A comparative view on mechanisms and functions of skeletal remodelling in teleost fish, with special emphasis on osteoclasts and their function

Zebrafish Bone and General Physiology Are Differently Affected by Hormones or Changes in Gravity

Teleost fish such as zebrafish (Danio rerio) are increasingly used for physiological, genetic and developmental studies. Our understanding of the physiological consequences of altered gravity in an entire organism is still incomplete. We used altered gravity and drug treatment experiments to evaluate their effects specifically on bone formation and more generally on whole genome gene expression. By combining morphometric tools with an objective scoring system for the state of development for each element in the head skeleton and specific gene expression analysis, we confirmed and characterized in detail the decrease or increase of bone formation caused by a 5 day treatment (from 5dpf to 10 dpf) of, respectively parathyroid hormone (PTH) or vitamin D3 (VitD3). Microarray transcriptome analysis after 24 hours treatment reveals a general effect on physiology upon VitD3 treatment, while PTH causes more specifically developmental effects. Hypergravity (3g from 5dpf to 9 dpf) exposure results in a significantly larger head and a significant increase in bone formation for a subset of the cranial bones. Gene expression analysis after 24 hrs at 3g revealed differential expression of genes involved in the development and function of the skeletal, muscular, nervous, endocrine and cardiovascular systems. Finally, we propose a novel type of experimental approach, the "Reduced Gravity Paradigm", by keeping the developing larvae at 3g hypergravity for the first 5 days before returning them to 1g for one additional day. 5 days exposure to 3g during these early stages also caused increased bone formation, while gene expression analysis revealed a central network of regulatory genes (hes5, sox10, lgals3bp, egr1, edn1, fos, fosb, klf2, gadd45ba and socs3a) whose expression was consistently affected by the transition from hyper- to normal gravity.

Zebrafish: A possible tool to evaluate bioactive ions

Zebrafish is a well-established model organism with a skeletal structure that highly resembles mammalian bone. Yet its use in the research field of biomaterials has been limited. One area that could benefit from this model system is the evaluation of ionic dissolution products from different materials. As a proof of concept we have evaluated the effect of silicate ions on the zebrafish larvae and compared it to a well-known osteoblastic cell line, MC3T3-E1 subclone 14. We have shown that sodium metasilicate (125 μM and 625 μM) induces more mineralisation in a dose-dependent manner in zebrafish larvae, 9 days post fertilisation as compared to the non-treated group. Moreover the same trends were seen when adding sodium metasilicate to MC3T3-E1 cultures, with more mineralisation and higher ALP levels with higher doses of silicate (25, 125 and 625 μM). These results indicate the feasibility of zebrafish larvae for ionic dissolution studies. The zebrafish model is superior to isolated cell cultures in the aspect that it includes the whole bone remodelling system, with osteoblasts, osteoclasts and osteocytes. Zebrafish could thus provide a powerful in vivo tool and be a bridge between cell culture systems and mammalian models.

Precursor cells from Atlantic salmon (Salmo salar) visceral fat holds the plasticity to differentiate into the osteogenic lineage

In order to study the potential plasticity of Atlantic salmon (Salmo salar) precursor cells (aSPCs) from the adipogenic mesenchyme cell lineage to differentiate to the osteogenic lineage, aSPCs were isolated and cultivated under either osteogenic or adipogenic promoting conditions. The results strengthen the hypothesis that aSPCs most likely are predestined to the adipogenic lineage, but they also hold the flexibility to turn into other lineages given the right stimuli. This assumption is supported by the fact that the transcription factor pparγ , important for regulation of adiopogenesis, was silent in aSPCs grown in osteogenic media, while runx2, important for osteogenic differentiation, was not expressed in aSPCs cultivated in adipogenic media. After 2 weeks in osteogenic promoting conditions the cells started to deposit extracellular matrix and after 4 weeks, the cells started mineralizing secreted matrix. Microarray analyses revealed large-scale transcriptome responses to osteogenic medium after 2 days, changes remained stable at day 15 and decreased by magnitude at day 30. Induction was observed in many genes involved in osteogenic differentiation, growth factors, regulators of development, transporters and production of extracellular matrix. Transcriptome profile in differentiating adipocytes was markedly different from differentiating osteoblasts with far fewer genes changing activity. The number of regulated genes slowly increased at the mature stage, when adipocytes increased in size and accumulated lipids. This is the first report on in vitro differentiation of aSPCs from Atlantic salmon to mineralizing osteogenic cells. This cell model system provides a new valuable tool for studying osteoblastogenesis in fish.

