Roles for Fgf signaling during zebrafish fin regeneration

A developmental transition in growth control during zebrafish caudal fin development

A long-standing question in developmental biology is how do growing and developing animals achieve form and then maintain it. We have revealed a critical transition in growth control during zebrafish caudal fin development, wherein a switch from allometric to isometric growth occurs. This morphological transition led us to hypothesize additional physiological changes in growth control pathways. To test this, we fasted juvenile and adult zebrafish. Juvenile fins continued allometric growth until development of the mature bi-lobed shape was completed. In contrast, the isometric growth of mature adult fins arrested within days of initiating a fast. We explored the biochemical basis of this difference in physiology between the two phases by assessing the sensitivity to rapamycin, a drug that blocks a nutrient-sensing pathway. We show that the nutrition-independent, allometric growth phase is resistant to rapamycin at 10-fold higher concentrations than are effective at arresting growth in the nutrition-dependent, isometric growth phase. We thus link a morphological transition in growth control between allometric and isometric growth mechanisms to different physiological responses to nutritional state of the animal and finally to different pharmacological responses to a drug (rapamycin) that affects the nutrition-sensing mechanism described from yeast to human.

Chemical modulation of receptor signaling inhibits regenerative angiogenesis in adult zebrafish

We examined the role of angiogenesis and the need for receptor signaling using chemical inhibition of the vascular endothelial growth factor receptor in the adult zebrafish tail fin. Using a small-molecule inhibitor, we were able to exert precise control over blood vessel regeneration. An angiogenic limit to tissue regeneration was determined, as avascular tissue containing skin, pigment, neuronal axons and bone precursors could regenerate up to about 1 mm. This indicates that tissues can regenerate without direct interaction with endothelial cells and at a distance from blood supply. We also investigated whether the effects of chemical inhibition could be enhanced in zebrafish vascular mutants. We found that adult zebrafish, heterozygous for a mutation in the critical receptor effector phospholipase Cgamma1, show a greater sensitivity to chemical inhibition. This study illustrates the utility of the adult zebrafish as a new model system for receptor signaling and chemical biology.

Characterization of two new zebrafish members of the hedgehog family: atypical expression of a zebrafish indian hedgehog gene in skeletal elements of both endochondral and dermal origins

We have characterized two new members of the Hedgehog (Hh) family in zebrafish, ihha and dhh, encoding for orthologues of the tetrapod Indian Hedgehog (Ihh) and Desert Hedgehog (Dhh) genes, respectively. Comparison of ihha and Type X collagen (col10a1) expression during skeletal development show that ihha transcripts are located in hypertrophic chondrocytes of cartilaginous elements of the craniofacial and fin endoskeleton. Surprisingly, col10a1 expression was also detected in cells forming intramembranous bones of the head and in flat cells surrounding cartilaginous structures. The expression of col10a1 in both endochondral and intramembranous bones reflects an atypical composition of the extracellular matrix of the zebrafish craniofacial skeleton. In addition, during fin ray regeneration, both ihha and col10a1 are detected in scleroblasts, osteoblast-like cells secreting the matrix of the dermal bone fin ray. The presence of cartilage markers suggests that the dermal fin ray possesses an intermediate phenotype between cartilage and bone.

sall4 acts downstream of tbx5 and is required for pectoral fin outgrowth

Okihiro syndrome (OS) is defined by forelimb defects associated with the eye disorder Duane anomaly and results from mutations in the gene SALL4. Forelimb defects in individuals with OS range from subtle thumb abnormalities to truncated limbs. Mutations in the T-box transcription factor TBX5 cause Holt-Oram syndrome (HOS), which results in forelimb and heart defects. Although mutations in TBX5 result in HOS, it has been predicted that these mutations account for only ∼30% of all individuals with HOS. Individuals with OS and HOS limb defects are very similar, in fact, individuals with mutations in SALL4 have in some cases previously been diagnosed with HOS. Using zebrafish as a model, we have investigated the function of sall4 and the relationship between sall4 and tbx5, during forelimb development. We demonstrate that sall4 and a related gene sall1 act downstream of tbx5 and are required for pectoral fin development. Our studies of Sall gene family redundancy and tbx5 offer explanations for the similarity of individuals with OS and HOS limb defects.

Cellular plasticity in vertebrate regeneration

Within the animal kingdom, there are several examples of organisms with remarkable regenerative abilities. Among vertebrates, newts appear to be the most adept at replacing lost structures and injured organs and can regenerate their limbs, tails, spinal cords, jaws, retinas, lenses, optic nerves, intestines, and heart ventricles. This regenerative ability is dependent on the induction of an unusual degree of cellular plasticity near the site of injury. Mature cells lose their differentiated characteristics and revert to proliferating progenitor cells that will later redifferentiate to replace the lost or injured tissues. This degree of cellular plasticity appears to be restricted to those vertebrates with the most remarkable regenerative abilities and is not observed in mammals. However, in the last several years, there have been a few studies suggesting that certain factors present in newt tissues can induce a dedifferentiation response in mammalian cells. These results suggest that the knowledge gained from studying the molecular basis of cellular plasticity in newts and other regeneration-competent model organisms might one day be used to enhance the regenerative potential in mammals.

Inhibition of zebrafish fin regeneration using in vivo electroporation of morpholinos againstfgfr1 andmsxb

Increased interest in using zebrafish as a model organism has led to a resurgence of fin regeneration studies. This has allowed for the identification of a large number of gene families, including signaling molecules and transcription factors, which are expressed during regeneration. However, in cases where no specific inhibitor is available for the gene product of interest, determination of a functional role for these genes has been difficult. Here we demonstrate that in vivo electroporation of morpholino oligonucleotides is a feasible approach for protein knock-down during fin regeneration. Morpholino oligonucleotides against fgfr1 and msxb were utilized and knock-down of both proteins resulted in reduced fin outgrowth. Importantly, Fgfr1 knock-down phenocopied outgrowth inhibition obtained with an Fgfr1 inhibitor. Furthermore, this method provided direct evidence for a functional role for msxb in caudal fin regeneration. Finally, knock-down of Fgfr1, but not Msxb, affected the blastemal expression of msxc, suggesting this technique can be used to determine epistasis in genetic pathways affecting regeneration. Thus, this convenient reverse genetic approach allows researchers to quickly (1) assess the function of genes known to be expressed during fin regeneration, (2) screen genes for functional relevance during fin regeneration, and (3) assign genes to the molecular pathways underlying fin regeneration.

