin regeneration

Understanding how mechanical stimulation from exercise influences cellular responses during tissue repair could enhance therapeutic strategies. We explored zebrafish caudal fin regeneration to study exercise impacts on a robust model of tissue regeneration. We used a swim tunnel to determine that exercise initiated during but not after blastema establishment impaired fin regeneration, including of the bony ray skeleton. Long-term tracking of fluorescently labeled cell lineages showed exercise disrupted blastemal mesenchyme formation. Transcriptomic profiling and section staining indicated exercise reduced an extracellular matrix (ECM) gene expression program, including for hyaluronic acid (HA) synthesis. Like exercise, HA synthesis inhibition or blastemal HA depletion disrupted blastema formation. We considered if injury-upregulated HA establishes a pro-regenerative environment facilitating mechanotransduction. HA density across the blastema correlated with nuclear localization of the mechanotransducer Yes-associated protein (Yap). Further, exercise loading or reducing HA decreased nuclear Yap and cell proliferation. We conclude early exercise during fin regeneration disrupts expression of an HA-rich ECM supporting blastema expansion. These results highlight the interface between mechanotransduction and ECM as consideration for timing exercise interventions and developing regenerative therapies.

Significance Statement

Controlled exercise promotes healing and recovery from severe skeletal injuries. However, properly timed interventions are essential to promote recovery and prevent further damage. We use zebrafish caudal fin regeneration to mechanistically study exercise impacts on a naturally robust and experimentally accessible model of tissue repair. We link detrimental early exercise effects during fin regeneration to impaired ECM synthesis, mechanotransduction, and cell proliferation. These insights could explain the value of delaying the onset of physical therapy and suggest pursuing therapies that maintain ECM integrity for regenerative rehabilitation.

CUT&Tag applied to zebrafish adult tail fins reveals a return of embryonic H3K4me3 patterns during regeneration

Regenerative potential is governed by a complex process of transcriptional reprogramming, involving chromatin reorganization and dynamics in transcription factor binding patterns throughout the genome. The degree to which chromatin and epigenetic changes contribute to this process remains only partially understood. Here we provide a modified CUT&Tag protocol suitable for improved characterization and interrogation of changes in chromatin modifications during adult fin regeneration in zebrafish. Our protocol generates data that recapitulates results from previously published ChIP-Seq methods, requires far fewer cells as input, and significantly improves signal to noise ratios. We deliver high-resolution enrichment maps for H3K4me3 of uninjured and regenerating fin tissues. During regeneration, we find that H3K4me3 levels increase over gene promoters which become transcriptionally active and genes which lose H3K4me3 become silenced. Interestingly, these reprogramming events recapitulate the H3K4me3 patterns observed in developing fin folds of 24-h old zebrafish embryos. Our results indicate that changes in genomic H3K4me3 patterns during fin regeneration occur in a manner consistent with reactivation of developmental programs, demonstrating CUT&Tag to be an effective tool for profiling chromatin landscapes in regenerating tissues.

CUT&Tag Applied to Zebrafish Adult Tail Fins Reveals a Return of Embryonic H3K4me3 Patterns During Regeneration

Regenerative potential is governed by a complex process of transcriptional reprogramming, involving chromatin reorganization and dynamics in transcription factor binding patterns throughout the genome. The degree to which chromatin and epigenetic changes contribute to this process remains partially understood. Here we provide a modified CUT&Tag protocol suitable for improved characterization and interrogation of epigenetic changes during adult fin regeneration in zebrafish. Our protocol generates data that recapitulates results from previously published ChIP-Seq methods, requires far fewer cells as input, and significantly improves signal to noise ratios. We deliver high-resolution enrichment maps for H3K4me3 of uninjured and regenerating fin tissues. During regeneration, we find that H3K4me3 levels increase over gene promoters which become transcriptionally active and genes which lose H3K4me3 become silenced. Interestingly, these epigenetic reprogramming events recapitulate the H3K4me3 patterns observed in developing fin folds of 24-hour old zebrafish embryos. Our results indicate that changes in genomic H3K4me3 patterns during fin regeneration occur in a manner consistent with reactivation of developmental programs, demonstrating CUT&Tag to be an effective tool for profiling chromatin landscapes in regenerating tissues.

Compartmentalization and synergy of osteoblasts drive bone formation in the regenerating fin

Zebrafish regenerate their fins which involves a component of cell plasticity. It is currently unclear how regenerate cells divide labor to allow for appropriate growth and patterning. Here, we studied lineage relationships of fluorescence-activated cell sorting-enriched epidermal, bone-forming (osteoblast), and (non-osteoblast) blastemal fin regenerate cells by single-cell RNA sequencing, lineage tracing, targeted osteoblast ablation, and electron microscopy. Most osteoblasts in the outgrowing regenerate derive from osterix+ osteoblasts, while mmp9+ cells reside at segment joints. Distal blastema cells contribute to distal osteoblast progenitors, suggesting compartmentalization of the regenerating appendage. Ablation of osterix+ osteoblasts impairs segment joint and bone matrix formation and decreases regenerate length which is partially compensated for by distal regenerate cells. Our study characterizes expression patterns and lineage relationships of rare fin regenerate cell populations, indicates inherent detection and compensation of impaired regeneration, suggests variable dependence on growth factor signaling, and demonstrates zonation of the elongating fin regenerate.