Power and challenges of using zebrafish as a model for skeletal tissue imaging

The zebrafish (Danio rerio) is now a widely used model organism in biomedical research. The species is also increasingly used for studying skeletal development and regeneration and for understanding human skeletal diseases. The small size of this model organism is an advantage and an extreme challenge for visualizing and diagnosing the animals' skeleton. This applies especially to early stages of skeletal development. Similar challenges arise for the analysis of the skeleton of other small fish species, such as medaka (Oryzias latipes). High quality histological preparations and knowledge about the special quality of the zebrafish skeleton remain prerequisites for a correct analysis. In addition, new methods for fast and high-resolution 2D and 3D skeletal tissue screening are required for a maximal understanding of skeletal development. We, in this study, review advantages and limitations of adapting current visualization techniques for zebrafish skeletal research. We discuss the methods for in toto visualization, such as X-raying, micro-CT, Alizarin red staining and optical projection tomography. Techniques for in vivo imaging, such as second harmonic generation microscopy and two-photon excitation fluorescence, are also discussed. Finally, we explore the possibilities of light-sheet microscopy for the analysis of the zebrafish skeleton.

Fin spine bone resorption in atlantic bluefin tuna, Thunnus thynnus, and comparison between wild and captive-reared specimens

Bone resorption in the first spine of the first dorsal fin of Atlantic bluefin tuna (ABFT) has long been considered for age estimation studies. In the present paper spine bone resorption was assessed in wild (aged 1 to 13 years) and captive-reared (aged 2 to 11 years) ABFT sampled from the Mediterranean Sea. Total surface (TS), solid surface (SS) and reabsorbed surface (RS) were measured in spine transverse sections in order to obtain proportions of SS and RS. The spine section surface was found to be isometrically correlated to the fish fork length by a power equation. The fraction of solid spine bone progressively decreased according to a logarithmic equation correlating SS/TS to both fish size and age. The values ranged from 57% in the smallest examined individuals to 37% in the largest specimens. This phenomenon was further enhanced in captive-reared ABFT where SS/TS was 22% in the largest measured specimen. The difference between the fraction of SS of wild and captive-reared ABFT was highly significant. In each year class from 1- to 7-year-old wild specimens, the fraction of spine reabsorbed surface was significantly higher in specimens collected from March to May than in those sampled during the rest of the year. In 4-year-old fish the normal SS increase during the summer did not occur, possibly coinciding with their first sexual maturity. According to the correlations between SS/TS and age, the rate of spine bone resorption was significantly higher, even almost double, in captive-reared specimens. This could be attributed to the wider context of systemic dysfunctions occurring in reared ABFT, and may be related to a number of factors, including nutritional deficiencies, alteration of endocrine profile, cortisol-induced stress, and loss of spine functions during locomotion in rearing conditions.

Molecular characterization of cbfβ gene and identification of new transcription variants: implications for function

The CBFβ gene encodes a transcription factor that, in combination with CBFα (also called Runx, runt-related transcription factor) regulates expression of several target genes. CBFβ interacts with all Runx family members, such as RUNX2, a regulator of bone-related gene transcription that contains a conserved DNA-binding domain. CBFβ stimulates DNA binding of the Runt domain, and is essential for most of the known functions of RUNX2. A comparative analysis of the zebrafish cbfβ gene and protein, and of its orthologous identified homologous proteins in different species indicates a highly conserved function. We cloned eleven zebrafish cbfβ gene transcripts, one resulting in the known Cbfβ protein (with 187 aa), and three additional variants resulting from skipping exon 5a (resulting in a protein with 174 aa) or exon 5b (resulting in a protein with 201 aa), both observed for the first time in zebrafish, and a completely novel isoform containing both exon 5a and 5b (resulting in a protein with 188 aa). Functional analysis of these isoforms provides insight into their role in regulating gene transcription. From the other variants two are premature termination Cbfβ forms, while the others show in-frame exon-skipping causing changes in the Cbfβ domain that may affect its function.