Fgf signaling instructs position-dependent growth rate during zebrafish fin regeneration

During appendage regeneration in urodeles and teleosts, tissue replacement is precisely regulated such that only the appropriate structures are recovered, a phenomenon referred to as positional memory. It is believed that there exists, or is quickly established after amputation, a dynamic gradient of positional information along the proximodistal (PD) axis of the appendage that assigns region-specific instructions to injured tissue. These instructions specify the amount of tissue to regenerate, as well as the rate at which regenerative growth is to occur. A striking theme among many species is that the rate of regeneration is more rapid in proximally amputated appendages compared with distal amputations. However, the underlying molecular regulation is unclear. Here, we identify position-dependent differences in the rate of growth during zebrafish caudal fin regeneration. These growth rates correlate with position-dependent differences in blastemal length, mitotic index and expression of the Fgf target genes mkp3, sef and spry4. To address whether PD differences in amounts of Fgf signaling are responsible for position-dependent blastemal function, we have generated transgenic fish in which Fgf receptor activity can be experimentally manipulated. We find that the level of Fgf signaling exhibits strict control over target gene expression, blastemal proliferation and regenerative growth rate. Our results demonstrate that Fgf signaling defines position-dependent blastemal properties and growth rates for the regenerating zebrafish appendage.

Aryl hydrocarbon receptor activation inhibits regenerative growth

There is considerable literature supporting the conclusion that inappropriate activation of the aryl hydrocarbon receptor (AHR) alters cellular signaling. We have established previously that fin regeneration is specifically inhibited by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in adult zebrafish and have used this in vivo endpoint to evaluate interactions between AHR and growth-controlling pathways. Because there are experimental limitations in studying regeneration in adult animals, we have developed a larval model to evaluate the effect of AHR activation on tissue regeneration. Two-day-old zebrafish regenerate their amputated caudal fins within 3 days. Here, we demonstrate that TCDD specifically blocks regenerative growth in larvae. The AHR pathway in zebrafish is considerably more complex than in mammals, with at least three zebrafish AHR genes (zfAHR1a, zfAHR1b, and zfAHR2) and two ARNT genes (zfARNT1 and zfARNT2). Although it was presumed that the block in regeneration was mediated by AHR activation, it had not been experimentally demonstrated. Using antisense morpholinos and mutant fish lines, we report that zfAHR2 and zfARNT1 are the in vivo dimerization partners that are required for inhibition of regeneration by TCDD. Several pathways including fibroblast growth factor (FGF) signaling are essential for fin regeneration. Even though impaired FGF signaling and TCDD exposure both inhibit fin regeneration, their morphometric response is distinct, suggesting that the mechanisms of impairment are different. With the plethora of molecular and genetic techniques that can be applied to larval-stage embryos, this in vivo regeneration system can be further exploited to understand cross-talk between AHR and other signaling pathways.

Heat-shock protein 60 is required for blastema formation and maintenance during regeneration

Zebrafish fin regeneration requires the formation and maintenance of blastema cells. Blastema cells are not derived from stem cells but behave as such, because they are slow-cycling and are thought to provide rapidly proliferating daughter cells that drive regenerative outgrowth. The molecular basis of blastema formation is not understood. Here, we show that heat-shock protein 60 (hsp60) is required for blastema formation and maintenance. We used a chemical mutagenesis screen to identify no blastema (nbl), a zebrafish mutant with an early fin regeneration defect. Fin regeneration failed in nbl due to defective blastema formation. nbl also failed to regenerate hearts. Positional cloning and mutational analyses revealed that nbl results from a V324E missense mutation in hsp60. This mutation reduced hsp60 function in binding and refolding denatured proteins. hsp60 expression is increased during formation of blastema cells, and dysfunction leads to mitochondrial defects and apoptosis in these cells. These data indicate that hsp60 is required for the formation and maintenance of regenerating tissue.

Cytoskeletal dynamics of the teleostean fin ray during fin epimorphic regeneration

Teleost fishes can regenerate their fins by epimorphic regeneration, a process that involves the transition of the formerly quiescent tissues of the stump to an active, growing state. This involves dynamic modifications of cell phenotype and behavior that must rely on alterations of the cytoskeleton. We have studied the spatial and temporal distribution of three main components of the cytoskeleton (actin, keratin and vimentin) in the regenerating fin, in order to establish putative relationships between cell cytoskeleton and cell behavior. According to our results, the massive rearrangement undergone by the epidermis right after injury, which takes place by cell migration, correlates with a transient down-regulation of keratin and a strong up-regulation of actin in the epidermal cells. During the subsequent epidermal growth, based on cell proliferation, keratin normal pattern is recovered while actin is down-regulated, although not to normal (quiescent) levels. The epidermal basal layer in contact with the blastema displays a particular cytoskeletal profile, different to that of the rest of the epidermal cells, which reflects its special features. In the connective tissue compartment, somatic cells do not contain vimentin, but keratin, as intermediate filament. Proliferative and migrative activation of these cells after injury correlates with actin up-regulation. Although this initial activation does not involve keratin down-regulation, blastemal cells were later observed to lack keratin, suggesting that such cytoskeletal modification might be needed for connective tissue cells to dedifferentiate and form the blastema. Cell differentiation in the newly formed, regenerated ray is accompanied by actin down-regulation and keratin up-regulation.

Isolation and expression patterns of genes for three angiopoietin-like proteins, Angptl1, 2 and 6 in zebrafish

Angiopoietin-like proteins (Angptls) are known to possess biological activities not only in the vascular system, but in the other mammalian tissues; however, their expression patterns and function in embryogenesis have not been extensively characterized. Here, we identify three zebrafish genes (Zangptl1, Zangptl2 and Zangptl6) highly homologous to mammalian Angptl1/ARP1, Angptl2/ARP2 and Angptl6/AGF, and describe their adult and embryonic temporal and spatial expression patterns. Zangptl1 is expressed faintly in the somites, while Zangptl2 is first detected in the yolk sac extension, spinal cord and branchial arches and is later expressed in the liver primordium and pectoral fin buds. Zangptl6 is expressed in the notochord. In addition to its embryonic expression, Zangptl2 is induced in adult fish during fin regeneration.