Independent mesenchymal progenitor pools respectively produce and maintain osteogenic and chondrogenic cells in zebrafish

Skeletal tissues including cartilage and bones are characteristic features of vertebrates that are crucial for supporting body morphology and locomotion. Studies mainly in mice have shown that osteoblasts and chondroblasts are supplied from several progenitors like the sclerotome cells in the embryonic stage, osteo-chondroprogenitors in growing long bones, and skeletal stem cells of bone marrow in the postnatal period. However, the exact origins of progenitor cells, their lineage relationships, and their potential to differentiate into osteoblasts and chondroblasts from embryos to adult tissues are not well understood. In this study, we conducted clonal cell tracking in zebrafish and showed that sox9a+ cells are already committed to either chondrogenic or osteogenic fates during embryonic stages and that respective progenies are independently maintained as mesenchymal progenitor pools. Once committed, they never change their lineage identities throughout animal life, even through regeneration. In addition, we further revealed that only osteogenic mesenchymal cells replenish the osteoblast progenitor cells (OPCs), a population of reserved tissue stem cells found to be involved in the de novo production of osteoblasts during regeneration and homeostasis in zebrafish. Thus, our clonal cell tracking study in zebrafish firstly revealed that the mesenchymal progenitor cells that are fated to develop into either chondroblasts or osteoblasts serve as respective tissue stem cells to maintain skeletal tissue homeostasis. Such mesenchymal progenitors dedicated to producing either chondroblasts or osteoblasts would be important targets for skeletal tissue regeneration.

Section Immunostaining for Protein Expression and Cell Proliferation Studies of Regenerating Fins

Adult zebrafish fins fully regenerate after resection, providing a highly accessible and remarkable vertebrate model of organ regeneration. Fin injury triggers wound epidermis formation and the dedifferentiation of injury-adjacent mature cells to establish an organized blastema of progenitor cells. Balanced cell proliferation and redifferentiation along with cell movements then progressively reestablish patterned tissues and restore the fin to its original size and shape. A mechanistic understanding of these coordinated cell behaviors and transitions requires direct knowledge of proteins in their physiological context, including expression, subcellular localization, and activity. Antibody-based staining of sectioned fins facilitates such high-resolution analyses of specific, native proteins. Therefore, such methods are mainstays of comprehensive, hypothesis-driven fin regeneration studies. However, section immunostaining requires labor-intensive, empirical optimization. Here, we present detailed, multistep procedures for antibody staining and co-detecting proliferating cells using paraffin and frozen fin sections. We include suggestions to avoid common pitfalls and to streamline the development of optimized, validated protocols for new and challenging antibodies.

Primary cilia in skeletal development and disease

Primary cilia are non-motile, microtubule-based sensory organelle present in most vertebrate cells with a fundamental role in the modulation of organismal development, morphogenesis, and repair. Here we focus on the role of primary cilia in embryonic and postnatal skeletal development. We examine evidence supporting its involvement in physiochemical and developmental signaling that regulates proliferation, patterning, differentiation and homeostasis of osteoblasts, chondrocytes, and their progenitor cells in the skeleton. We discuss how signaling effectors in mechanotransduction and bone development, such as Hedgehog, Wnt, Fibroblast growth factor and second messenger pathways operate at least in part at the primary cilium. The relevance of primary cilia in bone formation and maintenance is underscored by a growing list of rare genetic skeletal ciliopathies. We collate these findings and summarize the current understanding of molecular factors and mechanisms governing primary ciliogenesis and ciliary function in skeletal development and disease.

Insulin-like growth factor receptor / mTOR signaling elevates global translation to accelerate zebrafish fin regenerative outgrowth

Zebrafish robustly regenerate fins, including their characteristic bony ray skeleton. Amputation activates intra-ray fibroblasts and dedifferentiates osteoblasts that migrate under a wound epidermis to establish an organized blastema. Coordinated proliferation and re-differentiation across lineages then sustains progressive outgrowth. We generate a single cell transcriptome dataset to characterize regenerative outgrowth and explore coordinated cell behaviors. We computationally identify sub-clusters representing most regenerative fin cell lineages, and define markers of osteoblasts, intra- and inter-ray fibroblasts and growth-promoting distal blastema cells. A pseudotemporal trajectory and in vivo photoconvertible lineage tracing indicate distal blastemal mesenchyme restores both intra- and inter-ray fibroblasts. Gene expression profiles across this trajectory suggest elevated protein production in the blastemal mesenchyme state. O-propargyl-puromycin incorporation and small molecule inhibition identify insulin growth factor receptor (IGFR)/mechanistic target of rapamycin kinase (mTOR)-dependent elevated bulk translation in blastemal mesenchyme and differentiating osteoblasts. We test candidate cooperating differentiation factors identified from the osteoblast trajectory, finding IGFR/mTOR signaling expedites glucocorticoid-promoted osteoblast differentiation in vitro. Concordantly, mTOR inhibition slows but does not prevent fin regenerative outgrowth in vivo. IGFR/mTOR may elevate translation in both fibroblast- and osteoblast-lineage cells during the outgrowth phase as a tempo-coordinating rheostat.

Hallmarks of peripheral nerve function in bone regeneration

Skeletal tissue is highly innervated. Although different types of nerves have been recently identified in the bone, the crosstalk between bone and nerves remains unclear. In this review, we outline the role of the peripheral nervous system (PNS) in bone regeneration following injury. We first introduce the conserved role of nerves in tissue regeneration in species ranging from amphibians to mammals. We then present the distribution of the PNS in the skeletal system under physiological conditions, fractures, or regeneration. Furthermore, we summarize the ways in which the PNS communicates with bone-lineage cells, the vasculature, and immune cells in the bone microenvironment. Based on this comprehensive and timely review, we conclude that the PNS regulates bone regeneration through neuropeptides or neurotransmitters and cells in the peripheral nerves. An in-depth understanding of the roles of peripheral nerves in bone regeneration will inform the development of new strategies based on bone-nerve crosstalk in promoting bone repair and regeneration.