Alendronate rescued osteoporotic phenotype in a model of glucocorticoid-induced osteoporosis in adult zebrafish scale

Long-term effects of glucocorticoid treatment in humans induce bone loss and increase the risk of fracture in the skeleton. The pathogenic mechanisms of glucocorticoid-induced osteoporosis (GIOP) are still unclear. The GIOP and its effects have been reproduced in several animal models including Danio rerio (zebrafish) embryo. The treatment of adult fish with prednisolone (PN) has shown a dose-dependent decrease of mineralized matrix in the scales. Large resorption lacunae are characterized by single TRAP-positive cells which migrate to the margin of the scale merging into a multinucleated structures. The treatment with PN of cultured scales did not increase TRAP activity suggesting that the massive presence of osteoclasts in the resorption sites could be likely the result of a systemic recruitment of monocyte-macrophage precursors. We observed that treatment with PN induced a significant decrease of the alkaline phosphatase (ALP) activity in scale scleroblasts if compared with untreated controls. Then, we investigated the total mineral balance under prednisolone treatment using a time-dependent double live staining. The untreated fish fully repaired the resorption lacuna induced by prednisolone, whereas treated fish failed. The presence of osteoclast resorption fingerprints on new matrix suggested that the osteoclast activity counterbalances the osteodepositive activity exerted by scleroblasts. The treatment with PN in association with alendronate (AL) has surprisingly resulted in a significant decrease of TRAP activity and increase of ALP compared to PN-treated fish in biochemical and histological assays confirming the action of alendronate against GIOP in fish as well in humans.

Evolutionary Physiology of Bone: Bone Metabolism in Changing Environments

Bone evolved to serve many mechanical and physiological functions. Osteocytes and bone remodeling first appeared in the dermal skeleton of fish, and subsequently adapted to various challenges in terrestrial animals occupying diverse environments. This review discusses the physiology of bone and its role in mechanical and calcium homeostases from an evolutionary perspective. We review how bone physiology responds to changing environments and the adaptations to unique and extreme physiological conditions.

Blocking VEGF signaling delays development of replacement teeth in zebrafish

The dentition in zebrafish is extremely and richly vascularized, but the function of the vasculature, in view of the continuous replacement of the teeth, remains elusive. Through application of SU5416, a vascular endothelial growth factor receptor inhibitor, we studied the role of the blood vessels in the dentition of the zebrafish. We were unable to show an effect on the development of first-generation teeth as well as first tooth replacement. However, in juvenile fish, a delay was observed in the developmental state of the replacement tooth compared with what was expected based on the maturation state of the functional tooth. Furthermore, we observed a difference between treated and nontreated fish in the distance between blood vessels and developing replacement teeth. In conclusion, our results provide support for a nutritive, rather than an inductive, function of the vasculature in the process of tooth development and replacement.

Remodeling in bone without osteocytes: billfish challenge bone structure-function paradigms

A remarkable property of tetrapod bone is its ability to detect and remodel areas where damage has accumulated through prolonged use. This process, believed vital to the long-term health of bone, is considered to be initiated and orchestrated by osteocytes, cells within the bone matrix. It is therefore surprising that most extant fishes (neoteleosts) lack osteocytes, suggesting their bones are not constantly repaired, although many species exhibit long lives and high activity levels, factors that should induce considerable fatigue damage with time. Here, we show evidence for active and intense remodeling occurring in the anosteocytic, elongated rostral bones of billfishes (e.g., swordfish, marlins). Despite lacking osteocytes, this tissue exhibits a striking resemblance to the mature bone of large mammals, bearing structural features (overlapping secondary osteons) indicating intensive tissue repair, particularly in areas where high loads are expected. Billfish osteons are an order of magnitude smaller in diameter than mammalian osteons, however, implying that the nature of damage in this bone may be different. Whereas billfish bone material is as stiff as mammalian bone (unlike the bone of other fishes), it is able to withstand much greater strains (relative deformations) before failing. Our data show that fish bone can exhibit far more complex structure and physiology than previously known, and is apparently capable of localized repair even without the osteocytes believed essential for this process. These findings challenge the unique and primary role of osteocytes in bone remodeling, a basic tenet of bone biology, raising the possibility of an alternative mechanism driving this process.