Screen for genes differentially expressed during regeneration of the zebrafish caudal fin

The zebrafish caudal fin constitutes an important model for studying the molecular basis of tissue regeneration. The cascade of genes induced after amputation or injury, leading to restoration of the lost fin structures, include those responsible for wound healing, blastema formation, tissue outgrowth, and patterning. We carried out a systematic study to identify genes that are up-regulated during "initiation" (1 day) and "outgrowth and differentiation" (4 days) of fin regeneration by using two complementary methods, suppression subtraction hybridization (SSH) and differential display reverse transcriptase polymerase chain reaction (DDRT-PCR). We obtained 298 distinct genes/sequences from SSH libraries and 24 distinct genes/sequences by DDRT-PCR. We determined the expression of 54 of these genes using in situ hybridization. In parallel, gene expression analyses were done in zebrafish embryos and early larvae. The information gathered from the present study provides resources for further investigations into the molecular mechanisms of fin development and regeneration.

Regeneration in the ears of immunodeficient mice: identification and lineage analysis of mesenchymal stem cells

Wound healing in the ears of Athymic Nude-nu mice resembles regeneration. Histological analysis of the ear-punched tissues revealed the initial formation of a blastema-like structure followed by dermal, vascular, cartilage, and muscle regrowth exclusively in Athymic Nude-nu mice but not in wild-type controls (C57BL/6J). A subset of stem cells referred to here as ear mesenchymal stem cells (EMSCs) has been isolated from the external ears of regenerative (Athymic Nude-nu) and nonregenerative strains of mice. Morphological, histochemical, and molecular analysis after the induction of EMSC differentiation revealed multiple mesenchymal cell lineages (adipocytes, chondrocytes, and osteocytes) in all murine strains independent of their ability for regeneration. Thus, the absence of regeneration in wounded ears of C57BL/6J wild-type mice is not related to the absence of mesenchymal stem cell differentiation in tissue culture. Because nude mice lack T lymphocytes, it appears that in this model the absence of T lymphocytes in the wounded ears provides a microenvironment conducive to regeneration of mesenchymal tissues. These findings provide a new model to study the influence of the immune system on tissue regeneration.

Msx1 and Pax3 cooperate to mediate FGF8 and WNT signals during Xenopus neural crest induction

FGF, WNT, and BMP signaling promote neural crest formation at the neural plate boundary in vertebrate embryos. To understand how these signals are integrated, we have analyzed the role of the transcription factors Msx1 and Pax3. Using a combination of overexpression and morpholino-mediated knockdown strategies in Xenopus, we show that Msx1 and Pax3 are both required for neural crest formation, display overlapping but nonidentical activities, and that Pax3 acts downstream of Msx1. In neuralized ectoderm, Msx1 is sufficient to induce multiple early neural crest genes. Msx1 induces Pax3 and ZicR1 cell autonomously, in turn, Pax3 combined with ZicR1 activates Slug in a WNT-dependent manner. Upstream of this, WNTs initiate Slug induction through Pax3 activity, whereas FGF8 induces neural crest through both Msx1 and Pax3 activities. Thus, WNT and FGF8 signals act in parallel at the neural border and converge on Pax3 activity during neural crest induction.

Mutations in connexin43 (GJA1) perturb bone growth in zebrafish fins

Mechanisms that regulate the size and shape of bony structures are largely unknown. The molecular identification of the fin length mutant short fin (sof), which causes defects in the length of bony fin ray segments, may provide insights regarding the regulation of bone growth. In this report, we demonstrate that the sof phenotype is caused by mutations in the connexin43 (cx43) gene. This conclusion is supported by genetic mapping, reduced expression of cx43 in the original sof allele (sofb123), identification of missense mutations in three ENU-induced alleles, and by demonstration of partially abrogated cx43 function in sofb123 embryos. Expression of cx43 was identified in cells flanking the germinal region of newly growing segments as well as in the osteoblasts at segment boundaries. This pattern of cx43 expression in cells lateral to new segment growth is consistent with a model where cx43-expressing cells represent a biological ruler that measures segment size. This report identifies the first gene identification for a fin length mutation (sof) as well as the first connexin mutations in zebrafish, and therefore reveals a critical role for local cell-cell communication in the regulation of bone size and growth.

Early fin primordia of zebrafish larvae regenerate by a similar growth control mechanism with adult regeneration

Some vertebrate species, including urodele amphibians and teleost fish, have the remarkable ability of regenerating lost body parts. Regeneration studies have been focused on adult tissues, because it is unclear whether or not the repairs of injured tissues during early developmental stages have the same molecular base as that of adult regeneration. Here, we present evidence that a similar cellular and molecular mechanism to adult regeneration operates in the repair process of early zebrafish fin primordia, which are composed of epithelial and mesenchymal cells. We show that larval fin repair occurs through the formation of wound epithelium and blastema-like proliferating cells. Cell proliferation is first induced in the distal-most region and propagates to more proximal regions, as in adult regeneration. We also show that fibroblast growth factor signaling helps induce cell division. Our results suggest that the regeneration machinery directing cell proliferation in response to injury may exist from the early developmental stages.

Msx1 expression in the adult mouse brain: characterization of populations of beta-galactosidase-positive cells in the hippocampus and fimbria

We have analyzed Msx1 expression in the mature mouse brain using in situ hybridization and beta-galactosidase activity in Msx1(nLacZ) mice. The study revealed that Msx1 is strongly expressed in the circumventricular organs, such as the subcommissural organ and choroid plexus, and in some epithelia, such as that of the dorsal, but not the ventral part of the third ventricle. Immunohistochemical analysis revealed that the Msx1-expressing cells of the hippocampus and fimbria are astrocytes, oligodendrocytes or immature oligodendrocytes. In contrast, no co-expression was detected in these structures using several neuronal markers. These results were confirmed, using transmission electron microscopy, by the presence of 5-bromo-3-indolyl-beta-D-galactopyranosideprecipitates in astrocytes and oligodendrocytes in both sites. Moreover, using an anti-glial fibrillary acidic protein antibody (GFAP), our study reveals two populations of astrocytes in the adult hippocampus and other areas, such as the fimbria, namely Msx1+/GFAP+ and Msx1-/GFAP+. Beta-galactosidase activity was also observed in endothelial cells of hippocampal fissure blood vessels. We also observed co-localization of polysialic acid neural cell adhesion molecule, a marker of the polysialylated form of the neural cell adhesion molecule, in Msx1-expressing cells in the fimbria. These cells may be precursors of glial cells and originate from the epithelium of the fimbria. The present study indicates, in the mature mouse brain, that Msx1 may be linked to secretory activity in circumventricular organs, and to glial proliferation and differentiation in the hippocampus and fimbria, and presumably also in other cerebral areas. We suggest that Msx1 could be associated with brain homeostasis and blood-brain barrier function.