Fin ray branching is defined by TRAP+ osteolytic tubules in zebrafish

The shaping of bone structures relies on various cell types and signaling pathways. Here, we use the zebrafish bifurcating fin rays during regeneration to investigate bone patterning. We found that the regenerating fin rays form via two mineralization fronts that undergo an osteoblast-dependent fusion/stitching until the branchpoint, and that bifurcation is not simply the splitting of one unit into two. We identified tartrate-resistant acid phosphatase-positive osteolytic tubular structures at the branchpoints, hereafter named osteolytic tubules (OLTs). Chemical inhibition of their bone-resorbing activity strongly impairs ray bifurcation, indicating that OLTs counteract the stitching process. Furthermore, by testing different osteoactive compounds, we show that the position of the branchpoint depends on the balance between bone mineralization and resorption activities. Overall, these findings provide a unique perspective on fin ray formation and bifurcation, and reveal a key role for OLTs in defining the proximo-distal position of the branchpoint.

A regeneration-triggered metabolic adaptation is necessary for cell identity transitions and cell cycle re-entry to support blastema formation and bone regeneration

Regeneration depends on the ability of mature cells at the injury site to respond to injury, generating tissue-specific progenitors that incorporate the blastema and proliferate to reconstitute the original organ architecture. The metabolic microenvironment has been tightly connected to cell function and identity during development and tumorigenesis. Yet, the link between metabolism and cell identity at the mechanistic level in a regenerative context remains unclear. The adult zebrafish caudal fin, and bone cells specifically, have been crucial for the understanding of mature cell contribution to tissue regeneration. Here, we use this model to explore the relevance of glucose metabolism for the cell fate transitions preceding new osteoblast formation and blastema assembly. We show that injury triggers a modulation in the metabolic profile at early stages of regeneration to enhance glycolysis at the expense of mitochondrial oxidation. This metabolic adaptation mediates transcriptional changes that make mature osteoblast amenable to be reprogramed into pre-osteoblasts and induces cell cycle re-entry and progression. Manipulation of the metabolic profile led to severe reduction of the pre-osteoblast pool, diminishing their capacity to generate new osteoblasts, and to a complete abrogation of blastema formation. Overall, our data indicate that metabolic alterations have a powerful instructive role in regulating genetic programs that dictate fate decisions and stimulate proliferation, thereby providing a deeper understanding on the mechanisms regulating blastema formation and bone regeneration.

Spinal cord regeneration - the origins of progenitor cells for functional rebuilding

The spinal cord is one of the most important structures for all vertebrate animals as it connects almost all parts of the body to the brain. Injury to the mammalian spinal cord has devastating consequences, resulting in paralysis with little to no hope of recovery. In contrast, other vertebrate animals have been known for centuries to be capable of functionally regenerating large lesions in the spinal cord. Here, we will review the current knowledge of spinal cord regeneration and recent work in different proregenerative animals that has begun to shed light on the cellular and molecular mechanisms these animals use to direct cells to rebuild a complex, functional spinal cord.

Zebrafish Fin: Complex Molecular Interactions and Cellular Mechanisms Guiding Regeneration

The zebrafish caudal fin has become a popular model to study cellular and molecular mechanisms of regeneration due to its high regenerative capacity, accessibility for experimental manipulations, and relatively simple anatomy. The formation of a regenerative epidermis and blastema are crucial initial events and tightly regulated. Both the regenerative epidermis and the blastema are highly organized structures containing distinct domains, and several signaling pathways regulate the formation and interaction of these domains. Bone is the major tissue regenerated from the progenitor cells of the blastema. Several cellular mechanisms can provide source cells for blastemal (pre-)osteoblasts, including dedifferentiation of differentiated osteoblasts and de novo formation from other cell types, providing intriguing examples of cellular plasticity. In recent years, omics analyses and single-cell approaches have elucidated genetic and epigenetic regulation, increasing our knowledge of the surprisingly complex coordination of various mechanisms to achieve successful restoration of a seemingly simple structure.

Zebrafish fin regeneration involves generic and regeneration-specific osteoblast injury responses

Successful regeneration requires the coordinated execution of multiple cellular responses to injury. In amputated zebrafish fins, mature osteoblasts dedifferentiate, migrate towards the injury and form proliferative osteogenic blastema cells. We show that osteoblast migration is preceded by cell elongation and alignment along the proximodistal axis, which require actomyosin, but not microtubule turnover. Surprisingly, osteoblast dedifferentiation and migration can be uncoupled. Using pharmacological and genetic interventions, we found that NF-ĸB and retinoic acid signalling regulate dedifferentiation without affecting migration, while the complement system and actomyosin dynamics affect migration but not dedifferentiation. Furthermore, by removing bone at two locations within a fin ray, we established an injury model containing two injury sites. We found that osteoblasts dedifferentiate at and migrate towards both sites, while accumulation of osteogenic progenitor cells and regenerative bone formation only occur at the distal-facing injury. Together, these data indicate that osteoblast dedifferentiation and migration represent generic injury responses that are differentially regulated and can occur independently of each other and of regenerative growth. We conclude that successful fin bone regeneration appears to involve the coordinated execution of generic and regeneration-specific responses of osteoblasts to injury.