PDGF-BB secreted by preosteoclasts induces angiogenesis during coupling with osteogenesis

Osteogenesis during bone modeling and remodeling is coupled with angiogenesis. A recent study shows that the specific vessel subtype, strongly positive for CD31 and Endomucin (CD31^hiEmcn^hi), couples angiogenesis and osteogenesis. We found that preosteoclasts secrete platelet derived growth factor-BB (PDGF-BB), inducing CD31^hiEmcn^hi vessels during bone modeling and remodeling. Mice with depletion of PDGF-BB in tartrate-resistant acid phosphatase positive (TRAP⁺) cell lineage (Pdgfb^–/–) show significantly lower trabecular and cortical bone mass, serum and bone marrow PDGF-BB concentrations, and CD31^hiEmcn^hi vessels compared to wild-type mice. In the ovariectomized (OVX) osteoporotic mouse model, concentrations of serum and bone marrow PDGF-BB and CD31^hiEmcn^hi vessels are significantly decreased. Inhibition of cathepsin K (CTSK) increases preosteoclast numbers, resulting in higher levels of PDGF-BB to stimulate CD31^hiEmcn^hi vessels and bone formation in OVX mice. Thus, pharmacotherapies that increase PDGF-BB secretion from preosteoclasts offer a novel therapeutic target for osteoporosis to promote angiogenesis for bone formation.

Fish is Fish: the use of experimental model species to reveal causes of skeletal diversity in evolution and disease

Fishes are wonderfully diverse. This variety is a result of the ability of ray-finned fishes to adapt to a wide range of environments, and has made them more specious than the rest of vertebrates combined. With such diversity it is easy to dismiss comparisons between distantly related fishes in efforts to understand the biology of a particular fish species. However, shared ancestry and the conservation of developmental mechanisms, morphological features and physiology provide the ability to use comparative analyses between different organisms to understand mechanisms of development and physiology. The use of species that are amenable to experimental investigation provides tools to approach questions that would not be feasible in other 'non-model' organisms. For example, the use of small teleost fishes such as zebrafish and medaka has been powerful for analysis of gene function and mechanisms of disease in humans, including skeletal diseases. However, use of these fish to aid in understanding variation and disease in other fishes has been largely unexplored. This is especially evident in aquaculture research. Here we highlight the utility of these small laboratory fishes to study genetic and developmental factors that underlie skeletal malformations that occur under farming conditions. We highlight several areas in which model species can serve as a resource for identifying the causes of variation in economically important fish species as well as to assess strategies to alleviate the expression of the variant phenotypes in farmed fish. We focus on genetic causes of skeletal deformities in the zebrafish and medaka that closely resemble phenotypes observed both in farmed as well as natural populations of fishes.

Intraspecific variability modulates interspecific variability in animal organismal stoichiometry

Interspecific differences in organismal stoichiometry (OS) have been documented in a wide range of animal taxa and are of significant interest for understanding evolutionary patterns in OS. In contrast, intraspecific variation in animal OS has generally been treated as analytical noise or random variation, even though available data suggest intraspecific variability in OS is widespread. Here, we assess how intraspecific variation in OS affects inferences about interspecific OS differences using two co-occurring Neotropical fishes: Poecilia reticulata and Rivulus hartii. A wide range of OS has been observed within both species and has been attributed to environmental differences among stream systems. We assess the contributions of species identity, stream system, and the interactions between stream and species to variability in N:P, C:P, and C:N. Because predation pressure can impact the foraging ecology and life-history traits of fishes, we compare predictors of OS between communities that include predators, and communities where predators are absent. We find that species identity is the strongest predictor of N:P, while stream or the interaction of stream and species contribute more to the overall variation in C:P and C:N. Interspecific differences in N:P, C:P, and C:N are therefore not consistent among streams. The relative contribution of stream or species to OS qualitatively changes between the two predation communities, but these differences do not have appreciable effects in interspecific patterns. We conclude that although species identity is a significant predictor of OS, intraspecific OS is sometimes sufficient to overwhelm or obfuscate interspecific differences in OS.