Fgf signaling is required for zebrafish tooth development

We have investigated fibroblast growth factor (FGF) signaling during the development of the zebrafish pharyngeal dentition with the goal of uncovering novel roles for FGFs in tooth development as well as phylogenetic and topographic diversity in the tooth developmental pathway. We found that the tooth-related expression of several zebrafish genes is similar to that of their mouse orthologs, including both epithelial and mesenchymal markers. Additionally, significant differences in gene expression between zebrafish and mouse teeth are indicated by the apparent lack of fgf8 and pax9 expression in zebrafish tooth germs. FGF receptor inhibition with SU5402 at 32 h blocked dental epithelial morphogenesis and tooth mineralization. While the pharyngeal epithelium remained intact as judged by normal pitx2 expression, not only was the mesenchymal expression of lhx6 and lhx7 eliminated as expected from mouse studies, but the epithelial expression of dlx2a, dlx2b, fgf3, and fgf4 was as well. This latter result provides novel evidence that the dental epithelium is a target of FGF signaling. However, the failure of SU5402 to block localized expression of pitx2 suggests that the earliest steps of tooth initiation are FGF-independent. Investigations of specific FGF ligands with morpholino antisense oligonucleotides revealed only a mild tooth shape phenotype following fgf4 knockdown, while fgf8 inhibition revealed only a subtle down-regulation of dental dlx2b expression with no apparent effect on tooth morphology. Our results suggest redundant FGF signals target the dental epithelium and together are required for dental morphogenesis. Further work will be required to elucidate the nature of these signals, particularly with respect to their origins and whether they act through the mesenchyme.

Genetic approaches to disease and regeneration

Cardiovascular disease is largely a consequence of coronary artery blockage through excessive proliferation of smooth muscle cells. It in turn leads to myocardial infarction and permanent and functionally devastating tissue damage to the heart wall. Our studies have revealed that elastin is a primary player in maintaining vascular smooth muscle cells in their dormant state and thus may be a useful therapeutic in vascular disease. By studying zebrafish, which unlike humans, can repair damage to heart muscle, we have begun to uncover some of the genes that seem necessary to undertake the de-differentiation steps that currently fail and prevent the formation of new proliferating cardiomyocytes at the site of damage in a mammalian heart.

The cusp of evolution and development: a model of cichlid tooth shape diversity

Tooth shape is a hallmark of repeated evolutionary radiations among cichlid fishes from East Africa. Cusp shape and number vary both within populations and among closely related species with different feeding behaviors and ecologies. Here, we use histology and scanning electron microscopy to chart the developmental trajectory of tooth shape differences in fishes from Lake Malawi. We demonstrate that species with bi- or tricuspid adult (replacement) teeth initially possess a first-generation unicuspid dentition. Notably, the timing of turnover from first-generation to replacement teeth differs among species and is correlated with feeding ecology. Next, we use field data for cichlid species with adult unicuspid, bicuspid, and tricuspid teeth to demonstrate a strong and positive relationship between the number of teeth in a row and tooth shape. We discuss cichlid tooth ontogeny in the context of morphogenetic models designed to explain the developmental basis of tooth shape variation in mammals. We suggest that the dramatic differences in cichlid dentitions can be explained by variation in the expression of common activators and inhibitors acting at multiple stages of odontogenesis.

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.

Deciphering the swordtail's tale: a molecular and evolutionary quest

The power of sexual selection to influence the evolution of morphological traits was first proposed more than 130 years ago by Darwin. Though long a controversial idea, it has been documented in recent decades for a host of animal species. Yet few of the established sexually selected features have been explored at the level of their genetic or molecular foundations. In a recent report, Zauner et al.1 describe some of the molecular features associated with one of the best characterized of sexually selected traits, the male-specific tail "sword" seen in certain species of the fish genus Xiphophorus. Zauner et al. find that the msxC gene, a gene previously implicated in fin development from work in zebrafish, is dramatically and specifically upregulated in the development of the ventral caudal fin rays, which give rise to the sword, in males. The results provide the first molecular insight into the development of this sexually selected trait while prompting new questions about the structure of the entire genetic network that underlies this trait. To fully understand the molecular-genetic and evolutionary history of this network, however, it will be essential to determine whether sword-development is a basal or derived trait in Xiphophorus.

reg6 is required for branching morphogenesis during blood vessel regeneration in zebrafish caudal fins

Postnatal neovascularization is essential for wound healing, cancer progression, and many other physiological functions. However, its genetic mechanism is largely unknown. In this report, we study neovascularization in regenerating adult zebrafish fins using transgenic fish that express EGFP in blood vessel endothelial cells. We first describe the morphogenesis of regenerating vessels in wild-type animals and then the phenotypic analysis of a genetic mutation that disrupts blood vessel regeneration. In wild-type zebrafish caudal fins, amputated blood vessels heal their ends by 24 h postamputation (hpa) and then reconnect arteries and veins via anastomosis, to resume blood flow at wound sites by 48 hpa. The truncated vessels regenerate by first growing excess vessels to form unstructured plexuses, resembling the primary capillary plexuses formed during embryonic vasculogenesis. Interestingly, this mode of vessel growth switches by 8 days postamputation (dpa) to growth without a plexus intermediate. During blood vessel regeneration, vessel remodeling begins during early plexus formation and continues until the original vasculature pattern is reestablished at approximately 35 dpa. Temperature-sensitive mutants for reg6 have profound defects in blood vessel regeneration. At the restrictive temperature, reg6 regenerating blood vessels first fail to make reconnections between severed arteries and veins, and then form enlarged vascular sinuses rather than branched vascular plexuses. Reciprocal temperature-shift experiments show that reg6 function is required throughout plexus formation, but not during later growth. Our results suggest that the reg6 mutation causes defects in branch formation and/or angiogenic sprouting.