An Emerging Frontier in Intercellular Communication: Extracellular Vesicles in Regeneration

Regeneration requires cellular proliferation, differentiation, and other processes that are regulated by secreted cues originating from cells in the local environment. Recent studies suggest that signaling by extracellular vesicles (EVs), another mode of paracrine communication, may also play a significant role in coordinating cellular behaviors during regeneration. EVs are nanoparticles composed of a lipid bilayer enclosing proteins, nucleic acids, lipids, and other metabolites, and are secreted by most cell types. Upon EV uptake by target cells, EV cargo can influence diverse cellular behaviors during regeneration, including cell survival, immune responses, extracellular matrix remodeling, proliferation, migration, and differentiation. In this review, we briefly introduce the history of EV research and EV biogenesis. Then, we review current understanding of how EVs regulate cellular behaviors during regeneration derived from numerous studies of stem cell-derived EVs in mammalian injury models. Finally, we discuss the potential of other established and emerging research organisms to expand our mechanistic knowledge of basic EV biology, how injury modulates EV biogenesis, cellular sources of EVs in vivo, and the roles of EVs in organisms with greater regenerative capacity.

Single-cell resolution of MET- and EMT-like programs in osteoblasts during zebrafish fin regeneration

Zebrafish regenerate fin rays following amputation through epimorphic regeneration, a process that has been proposed to involve the epithelial-to-mesenchymal transition (EMT). We performed single-cell RNA sequencing (scRNA-seq) to elucidate osteoblastic transcriptional programs during zebrafish caudal fin regeneration. We show that osteoprogenitors are enriched with components associated with EMT and its reverse, mesenchymal-to-epithelial transition (MET), and provide evidence that the EMT markers cdh11 and twist2 are co-expressed in dedifferentiating cells at the amputation stump at 1 dpa, and in differentiating osteoblastic cells in the regenerate, the latter of which are enriched in EMT signatures. We also show that esrp1, a regulator of alternative splicing in epithelial cells that is associated with MET, is expressed in a subset of osteoprogenitors during outgrowth. This study provides a single cell resource for the study of osteoblastic cells during zebrafish fin regeneration, and supports the contribution of MET- and EMT-associated components to this process.

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.

The Warburg effect is necessary to promote glycosylation in the blastema during zebrafish tail regeneration

Throughout their lifetime, fish maintain a high capacity for regenerating complex tissues after injury. We utilized a larval tail regeneration assay in the zebrafish Danio rerio, which serves as an ideal model of appendage regeneration due to its easy manipulation, relatively simple mixture of cell types, and superior imaging properties. Regeneration of the embryonic zebrafish tail requires development of a blastema, a mass of dedifferentiated cells capable of replacing lost tissue, a crucial step in all known examples of appendage regeneration. Using this model, we show that tail amputation triggers an obligate metabolic shift to promote glucose metabolism during early regeneration similar to the Warburg effect observed in tumor forming cells. Inhibition of glucose metabolism did not affect the overall health of the embryo but completely blocked the tail from regenerating after amputation due to the failure to form a functional blastema. We performed a time series of single-cell RNA sequencing on regenerating tails with and without inhibition of glucose metabolism. We demonstrated that metabolic reprogramming is required for sustained TGF-β signaling and blocking glucose metabolism largely mimicked inhibition of TGF-β receptors, both resulting in an aberrant blastema. Finally, we showed using genetic ablation of three possible metabolic pathways for glucose, that metabolic reprogramming is required to provide glucose specifically to the hexosamine biosynthetic pathway while neither glycolysis nor the pentose phosphate pathway were necessary for regeneration.

Zebrafish as a Model for Germ Cell Regeneration

Germ cell acts as a link between transfer of genetic information and process of species evolution. Defects or malformations of germ cells can lead to infertility or tumors. Germ cell regeneration is one of the effective ways to treat the infertility. Therefore, it is of great scientific and clinical interests to dissect the cellular and molecular mechanisms underlying germ cell regeneration. Progress have already been achieved in germ cell regeneration using model organisms for decades. However, key open issues regarding the underpinning mechanisms still remain poorly understood. Zebrafish is well known for its powerful regenerative capacity to regenerate various tissues and organs. Recently, advances in genomics, genetics, microscopy, and single cell technologies have made zebrafish an attractive model to study germ cell development and regeneration. Here we review recent technologies for the study of germ cell regeneration in zebrafish, highlight the potential of germline stem cells (GSCs) in the contribution to reproductive system regeneration, and discuss the nanos. Wnt signaling and germ cell-specific factors involved in the regulation of germ cell regeneration.

longfin causes cis-ectopic expression of the kcnh2a ether-a-go-go K+ channel to autonomously prolong fin outgrowth

Organs stop growing to achieve a characteristic size and shape in scale with the body of an animal. Likewise, regenerating organs sense injury extents to instruct appropriate replacement growth. Fish fins exemplify both phenomena through their tremendous diversity of form and remarkably robust regeneration. The classic zebrafish mutant longfint2 develops and regenerates dramatically elongated fins and underlying ray skeleton. We show longfint2 chromosome 2 overexpresses the ether-a-go-go-related voltage-gated potassium channel kcnh2a. Genetic disruption of kcnh2a in cis rescues longfint2, indicating longfint2 is a regulatory kcnh2a allele. We find longfint2 fin overgrowth originates from prolonged outgrowth periods by showing Kcnh2a chemical inhibition during late stage regeneration fully suppresses overgrowth. Cell transplantations demonstrate longfint2-ectopic kcnh2a acts tissue autonomously within the fin intra-ray mesenchymal lineage. Temporal inhibition of the Ca2+-dependent phosphatase calcineurin indicates it likewise entirely acts late in regeneration to attenuate fin outgrowth. Epistasis experiments suggest longfint2-expressed Kcnh2a inhibits calcineurin output to supersede growth cessation signals. We conclude ion signaling within the growth-determining mesenchyme lineage controls fin size by tuning outgrowth periods rather than altering positional information or cell-level growth potency.