The fragile elderly hip: mechanisms associated with age-related loss of strength and toughness

Every hip fracture begins with a microscopic crack, which enlarges explosively over microseconds. Most hip fractures in the elderly occur on falling from standing height, usually sideways or backwards. The typically moderate level of trauma very rarely causes fracture in younger people. Here, this paradox is traced to the decline of multiple protective mechanisms at many length scales from nanometres to that of the whole femur. With normal ageing, the femoral neck asymmetrically and progressively loses bone tissue precisely where the cortex is already thinnest and is also compressed in a sideways fall. At the microscopic scale of the basic remodelling unit (BMU) that renews bone tissue, increased numbers of actively remodelling BMUs associated with the reduced mechanical loading in a typically inactive old age augments the numbers of mechanical flaws in the structure potentially capable of initiating cracking. Menopause and over-deep osteoclastic resorption are associated with incomplete BMU refilling leading to excessive porosity, cortical thinning and disconnection of trabeculae. In the femoral cortex, replacement of damaged bone or bone containing dead osteocytes is inefficient, impeding the homeostatic mechanisms that match strength to habitual mechanical usage. In consequence the participation of healthy osteocytes in crack-impeding mechanisms is impaired. Observational studies demonstrate that protective crack deflection in the elderly is reduced. At the most microscopic levels attention now centres on the role of tissue ageing, which may alter the relationship between mineral and matrix that optimises the inhibition of crack progression and on the role of osteocyte ageing and death that impedes tissue maintenance and repair. This review examines recent developments in the understanding of why the elderly hip becomes fragile. This growing understanding is suggesting novel testable approaches for reducing risk of hip fracture that might translate into control of the growing worldwide impact of hip fractures on our ageing populations.

Bone marrow mesenchymal stem cells and TGF-β signaling in bone remodeling

During bone resorption, abundant factors previously buried in the bone matrix are released into the bone marrow microenvironment, which results in recruitment and differentiation of bone marrow mesenchymal stem cells (MSCs) for subsequent bone formation, temporally and spatially coupling bone remodeling. Parathyroid hormone (PTH) orchestrates the signaling of many pathways that direct MSC fate. The spatiotemporal release and activation of matrix TGF-β during osteoclast bone resorption recruits MSCs to bone-resorptive sites. Dysregulation of TGF-β alters MSC fate, uncoupling bone remodeling and causing skeletal disorders. Modulation of TGF-β or PTH signaling may reestablish coupled bone remodeling and be a potential therapy.

Prednisolone induces osteoporosis-like phenotype in regenerating zebrafish scales

We demonstrate that glucocorticoids induce an osteoporotic phenotype in regenerating scales of zebrafish. Exposure to prednisolone results in altered mineral content, enhanced matrix breakdown, and an osteoporotic gene-expression profile in osteoblasts and osteoclasts. This highlights that the zebrafish scale provides a powerful tool for preclinical osteoporosis research.

INTRODUCTION

This study aims to evaluate whether glucocorticoid (prednisolone) treatment of zebrafish induces an osteoporotic phenotype in regenerating scales. Scales, a readily accessible dermal bone tissue, may provide a tool to study direct osteogenesis and its disturbance by glucocorticoids.

METHODS

In adult zebrafish, treated with 25 μM prednisolone phosphate via the water, scales were removed and allowed to regenerate. During regeneration scale morphology and the molar calcium/phosphorus ratio in scales were assessed and osteoblast and osteoclast activities were monitored by time profiling of cell-specific genes; mineralization was visualized by Von Kossa staining, osteoclast activity by tartrate-resistant acid phosphatase histochemistry.

RESULTS

Prednisolone (compared to controls) enhances osteoclast activity and matrix resorption and slows down the build up of the calcium/phosphorus molar ratio indicative of altered crystal maturation. Prednisolone treatment further impedes regeneration through a shift in the time profiles of osteoblast and osteoclast genes that commensurates with an osteoporosis-like imbalance in bone formation.