Vascular Endothelial Growth Factor and Its Receptors in Embryonic Zebrafish Blood Vessel Development

There is intense interest in how blood vessel development is regulated. A number of vascular growth factors and their receptors have been described. The vascular endothelial growth factor (VEGF) and its receptors are major contributors to normal mammalian vascular development. These receptors include VEGFR-1, VEGFR-2, VEGFR-3, neuropilin-1 (NRP1), and NRP2. The function of these genes have been determined to some degree in mouse gene targeting studies. These knockouts are embryonically lethal, and early death can be attributed in part to lack of normal blood and lymphatic vessel development. More recently, it has been demonstrated that zebrafish are an excellent model for studying the genes and proteins that regulate embryonic vascular development. Zebrafish have a number of advantages compared to mice, including rapid embryonic development and the ability to examine and manipulate embryos outside of the animal. In this review, we describe some of the earlier mouse VEGF/receptor functional studies and emphasize the development of the zebrafish vasculature. We describe the zebrafish vasculature, zebrafish VEGF and VEGF receptors, advantages of the zebrafish model, resources, and methods of determining growth factor and receptor function.

Seasonal changes in the lower jaw skeleton in male Atlantic salmon (Salmo salar L.): remodelling and regression of the kype after spawning

The return of Atlantic salmon (Salmon salar) to their home river for spawning coincides with drastic skeletal alterations in both sexes. Most prominent is the development of a kype (hook) at the tip of the lower jaw in males. Salmon that survive spawning have to cope with the kype throughout their life, unless it disappears after spawning, as was suggested in the early literature. To understand the fate of the kype skeleton, we compared morphological and histological features of kypes from pre-spawned mature anadromous males (grilse) with post-spawned males (kelts). The kype of male grilse is supported by fast-growing skeletal needles that differ from regular dentary bone. In kelts, growth of the kype skeleton has stopped and skeletal needles are resorbed apically by osteoclasts. Simultaneously, and despite the critical physiological condition of the animals, proximal parts of the kype skeleton are remodelled and converted into regular dentary bone. Apical resorption of the skeleton explains reports of a decrease of the kype in kelts. The conversion of basal kype skeleton into regular dentary bone contributes to the elongation of the dentary and probably also to the development of a larger kype in repetitive spawning males.

The complexity of alternative splicing of hagoromo mRNAs is increased in an explosively speciated lineage in East African cichlids

The adaptive radiation of cichlid fishes in the lakes of East Africa is a prime example of speciation. The choice of cichlid mates on the basis of a variety of coloration represents a potential basis for speciation that led to adaptive radiation. Here, we characterize the cichlid homolog of the zebrafish hagoromo (hag) gene that was recently cloned and characterized from a pigmentation mutant. Although only one hag mRNA was reported in zebrafish, cichlids express nine different hag mRNAs resulting from alternative splicing. The hag mRNAs are expressed between the myotome and the epidermis where pigment cells are located, suggesting the cichlid hag gene is involved in pigmentation. The hag mRNA splicing pattern does not fluctuate among individuals from each of two species, suggesting that alternative splice site choice is fixed within species. Furthermore, cichlids in lineages that underwent explosive speciation expressed a greater variety of hag mRNAs than those in lineages that did not undergo such a degree of speciation, suggesting that species in the explosively speciated lineage acquired a complex regulatory mechanism of alternative splicing over a very short evolutionary period. Here, we provide an example in which alternative splicing may play a role in mate choice, leading to cichlid speciation through diversification of gene function by production of multiple mRNAs from a single gene.

Differentiation of chondrocytes and scleroblasts during dorsal fin skeletogenesis in flounder larvae

In teleosts, the embryonic fin fold consists of a peridermis, an underlying epidermis and a small number of mesenchymal cells. Beginning from such a simple structure, the fin skeletons, including the proximal and distal radials and lepidotrichia (finrays), develop in the dorsal fin fold at the larval stage. Their process of skeletogenesis and embryonic origin are unclear. Using flounder larvae, we report the differentiation process for chondrocytes and scleroblasts prior to fin skeletogenesis and the effects of retinoic acid (RA) on it. In early larvae, the mesenchymal cells grow between the epidermis and spinal cord to form a line of periodical condensations, which are proximal radial primordia, to produce chondrocytes. The prescleroblasts, which ossify the proximal radial cartilages, differentiate in the mesenchymal cells remaining between the cartilages. Then, mesenchymal condensations occur between the distal ends of the proximal radials, forming distal radial primordia, to produce chondrocytes. Simultaneously, condensations occur between the distal radial primordia and peridermis, which are lepidotrichia primordia, to produce prescleroblasts. Exogenous RA specifically inhibits the mesenchymal condensation prior to the proximal radial formation together with the down-regulation of sonic hedgehog (shh) and patched (pta) expression, resulting in the loss of proximal radials. Thus, it was indicated that differentiation of the precursor cells of radials and lepidotrichia begins in the proximal part of the fin fold and that the initial mesenchymal condensation prior to the proximal radial formation is highly susceptible to the effects of RA. Lepidotrichia formation does not occur where proximal radials are absent, indicating that lepidotrichia differentiation requires interaction with the radial cartilages. To examine the suggestion that neural crest cells contribute to the medial fin skeletons, we localized the HNK-1 positive cells in flounder embryos and slug and msxb-positive cells in pufferfish, Fugu rubripes, embryos. That the positive cells commonly arrive at the proximal part of the fin fold does not contradict the suggestion, but their final destiny as radial chondrocytes or lepidotrichia scleroblasts, should be further investigated.

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.