Animal regeneration in the era of transcriptomics

Animal regeneration, the ability to restore a lost body part, is a process that has fascinated scientists for centuries. In this review, we first present what regeneration is and how it relates to development, as well as the widespread and diverse nature of regeneration in animals. Despite this diversity, animal regeneration includes three common mechanistic steps: initiation, induction and activation of progenitors, and morphogenesis. In this review article, we summarize and discuss, from an evolutionary perspective, the recent data obtained for a variety of regeneration models which have allowed to identify key shared mechanisms that control these main steps of animal regeneration. This review also synthesizes the wealth of high-throughput mRNA sequencing data (bulk mRNA-seq) concerning regeneration which have been obtained in recent years, highlighting the major advances in the regeneration field that these studies have revealed. We stress out that, through a comparative approach, these data provide opportunities to further shed light on the evolution of regeneration in animals. Finally, we point out how the use of single-cell mRNA-seq technology and integration with epigenomic approaches may further help researchers to decipher mechanisms controlling regeneration and their evolution in animals.

From Cell Death to Regeneration: Rebuilding After Injury

The ability to regrow lost or damaged tissues is widespread, but highly variable among animals. Understanding this variation remains a challenge in regeneration biology. Numerous studies from Hydra to mouse have shown that apoptosis acts as a potent and necessary mechanism in regeneration. Much is known about the involvement of apoptosis during normal development in regulating the number and type of cells in the body. In the context of regeneration, apoptosis also regulates cell number and proliferation in tissue remodeling. Apoptosis acts both early in the process to stimulate regeneration and later to regulate regenerative patterning. Multiple studies indicate that apoptosis acts as a signal to stimulate proliferation within the regenerative tissues, producing the cells needed for full regeneration. The conservation of apoptosis as a regenerative mechanism demonstrated across species highlights its importance and motivates the continued investigation of this important facet of programmed cell death. This review summarizes what is known about the roles of apoptosis during regeneration, and compares regenerative apoptosis with the mechanisms and function of apoptosis in development. Defining the complexity of regenerative apoptosis will contribute to new knowledge and perspectives for understanding mechanisms of apoptosis induction and regulation.

Skeletal Biology and Disease Modeling in Zebrafish

Zebrafish are teleosts (bony fish) that share with mammals a common ancestor belonging to the phylum Osteichthyes, from which their endoskeletal systems have been inherited. Indeed, teleosts and mammals have numerous genetically conserved features in terms of skeletal elements, ossification mechanisms, and bone matrix components in common. Yet differences related to bone morphology and function need to be considered when investigating zebrafish in skeletal research. In this review, we focus on zebrafish skeletal architecture with emphasis on the morphology of the vertebral column and associated anatomical structures. We provide an overview of the different ossification types and osseous cells in zebrafish and describe bone matrix composition at the microscopic tissue level with a focus on assessing mineralization. Processes of bone formation also strongly depend on loading in zebrafish, as we elaborate here. Furthermore, we illustrate the high regenerative capacity of zebrafish bones and present some of the technological advantages of using zebrafish as a model. We highlight zebrafish axial and fin skeleton patterning mechanisms, metabolic bone disease such as after immunosuppressive glucocorticoid treatment, as well as osteogenesis imperfecta (OI) and osteopetrosis research in zebrafish. We conclude with a view of why larval zebrafish xenografts are a powerful tool to study bone metastasis. © 2021 American Society for Bone and Mineral Research (ASBMR).

Cellular and molecular mechanisms that regulate mammalian digit tip regeneration

Digit tip regeneration is one of the few examples of true multi-tissue regeneration in an adult mammal. The key step in this process is the formation of the blastema, a transient proliferating cell mass that generates the different cell types of the digit to replicate the original structure. Failure to form the blastema results in a lack of regeneration and has been postulated to be the reason why mammalian limbs cannot regrow following amputation. Understanding how the blastema forms and functions will help us to determine what is required for mammalian regeneration to occur and will provide insights into potential therapies for mammalian tissue regeneration and repair. This review summarizes the cellular and molecular mechanisms that influence murine blastema formation and govern digit tip regeneration.

Cellular diversity of the regenerating caudal fin

Zebrafish faithfully regenerate their caudal fin after amputation. During this process, both differentiated cells and resident progenitors migrate to the wound site and undergo lineage-restricted, programmed cellular state transitions to populate the new regenerate. Until now, systematic characterizations of cells comprising the new regenerate and molecular definitions of their state transitions have been lacking. We hereby characterize the dynamics of gene regulatory programs during fin regeneration by creating single-cell transcriptome maps of both preinjury and regenerating fin tissues at 1/2/4 days post-amputation. We consistently identified epithelial, mesenchymal, and hematopoietic populations across all stages. We found common and cell type-specific cell cycle programs associated with proliferation. In addition to defining the processes of epithelial replenishment and mesenchymal differentiation, we also identified molecular signatures that could better distinguish epithelial and mesenchymal subpopulations in fish. The insights for natural cell state transitions during regeneration point to new directions for studying this regeneration model.