CONCLUSIONS

A glucocorticoid-induced osteoporosis phenotype as seen in mammals was induced in regenerating scalar bone of zebrafish treated with prednisolone. An unsurpassed convenience and low cost then make the zebrafish scale a superior model for preclinical studies in osteoporosis research.

Vitamin D: calcium and bone homeostasis during evolution

Vitamin D3 is already found early in the evolution of life but essentially as inactive end products of the photochemical reaction of 7-dehydrocholestol with ultraviolet light B. A full vitamin D (refers to vitamin D2 and D3) endocrine system, characterized by a specific VDR (vitamin D receptor, member of the nuclear receptor family), specific vitamin D metabolizing CYP450 enzymes regulated by calciotropic hormones and a dedicated plasma transport-protein is only found in vertebrates. In the earliest vertebrates (lamprey), vitamin D metabolism and VDR may well have originated from a duplication of a common PRX/VDR ancestor gene as part of a xenobiotic detoxification pathway. The vitamin D endocrine system, however, subsequently became an important regulator of calcium supply for an extensive calcified skeleton. Vitamin D is essential for normal calcium and bone homeostasis as shown by rickets in vitamin D-deficient growing amphibians, reptiles, birds and mammals. From amphibians onward, bone is gradually more dynamic with regulated bone resorption, mainly by combined action of PTH and 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) on the generation and function of multinucleated osteoclasts. Therefore, bone functions as a large internal calcium reservoir, under the control of osteoclasts. Osteocytes also display a remarkable spectrum of activities, including mechanical sensing and regulating mineral homeostasis, but also have an important role in global nutritional and energy homeostasis. Mineralization from reptiles onward is under the control of well-regulated SIBLING proteins and associated enzymes, nearly all under the control of 1,25(OH)2D3. The vitamin D story thus started as inert molecule but gained an essential role for calcium and bone homeostasis in terrestrial animals to cope with the challenge of higher gravity and calcium-poor environment.

Probiotic supplementation promotes calcification in Danio rerio larvae: a molecular study

A growing number of studies have been showing that dietary probiotics can exert beneficial health effects in both humans and animals. We previously demonstrated that dietary supplementation with Lactobacillus rhamnosus - a component of the human gut microflora - enhances reproduction, larval development, and the biomineralization process in Danio rerio (zebrafish). The aim of this study was to identify the pathways affected by L. rhamnosus during zebrafish larval development. Our morphological and histochemical findings show that L. rhamnosus accelerates bone deposition through stimulation of the expression of key genes involved in ossification, e.g. runt-related transcription factor 2 (runx2), Sp7 transcription factor (sp7), matrix Gla protein (mgp), and bone gamma-carboxyglutamate (gla) protein (bglap) as well as through inhibition of sclerostin (sost), a bone formation inhibitor. Western blot analysis of mitogen-activated protein kinase 1 and 3-(Mapk1 and Mapk3), which are involved in osteoblast and osteocyte differentiation, documented an increase in Mapk1 16 days post fertilization (dpf) and of Mapk3 23 dpf in individuals receiving L. rhamnosus supplementation. Interestingly, a reduction of sost detected in the same individuals suggests that the probiotic may help treat bone disorders.

Development of an in vitro cell system from zebrafish suitable to study bone cell differentiation and extracellular matrix mineralization

Mechanisms of bone formation and skeletal development have been successfully investigated in zebrafish using a variety of in vivo approaches, but in vitro studies have been hindered due to a lack of homologous cell lines capable of producing an extracellular matrix (ECM) suitable for mineral deposition. Here we describe the development and characterization of a new cell line termed ZFB1, derived from zebrafish calcified tissues. ZFB1 cells have an epithelium-like phenotype, grow at 28°C in a regular L-15 medium supplemented with 15% of fetal bovine serum, and are maintained and manipulated using standard methods (e.g., trypsinization, cryopreservation, and transfection). They can therefore be propagated and maintained easily in most cell culture facilities. ZFB1 cells show aneuploidy with 2n=78 chromosomes, indicative of cell transformation. Furthermore, because DNA can be efficiently delivered into their intracellular space by nucleofection, ZFB1 cells are suitable for gene targeting approaches and for assessing gene promoter activity. ZFB1 cells can also differentiate toward osteoblast or chondroblast lineages, as demonstrated by expression of osteoblast- and chondrocyte-specific markers, they exhibit an alkaline phosphatase activity, a marker of bone formation in vivo, and they can mineralize their ECM. Therefore, they represent a valuable zebrafish-derived in vitro system for investigating bone cell differentiation and extracellular matrix mineralization.