Postnatal recapitulation of embryonic hedgehog pathway in response to skeletal muscle ischemia

BACKGROUND

Hedgehog (Hh) proteins are morphogens regulating epithelial-mesenchymal signaling during several crucial processes of embryonic development, including muscle patterning. Sonic (Shh), Indian (Ihh), and Desert (Dhh) hedgehog constitute the repertoire of Hh genes in humans. The activities of all 3 are transduced via the Patched (Ptc1) receptor. Recent observations indicate that exogenous administration of Shh induces angiogenesis. Here, we studied whether the endogenous Hh pathway, in addition to its functions during embryogenesis, plays a physiological role in muscle regeneration after ischemia in adults.

METHODS AND RESULTS

We found that skeletal muscle ischemia induces strong local upregulation of Shh mRNA and protein. In addition, the Ptc1 receptor is activated in interstitial mesenchymal cells within the ischemic area, indicating that these cells respond to Shh and that the Shh pathway is functional. We also found that Shh-responding cells produce vascular endothelial growth factor under ischemic conditions and that systemic treatment with a Shh-blocking antibody inhibits the local angiogenic response and the upregulation of vascular endothelial growth factor.

CONCLUSIONS

Our study shows that the Hh signaling may be recapitulated postnatally in adult and fully differentiated muscular tissues and has a regulatory role on angiogenesis during muscle regeneration after ischemia. These findings demonstrate a novel biological activity for the Hh pathway with both fundamental and potential therapeutic implications.

Saltatory control of isometric growth in the zebrafish caudal fin is disrupted in long fin and rapunzel mutants

Zebrafish fins grow by sequentially adding new segments of bone to the distal end of each fin ray. In wild type zebrafish, segment addition is regulated such that an isometric relationship is maintained between fin length and body length over the lifespan of the growing fish. Using a novel, surrogate marker for fin growth in conjunction with cell proliferation assays, we demonstrate here that segment addition is not continuous, but rather proceeds by saltation. Saltation is a fundamental growth mechanism shared by disparate vertebrates, including humans. We further demonstrate that segment addition proceeds in conjunction with cyclic bursts of cell proliferation in the distal fin ray mesenchyme. In contrast, cells in the distal fin epidermis proliferate at a constant rate throughout the fin ray growth cycle. Finally, we show that two separate fin overgrowth mutants, long fin and rapunzel, bypass the stasis phase of the fin ray growth cycle to develop asymmetrical and symmetrical fin overgrowth, respectively.

Chemical genetics: adding to the developmental biology toolbox

Recent advances in cell and molecular biology have generated important tools to probe developmental questions. In addition, new genetic model systems such as Danio rerio now make large-scale vertebrate early developmental mutant screens feasible. Nonetheless, some developmental questions remain difficult to study because of the need for finer temporal, spatial, or tuneable control of gene function within a developmental system. New uses for old teratogens as well as novel chemical modulators of development have begun to fill this void.

Positional cloning of a temperature-sensitive mutant emmental reveals a role for sly1 during cell proliferation in zebrafish fin regeneration

Here, we used classical genetics in zebrafish to identify temperature-sensitive mutants in caudal fin regeneration. Gross morphological, histological, and molecular analyses revealed that one of these strains, emmental (emm), failed to form a functional regeneration blastema. Inhibition of emm function by heat treatment during regenerative outgrowth rapidly blocked regeneration. This block was associated with reduced proliferation in the proximal blastema and expansion of the nonproliferative distal blastemal zone. Positional cloning revealed that the emm phenotype is caused by a mutation in the orthologue of yeast sly1, a gene product involved in protein trafficking. sly1 is upregulated in the newly formed blastema as well as during regenerative outgrowth. Thus, sly1 is essential for blastemal organization and proliferation during two stages of fin regeneration.

Posterior hoxa genes expression during zebrafish bony fin ray development and regeneration suggests their involvement in scleroblast differentiation

Expression of two zebrafish developmental posterior hoxa genes, hoxa11b and hoxa13b, was studied by in situ hybridization during pectoral and caudal fin development and regeneration. Expression was restricted to cells of the bony rays region. During fin development, molecular cytological analysis revealed that a subpopulation of mesenchymal cells expressed these two hoxa genes during their early differentiation in the subapical region of the developing ray. These cells were identified as differentiating dermal bone making cells (scleroblasts). During fin regeneration, hoxa11b and hoxa13b genes are both induced in undifferentiated cells of the distalmost blastema region (DMB) and the proliferating zone (PZ) and later in differentiating bone-forming cells. In addition, the transient regionalization of the hoxa13b expression pattern in differentiated bone-forming cells along the proximodistal axis of the regenerating ray suggests that hoxa13b could participate in ray patterning. This study is the first to establish a correlation between hoxa gene expression and dermal bone cell differentiation.

[Regeneration of vertebrate appendage: an old experimental model to study stem cells in the adult]

The application of stem cell therapy to cure degenerative diseases offers immense possibilities, but the research in this field is the subject of ethical debates raised by the question of destructive research on early human embryos. Stem cells taken in the adult constitute an alternative to human embryonic stem cells, but our knowledge on totipotent or pluripotent cells is currently insufficient. Furthermore, many questions must be solved before selection and differentiation of these cells in a given cellular type can be controlled on a routine basis. What are the molecular characteristics of an adult stem cell? What are the mechanisms involved in cell reprogramming? Which signals control stem cell replication and differentiation? Basic research activities must be carried out in order to clarify all these points. In this context, the regeneration of vertebrate appendages provides a model for this type of research. The regeneration process is defined by both the morphological and functional reconstruction of a part of a living organism, which has previously been destroyed. But why are some vertebrates able to regenerate complex structures and others apparently not? Among most vertebrates, the capacity to regenerate is limited to some tissues. It is however possible to observe the regeneration of appendages (limb, tail, fin, jaw, etc.) among several amphibians and fish. This regeneration leads to re-forming of the amputated part with a complete restoration of its shape, segmentation and function. Why is the amputation of limbs not followed by regeneration in mammals and birds: absence of stem cells, absence of recruitment signals for these cells, or absence of signal receptivity? This review constitutes a report on the current understanding of the basis of on regeneration of legs in tetrapods and of fins in fish with an emphasis in the role of the nervous system in this process.

Tales of regeneration in zebrafish

Complex tissue regeneration involves exquisitely coordinated proliferation and patterning of adult cells after severe injury or amputation. Certain lower vertebrates such as urodele amphibians and teleost fish have a greater capacity for regeneration than mammals. However, little is known about molecular mechanisms of regeneration, and cellular mechanisms are incompletely defined. To address this deficiency, we and others have focused on the zebrafish model system. Several helpful tools and reagents are available for use with zebrafish, including the potential for genetic approaches to regeneration. Recent studies have shed light on the remarkable ability of zebrafish to regenerate fins.