Cells Isolated from Regenerating Caudal Fin of Sparus aurata Can Differentiate into Distinct Bone Cell Lineages

Teleosts have the ability to regenerate their caudal fin upon amputation. A highly proliferative mass of undifferentiated cells called blastema forms beneath wound epidermis and differentiates to regenerate all missing parts of the fin. To date, the origin and fate of the blastema is not completely understood. However, current hypotheses suggest that the blastema is comprised of lineage-restricted dedifferentiated cells. To investigate the differentiation capacity of regenerating fin-derived cells, primary cultures were initiated from the explants of 2-days post-amputation (dpa) regenerates of juvenile gilthead seabream (Sparus aurata). These cells were subcultured for over 30 passages and were named as BSa2. After 10 passages they were characterized for their ability to differentiate towards different bone cell lineages and mineralize their extracellular matrix, through immunocytochemistry, histology, and RT-PCR. Exogenous DNA was efficiently delivered into these cells by nucleofection. Assessment of lineage-specific markers revealed that BSa2 cells were capable of osteo/chondroblastic differentiation. BSa2 cells were also found to be capable of osteoclastic differentiation, as demonstrated through TRAP-specific staining and pit resorption assay. Here, we describe the development of the first successful cell line viz., BSa2, from S. aurata 2-dpa regenerating caudal fins, which has the ability of multilineage differentiation and is capable of in vitro mineralization. The availability of such in vitro cell systems has the potential to stimulate research on the mechanisms of cell differentiation during fin regeneration and provide new insights into the mechanisms of bone formation.

Twist3 is required for dedifferentiation during extraocular muscle regeneration in adult zebrafish

Severely damaged adult zebrafish extraocular muscles (EOMs) regenerate through dedifferentiation of residual myocytes involving a muscle-to-mesenchyme transition. Members of the Twist family of basic helix-loop-helix transcription factors (TFs) are key regulators of the epithelial-mesenchymal transition (EMT) and are also involved in craniofacial development in humans and animal models. During zebrafish embryogenesis, twist family members (twist1a, twist1b, twist2, and twist3) function to regulate craniofacial skeletal development. Because of their roles as master regulators of stem cell biology, we hypothesized that twist TFs regulate adult EOM repair and regeneration. In this study, utilizing an adult zebrafish EOM regeneration model, we demonstrate that inhibiting twist3 function using translation-blocking morpholino oligonucleotides (MOs) impairs muscle regeneration by reducing myocyte dedifferentiation and proliferation in the regenerating muscle. This supports our hypothesis that twist TFs are involved in the early steps of dedifferentiation and highlights the importance of twist3 during EOM regeneration.

Skeletal stem cells: insights into maintaining and regenerating the skeleton

Skeletal stem cells (SSCs) generate the progenitors needed for growth, maintenance and repair of the skeleton. Historically, SSCs have been defined as bone marrow-derived cells with inconsistent characteristics. However, recent in vivo tracking experiments have revealed the presence of SSCs not only within the bone marrow but also within the periosteum and growth plate reserve zone. These studies show that SSCs are highly heterogeneous with regard to lineage potential. It has also been revealed that, during digit tip regeneration and in some non-mammalian vertebrates, the dedifferentiation of osteoblasts may contribute to skeletal regeneration. Here, we examine how these research findings have furthered our understanding of the diversity and plasticity of SSCs that mediate skeletal maintenance and repair.

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

BACKGROUND

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

RESULTS

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

CONCLUSION

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

Acquisition of a Unique Mesenchymal Precursor-like Blastema State Underlies Successful Adult Mammalian Digit Tip Regeneration

Here, we investigate the origin and nature of blastema cells that regenerate the adult murine digit tip. We show that Pdgfra-expressing mesenchymal cells in uninjured digits establish the regenerative blastema and are essential for regeneration. Single-cell profiling shows that the mesenchymal blastema cells are distinct from both uninjured digit and embryonic limb or digit Pdgfra-positive cells. This unique blastema state is environmentally determined; dermal fibroblasts transplanted into the regenerative, but not non-regenerative, digit express blastema-state genes and contribute to bone regeneration. Moreover, lineage tracing with single-cell profiling indicates that endogenous osteoblasts or osteocytes acquire a blastema mesenchymal transcriptional state and contribute to both dermis and bone regeneration. Thus, mammalian digit tip regeneration occurs via a distinct adult mechanism where the regenerative environment promotes acquisition of a blastema state that enables cells from tissues such as bone to contribute to the regeneration of other mesenchymal tissues such as the dermis.

Zebrafish: A Resourceful Vertebrate Model to Investigate Skeletal Disorders

Animal models are essential tools for addressing fundamental scientific questions about skeletal diseases and for the development of new therapeutic approaches. Traditionally, mice have been the most common model organism in biomedical research, but their use is hampered by several limitations including complex generation, demanding investigation of early developmental stages, regulatory restrictions on breeding, and high maintenance cost. The zebrafish has been used as an efficient alternative vertebrate model for the study of human skeletal diseases, thanks to its easy genetic manipulation, high fecundity, external fertilization, transparency of rapidly developing embryos, and low maintenance cost. Furthermore, zebrafish share similar skeletal cells and ossification types with mammals. In the last decades, the use of both forward and new reverse genetics techniques has resulted in the generation of many mutant lines carrying skeletal phenotypes associated with human diseases. In addition, transgenic lines expressing fluorescent proteins under bone cell- or pathway- specific promoters enable in vivo imaging of differentiation and signaling at the cellular level. Despite the small size of the zebrafish, many traditional techniques for skeletal phenotyping, such as x-ray and microCT imaging and histological approaches, can be applied using the appropriate equipment and custom protocols. The ability of adult zebrafish to remodel skeletal tissues can be exploited as a unique tool to investigate bone formation and repair. Finally, the permeability of embryos to chemicals dissolved in water, together with the availability of large numbers of small-sized animals makes zebrafish a perfect model for high-throughput bone anabolic drug screening. This review aims to discuss the techniques that make zebrafish a powerful model to investigate the molecular and physiological basis of skeletal disorders.