A bone to pick with zebrafish

The development of high-throughput sequencing and genome-wide association studies allows us to deduce the genetic factors underlying diseases much more rapidly than possible through classical genetics, but a true understanding of the molecular mechanisms of these diseases still relies on integrated approaches including in vitro and in vivo model systems. One such model that is particularly suitable for studying bone diseases is the zebrafish (Danio rerio), a small fresh-water teleost that is highly amenable to genetic manipulation and in vivo imaging. Zebrafish physiology and genome organization are in many aspects similar to those of humans, and the skeleton and mineralizing tissues are no exception. In this review, we highlight some of the contributions that have been made through the study of mutant zebrafish that feature bone and/or mineralization disorders homologous to human diseases, including osteogenesis imperfecta, fibrodysplasia ossificans progressiva and generalized arterial calcification of infancy. The genomic and phenotypic similarities between the zebrafish and human cases are illustrated. We show that, despite some systemic physiological differences between mammals and teleosts, and a relative lack of a history as a model for bone research, the zebrafish represents a useful complement to mouse and tissue culture systems in the investigation of genetic bone disorders.

Characterization of zebrafish mutants with defects in bone calcification during development

Using the fluorescent dyes calcein and alcian blue, we stained the F3 generation of chemically (ENU) mutagenized zebrafish embryos and larvae, and screened for mutants with defects in bone development. We identified a mutant line, bone calcification slow (bcs), which showed delayed axial vertebra calcification during development. Before 4-5 days post-fertilization (dpf), the bcs embryos did not display obvious abnormalities in bone development (i.e., normal number, size and shape of cartilage and vertebrae). At 5-6 dpf, when vertebrae calcification starts, bcs embryos began to show defects. At 7 dpf, for example, in most of the bcs embryos examined, calcein staining revealed no signals of vertebrae mineralization, whereas during the same developmental stages, 2-14 mineralized vertebrae were observed in wild-type animals. Decreases in the number of calcified vertebrae were also observed in bcs mutants when examined at 9 and 11 dpf, respectively. Interestingly, by 13 dpf the defects in bcs mutants were no longer evident. There were no significant differences in the number of calcified vertebrae between wild-type and mutant animals. We examined the expression of bone development marker genes (e.g., Sox9b, Bmp2b, and Cyp26b1, which play important roles in bone formation and calcification). In mutant fish, we observed slight increases in Sox9b expression, no alterations in Bmp2b expression, but significant increases in Cyp26b1 expression. Together, the data suggest that bcs delays axial skeletal calcification, but does not affect bone formation and maturation.

Comparative analysis of a teleost skeleton transcriptome provides insight into its regulation

An articulated endoskeleton that is calcified is a unifying innovation of the vertebrates, however the molecular basis of the structural divergence between terrestrial and aquatic vertebrates, such as teleost fish, has not been determined. In the present study long-read next generation sequencing (NGS, Roche 454 platform) was used to characterize acellular perichondral bone (vertebrae) and chondroid bone (gill arch) in the gilthead sea bream (Sparus auratus). A total of 15.97 and 14.53Mb were produced, respectively from vertebrae and gill arch cDNA libraries and yielded 32,374 and 28,371 contigs (consensus sequences) respectively. 10,455 contigs from vertebrae and 10,625 contigs from gill arches were annotated with gene ontology terms. Comparative analysis of the global transcriptome revealed 4249 unique transcripts in vertebrae, 4201 unique transcripts in the gill arches and 3700 common transcripts. Several core gene networks were conserved between the gilthead sea bream and mammalian skeleton. Transcripts for putative endocrine factors were identified in acellular gilthead sea bream bone suggesting that in common with mammalian bone it can act as an endocrine tissue. The acellular bone of the vertebra, in contrast to current opinion based on histological analysis, was responsive to a short fast and significant (p<0.05) down-regulation of several transcripts identified by NGS, osteonectin, osteocalcin, cathepsin K and IGFI occurred. In gill arches fasting caused a significant (p<0.05) down-regulation of osteocalcin and up-regulation of MMP9.