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.

Ray-interray interactions during fin regeneration of Danio rerio

Teleost fin ray bifurcations are characteristic of each ray in each fin of the fishes. Control of the positioning of such morphological markers is not well understood. We present evidence suggesting that the interray blastema is necessary for a proper bifurcation of each ray during regeneration in Danio rerio (Hamilton-Buchanan) (Cyprinidae, Teleostei). We performed single ray ablations, heterotopical graftings of ray fragments and small holes in lateral rays which do not normally bifurcate, to generate recombinants in which the lateral rays are surrounded with ectopic interrays originating from different positions within the tail fin. These ray-interray recombinants do now bifurcate. Furthermore, we show that the interray tissue and surrounding epidermis can modulate the length of the ray. These results stress the role of the interray in inducing bifurcations of the ray blastema as well as modulating ray morphogenesis in general. In addition, gene expression analysis under these experimental conditions suggests that msxA and msxD expression in the ray and interray epidermis is controlled by the ray blastema and that bmp4 could be a candidate signal involved in these inductions.

From Zebrafish to human: modular medical models

Genetic screens in Drosophila melanogaster, Caenorhabditis elegans, and Danio rerio clarified the logic of metazoan development by revealing critical unitary steps and pathways to embryogenesis. Can genetic screens similarly organize medicine? We here examine human diseases that resemble mutations in Danio rerio, the zebrafish, the one vertebrate species for which large-scale genetic screens have been performed and extensively analyzed. Zebrafish mutations faithfully phenocopy many human disorders. Each mutation, once cloned, provides candidate genes and pathways for evaluation in the human. The collection of mutations in their entirety potentially provides a medical taxonomy, one based in developmental biology and genetics.

Mps1 defines a proximal blastemal proliferative compartment essential for zebrafish fin regeneration

One possible reason why regeneration remains enigmatic is that the dominant organisms used for studying regeneration are not amenable to genetic approaches. We mutagenized zebrafish and screened for temperature-sensitive defects in adult fin regeneration. The nightcap mutant showed a defect in fin regeneration that was first apparent at the onset of regenerative outgrowth. Positional cloning revealed that nightcapencodes the zebrafish orthologue of mps1, a kinase required for the mitotic checkpoint. mps1 expression was specifically induced in the proximal regeneration blastema, a group of cells that normally proliferate intensely during outgrowth. The nightcap mutation caused severe defects in these cells. However, msxb-expressing blastemal cells immediately distal to this proliferative region did not induce mps1and were retained in mutants. These results indicate that the proximal blastema comprises an essential subpopulation of the fin regenerate defined by the induction and function of Mps1. Furthermore, we show that molecular mechanisms of complex tissue regeneration can now be dissected using zebrafish genetics.

Bone patterning is altered in the regenerating zebrafish caudal fin after ectopic expression of sonic hedgehog and bmp2b or exposure to cyclopamine

Amputation of the zebrafish caudal fin stimulates regeneration of the dermal skeleton and reexpression of sonic hedgehog (shh)-signaling pathway genes. Expression patterns suggest a role for shh signaling in the secretion and patterning of the regenerating dermal bone, but a direct role has not been demonstrated. We established an in vivo method of gene transfection to express ectopically genes in the blastema of regenerating fins. Ectopic expression of shh or bmp2 in the blastema-induced excess bone deposition and altered patterning of the regenerate. The effects of shh ectopic expression could be antagonized by ectopic expression of chordin, an inhibitor of bone morphogenetic protein (bmp) signaling. We disrupted shh signaling in the regenerating fin by exposure to cyclopamine and found a dose-dependent inhibition of fin outgrowth, accumulation of melanocytes in the distal region of each fin ray, loss of actinotrichia, and reduction in cell proliferation in the mesenchyme. Morphological changes were accompanied by an expansion, followed by a reduction, in domains of shh expression and a rapid abolition of ptc1 expression. These results implicate shh and bmp2b signaling in the proliferation and/or differentiation of specialized bone-secreting cells in the blastema and suggest shh expression may be controlled by regulatory feedback mechanisms that define the region of bone secretion in the outgrowing fin.

A proliferation gradient between proximal and msxb-expressing distal blastema directs zebrafish fin regeneration

Previous studies of zebrafish fin regeneration led to the notion that the regeneration blastema is a homogeneous population of proliferating cells. Here, we show that the blastema consists of two components with markedly distinct proliferation properties. During early blastema formation, proliferating cells are evenly distributed. At the onset of regenerative outgrowth, however, blastemal cells are partitioned into two domains. Proximal blastemal cells proliferate at a high rate, shifting from a median G2 of more than 6 hours to approximately 1 hour. By contrast, the most distal blastemal cells do not proliferate. There is a gradient of proliferation between these extremes. Using bromodeoxyuridine incorporation and anti-phosphohistone H3 labeling, we find a 50-fold difference in proliferation across the gradient that extends approximately 50 μm, or ten cell diameters. We show that during early regeneration, proliferating blastemal cells express msxb, a homeodomain transcriptional repressor. While msxb is widely expressed among proliferating cells during blastema formation, its expression becomes restricted to a small number of non-proliferating, distal blastemal cells during regenerative outgrowth. Bromodeoxyuridine pulse-chase experiments show that distal and proximal blastemal cells are formed from proliferating, msxb-positive blastemal cells, not from preexisting slow-cycling cells. These data support the idea that blastema formation results from dedifferentiation of intraray mesenchymal cells. Based on these findings, we propose a new model of zebrafish fin regeneration in which the function of non-proliferating, msxb-expressing, distal blastemal cells is to specify the boundary of proliferation and provide direction for regenerative outgrowth.