New Insights into the Control of Cell Fate Choices and Differentiation by Retinoic Acid in Cranial, Axial and Caudal Structures

Retinoic acid (RA) signaling is an important regulator of chordate development. RA binds to nuclear RA receptors that control the transcriptional activity of target genes. Controlled local degradation of RA by enzymes of the Cyp26a gene family contributes to the establishment of transient RA signaling gradients that control patterning, cell fate decisions and differentiation. Several steps in the lineage leading to the induction and differentiation of neuromesodermal progenitors and bone-producing osteogenic cells are controlled by RA. Changes to RA signaling activity have effects on the formation of the bones of the skull, the vertebrae and the development of teeth and regeneration of fin rays in fish. This review focuses on recent advances in these areas, with predominant emphasis on zebrafish, and highlights previously unknown roles for RA signaling in developmental processes.

Regulation of Genomic Output and (Pluri)potency in Regeneration

Regeneration is a remarkable phenomenon that has been the subject of awe and bafflement for hundreds of years. Although regeneration competence is found in highly divergent organisms throughout the animal kingdom, recent advances in tools used for molecular and genomic characterization have uncovered common genes, molecular mechanisms, and genomic features in regenerating animals. In this review we focus on what is known about how genome regulation modulates cellular potency during regeneration. We discuss this regulation in the context of complex tissue regeneration in animals, from Hydra to humans, with reference to ex vivo-cultured cell models of pluripotency when appropriate. We emphasize the importance of a detailed molecular understanding of both the mechanisms that regulate genomic output and the functional assays that assess the biological relevance of such molecular characterizations.

Model systems for regeneration: zebrafish

Tissue damage can resolve completely through healing and regeneration, or can produce permanent scarring and loss of function. The response to tissue damage varies across tissues and between species. Determining the natural mechanisms behind regeneration in model organisms that regenerate well can help us develop strategies for tissue recovery in species with poor regenerative capacity (such as humans). The zebrafish (Danio rerio) is one of the most accessible vertebrate models to study regeneration. In this Primer, we highlight the tools available to study regeneration in the zebrafish, provide an overview of the mechanisms underlying regeneration in this system and discuss future perspectives for the field.

Distribution and Restoration of Serotonin-Immunoreactive Paraneuronal Cells During Caudal Fin Regeneration in Zebrafish

Aquatic vertebrates possess diverse types of sensory cells in their skin to detect stimuli in the water. In the adult zebrafish, a common model organism, the presence of such cells in fins has only rarely been studied. Here, we identified scattered serotonin (5-HT)-positive cells in the epidermis of the caudal fin. These cells were distinct from keratinocytes as revealed by their low immunoreactivity for cytokeratin and desmosome markers. Instead, they were detected by Calretinin (Calbindin-2) and Synaptic vesicle glycoprotein 2 (SV2) antibodies, indicating a calcium-regulated neurosecretory activity. Consistently, electron microscopy revealed abundant secretory organelles in desmosome-negative cells in the fin epidermis. Based on the markers, 5-HT, Calretinin and SV2, we referred to these cells as HCS-cells. We found that HCS-cells were spread throughout the entire caudal fin at an average density of 140 cells per mm² on each fin surface. These cells were strongly enriched at ray bifurcations in wild type fins, as well as in elongated fins of another longfin mutant fish. To determine whether hydrodynamics play a role in the distribution of HCS-cells, we used an interdisciplinary approach and performed kinematic analysis. Measurements of particle velocity with a fin model revealed differences in fluid velocities between bifurcated rods and adjacent non-bifurcated regions. Therefore the accumulation of HCS-cells near bone bifurcations may be a biological adaptation for sensing of water parameters. The significance of this HCS-cell pattern is reinforced by the fact, that it is reestablished in the regenerated fin after amputation. Regeneration of HCS-cells was not impaired by the chemical inhibition of serotonin synthesis, suggesting that this neurotransmitter is not essential for the restorative process. In conclusion, our study identified a specific population of solitary paraneurons in the zebrafish fin, whose distribution correlates with fluid dynamics.

Recording development with single cell dynamic lineage tracing

Every animal grows from a single fertilized egg into an intricate network of cell types and organ systems. This process is captured in a lineage tree: a diagram of every cell's ancestry back to the founding zygote. Biologists have long sought to trace this cell lineage tree in individual organisms and have developed a variety of technologies to map the progeny of specific cells. However, there are billions to trillions of cells in complex organisms, and conventional approaches can only map a limited number of clonal populations per experiment. A new generation of tools that use molecular recording methods integrated with single cell profiling technologies may provide a solution. Here, we summarize recent breakthroughs in these technologies, outline experimental and computational challenges, and discuss biological questions that can be addressed using single cell dynamic lineage tracing.

Fin healing and regeneration in sturgeon

Pectoral fin healing in fin spines and rays were examined in juvenile Atlantic sturgeon Acipenser oxyrinchus oxyrinchus following three different sampling techniques (n = 8-9 fish per treatment): entire leading fin spine removed, a 1-2 cm portion removed near the point of articulation, or a 1-2 cm portion removed from a secondary fin ray. Also, to determine whether antibiotic treatment influences healing, an additional group of fish (n = 8) was not given an injection of an oxytetracycline (OTC)-based antibiotic following removal of the entire leading fin spine. Following fin sampling, fish from different treatments were mixed equally between three large (4,000 I) recirculating systems and fin-ray healing and mortality were monitored over a 12 month period. To assess healing, blood samples were collected at 4 months to measure immune system responses, radiographs were taken at 4, 8 and 12 months to assess the degree of calcification in regions of damaged fins and fins were analyzed histologically at 12 months. Fish grew from a mean weight of 1.8 to 3.2 kg during the experiment and survival was near 100% in all treatments, with only one fish dying of unknown causes. Leukocyte counts, an indication of health status and survival were similar among treatments and in groups with or without antibiotic injection. Radiographs revealed mineralization took longer in fish with the entire leading fin spine removed and was the slowest near the point of articulation, presumably due to the greater structural support for the pectoral fin at this location. Histological sampling indicated spines and rays had similar healing patterns. Following injury, an orderly matrix of collagen bundles and many evenly spaced scleroblasts were present, transitioning to Sharpey fibres, with concentric layers forming lamellar bone. Healing and mineralization were characterized as periosteal osteogenesis and included embedded osteocytes surrounded by an osteoid seam. Chondroid formation was apparent in a few fractures not associated with treatments. The duration of time for external wound healing and internal mineralization of spines and rays depended on the fin treatment, with the slowest healing observed in fish with the most tissue removed, the entire leading fin spine.

Epigenetic reprogramming by histone acetyltransferase HAG1/AtGCN5 is required for pluripotency acquisition in Arabidopsis

Shoot regeneration can be achieved in vitro through a two-step process involving the acquisition of pluripotency on callus-induction media (CIM) and the formation of shoots on shoot-induction media. Although the induction of root-meristem genes in callus has been noted recently, the mechanisms underlying their induction and their roles in de novo shoot regeneration remain unanswered. Here, we show that the histone acetyltransferase HAG1/AtGCN5 is essential for de novo shoot regeneration. In developing callus, it catalyzes histone acetylation at several root-meristem gene loci including WOX5, WOX14, SCR, PLT1, and PLT2, providing an epigenetic platform for their transcriptional activation. In turn, we demonstrate that the transcription factors encoded by these loci act as key potency factors conferring regeneration potential to callus and establishing competence for de novo shoot regeneration. Thus, our study uncovers key epigenetic and potency factors regulating plant-cell pluripotency. These factors might be useful in reprogramming lineage-specified plant cells to pluripotency.

Towards a gene regulatory network shaping the fins of the Princess cichlid

Variation in fin shape and size contributes to the outstanding morphological diversity of teleost fishes, but the regulation of fin growth has not yet been studied extensively outside the zebrafish model. A previous gene expression study addressing the ornamental elongations of unpaired fins in the African cichlid fish Neolamprologus brichardi identified three genes (cx43, mmp9 and sema3d) with strong and consistent expression differences between short and elongated fin regions. Remarkably, the expression patterns of these genes were not consistent with inferences on their regulatory interactions in zebrafish. Here, we identify a gene expression network (GRN) comprising cx43, mmp9, and possibly also sema3d by a stepwise approach of identifying co-expression modules and predicting their upstream regulators. Among the transcription factors (TFs) predicted as potential upstream regulators of 11 co-expressed genes, six TFs (foxc1, foxp1, foxd3, myc, egr2, irf8) showed expression patterns consistent with their cooperative transcriptional regulation of the gene network. Some of these TFs have already been implicated in teleost fish fin regeneration and formation. We particularly discuss the potential function of foxd3 as driver of the network and its role in the unexpected gene expression correlations observed in N. brichardi.

VEGF, FGF-2 and TGFβ expression in the normal and regenerating epidermis of geckos: implications for epidermal homeostasis and wound healing in reptiles

The skin is a bilayered organ that serves as a key barrier between an organism and its environment. In addition to protecting against microbial invasion, physical trauma and environmental damage, skin participates in maintaining homeostasis. Skin is also capable of spontaneous self-repair following injury. These functions are mediated by numerous pleiotrophic growth factors, including members of the vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), and transforming growth factor β (TGFβ) families. Although growth factor expression has been well documented in mammals, particularly during wound healing, for groups such as reptiles less is known. Here, we investigate the spatio-temporal pattern of expression of multiple growth factors in normal skin and following a full-thickness cutaneous injury in the representative lizard Eublepharis macularius, the leopard gecko. Unlike mammals, leopard geckos can heal cutaneous wounds without scarring. We demonstrate that before, during and after injury, keratinocytes of the epidermis express a diverse panel of growth factor ligands and receptors, including: VEGF, VEGFR1, VEGFR2, and phosphorylated VEGFR2; FGF-2 and FGFR1; and phosphorylated SMAD2, TGFβ1, and activin βA. Unexpectedly, only the tyrosine kinase receptors VEGFR1 and FGFR1 were dynamically expressed, and only during the earliest phases of re-epithelization; otherwise all the proteins of interest were constitutively present. We propose that the ubiquitous pattern of growth factor expression by keratinocytes is associated with various roles during tissue homeostasis, including protection against ultraviolet photodamage and coordinated body-wide skin shedding.

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.