Cell proliferation during blastema formation in the regenerating teleost fin

Epimorphic regeneration in teleost fins occurs through the establishment of a balanced growth state in which a blastema gives rise to all the mesenchymal cells, whereas definite areas of the epidermis proliferate leading to its extension, thus, allowing the enlargement of the whole structure. This type of regeneration involves specific mechanisms that temporally and spatially regulate cell proliferation. To understand how the blastema is formed and how this growth situation is set up, we investigated cell proliferation patterns in the regenerating fin of the goldfish Carassius auratus from the time of amputation to that of blastema formation by using proliferating cell nuclear antigen immunostaining and bromodeoxyuridine labeling. Wound closure and apical epidermal cap formation took place by epidermal migration and re-arrangement, without the contribution of cell proliferation. As soon as the apical cap had formed, the epidermis started to proliferate at its lateral surfaces, in which all layers maintained cycling for the duration of the studied process. The distal epidermal cap, on the contrary, presented very few cycling cells, and its cytoarchitecture was indicative of continuous remodeling due to ray growth. The basal layer of this epidermal cap showed a typical morphology and remained nonproliferative whilst in contact with the proliferating blastema. Proliferation in the mesenchymal compartment of the ray started far from the amputation plane. Subsequently, cycling cells approached that location, until they formed the blastema in contact with the apical epidermal cap. Differences observed between the epidermis and mesenchyma, regarding activation of the cell cycle and the establishment of proliferative patterns, suggest that differential mechanisms regulate cell proliferation in each of these compartments during the initial stages of regeneration.

Patterning mechanisms controlling vertebrate limb development

Vertebrate limb buds are embryonic structures for which much molecular and cellular data are known regarding the mechanisms that control pattern formation during development. Specialized regions of the developing limb bud, such as the zone of polarizing activity (ZPA), the apical ectodermal ridge (AER), and the non-ridge ectoderm, direct and coordinate the development of the limb bud along the anterior-posterior (AP), dorsal-ventral (DV), and proximal-distal (PD) axes, giving rise to a stereotyped pattern of elements well conserved among tetrapods. In recent years, specific gene functions have been shown to mediate the organizing and patterning activities of the ZPA, the AER, and the non-ridge ectoderm. The analysis of these gene functions has revealed the existence of complex interactions between signaling pathways operated by secreted factors of the HH, TGF-beta/BMP, WNT, and FGF superfamilies, which interact with many other genetic networks to control limb positioning, outgrowth, and patterning. The study of limb development has helped to establish paradigms for the analysis of pattern formation in many other embryonic structures and organs.

High-efficiency gene transfer into adult fish: a new tool to study fin regeneration

Zebrafish represents an excellent model to study the function of vertebrate genes (e.g., well-developed genetics, large number of mutants, and genomic sequencing in progress), inasmuch as we have tools to manipulate gene expression. Recent use of injected morpholinos in eggs provides a good method to " knockdown " gene expression in early development (Nasevicius and Ekker, 2000), and the "caged" RNA injected in eggs allows to overexpress a gene in a specific set of cells (Ando et al., 2001). However, a method to specifically modify gene expression in the juvenile or in the adult is still missing. Such a method would be a very powerful tool to understand gene function in differentiated tissues. We describe here an electroporation-based approach, which allows gene transfer in adult tissues. Its efficiency was assessed using a GFP (green fluorescent protein) dependent assay. We then used this method to disrupt the Fgf signalling pathway during the process of regeneration.

The origin of skeletal muscle stem cells in the embryo and the adult

Skeletal muscle progenitors are specified during embryogenesis and in addition have recently been found to be generated from either mesenchymal or neural stem cells in the adult. We review recent progress in identifying the signals and transcription factors that control skeletal muscle formation during embryogenesis and in the adult.

Cell proliferation and movement during early fin regeneration in zebrafish

Cell proliferation and cell movement during early regeneration of zebrafish caudal fins were examined by injecting BrdU and Di-I, respectively. In normal fins of adult fish, a small number of proliferating cells are observed in the epidermis only. Shortly following amputation, epithelial cells covered the wound to form the epidermal cap but did not proliferate. However, by 24 hr, epithelial cells proximal to the level of amputation were strongly labeled with BrdU. Label incorporation was also detected in a few mesenchymal cells. Proliferating cells in the basal epithelial layer were first observed at 48 hr at the level of the newly formed lepidotrichia. At 72 hr, proliferating mesenchymal cells were found distal to the plane of amputation whereas more proximal labeled cells included mainly those located between the lepidotrichia and the basal membrane. When BrdU-injected fins were allowed to regenerate for longer periods, labeled cells were observed in the apical epidermal cap, a location where cells are not thought to proliferate. This result is suggestive of cell migration. Epithelial cells, peripheral to the rays or in the tissue between adjacent rays, were labeled with Di-I and were shown to quickly migrate towards the site of amputation, the cells closer to the wound migrating faster. Amputation also triggered migration of cells of the connective tissue located between the hemirays. Although cell movement was induced up to seven segments proximal from the level of amputation, cells located within two segments from the wound provided the main contribution to the blastema. Thus, cell proliferation and migration contribute to the early regeneration of zebrafish fins.

sprouty4 acts in vivo as a feedback-induced antagonist of FGF signaling in zebrafish

In looking for novel factors involved in the regulation of the fibroblast growth factor (FGF) signaling pathway, we have isolated a zebrafish sprouty4 gene, based on its extensive similarities with the expression patterns of both fgf8 and fgf3. Through gain- and loss-of-function experiments, we demonstrate that Fgf8 and Fgf3 act in vivo to induce the expression of Spry4, which in turn can inhibit activity of these growth factors. When overexpressed at low doses, Spry4 induces loss of cerebellum and reduction in size of the otic vesicle, thereby mimicking the fgf8/acerebellar mutant phenotype. Injections of high doses of Spry4 cause ventralization of the embryo, an opposite phenotype to the dorsalisation induced by overexpression of Fgf8 or Fgf3. Conversely we have shown that inhibition of Spry4 function through injection of antisense morpholino oligonucleotide leads to a weak dorsalization of the embryo, the phenotype expected for an upregulation of Fgf8 or Fgf3 signaling pathway. Finally, we show that Spry4 interferes with FGF signaling downstream of the FGF receptor 1 (FGFR1). In addition, our analysis reveals that signaling through FGFR1/Ras/mitogen-activated protein kinase pathway is involved, not in mesoderm induction, but in the control of the dorsoventral patterning via the regulation of bone morphogenetic protein (BMP) expression.