PTHrP-induced modifications of the sea bream (Sparus auratus) vertebral bone proteome

Endocrine factors play an essential role in the formation and turnover of the skeleton in vertebrates. In the present study sea bream vertebral bone transcripts for PTH1R and PTH3R were identified and the action of intermittent administration of parathyroid hormone related protein (PTHrP) on the proteome of vertebral bone was analysed. Treatment of immature sea bream (Sparus auratus, n=6) for 5days with homologous recombinant PTHrP(1-125; 150ng/g body weight) modified bone metabolism and caused a significant (p<0.05) reduction in both tartrate resistant acid phosphatase (TRACP) and alkaline phosphatase (ALP) in relation to control fish. However, the ratio of TRACP: ALP in PTHrP treated fish (1.3 to 2.2 cf. control) suggested it had an anabolic response. A sea bream vertebral bone proteome of 157 protein spots was generated and putative identity assigned to 118 (75.2%) proteins of which 72% had homology to proteins/transcripts from teleosts many of which have not previously been reported in teleost bone. Classification of bone proteins using gene ontology revealed those with protein or metal/ion (e.g., calcium, magnesium, zinc) binding (∼53%) activities were most abundant. The expression of eight proteins was significantly (p<0.05) modified in the vertebra of PTHrP treated compared to control fish; three were up-regulated, betainehomocystein S-methyltransferase, glial fibrillary acidic protein, parvalbumin beta and five were down-regulated, annexin A5, apolipoprotein A1, myosin light chain 2, fast skeletal myosin light chain 3, troponin C. In conclusion, intermittent administration of PTHrP to sea bream is associated with an anabolic response in vertebral bone metabolism and modifies calcium binding proteins in the proteome.

Shaping development through mechanical strain: the transcriptional basis of diet-induced phenotypic plasticity in a cichlid fish

Adaptive phenotypic plasticity, the ability of an organism to change its phenotype to match local environments, is increasingly recognized for its contribution to evolution. However, few empirical studies have explored the molecular basis of plastic traits. The East African cichlid fish Astatoreochromis alluaudi displays adaptive phenotypic plasticity in its pharyngeal jaw apparatus, a structure that is widely seen as an evolutionary key innovation that has contributed to the remarkable diversity of cichlid fishes. It has previously been shown that in response to different diets, the pharyngeal jaws change their size, shape and dentition: hard diets induce an adaptive robust molariform tooth phenotype with short jaws and strong internal bone structures, while soft diets induce a gracile papilliform tooth phenotype with elongated jaws and slender internal bone structures. To gain insight into the molecular underpinnings of these adaptations and enable future investigations of the role that phenotypic plasticity plays during the formation of adaptive radiations, the transcriptomes of the two divergent jaw phenotypes were examined. Our study identified a total of 187 genes whose expression differs in response to hard and soft diets, including immediate early genes, extracellular matrix genes and inflammatory factors. Transcriptome results are interpreted in light of expression of candidate genes-markers for tooth size and shape, bone cells and mechanically sensitive pathways. This study opens up new avenues of research at new levels of biological organization into the roles of phenotypic plasticity during speciation and radiation of cichlid fishes.

Vestiges, rudiments and fusion events: the zebrafish caudal fin endoskeleton in an evo-devo perspective

The vertebral column results from a controlled segmentation process associated with two main structures, the notochord and the somites. Pathological fusion of vertebral bodies can result from impaired segmentation during embryonic development or occur postnatally. Here, we explore the process of formation and subsequent fusion of the caudalmost vertebral bodies in zebrafish, where fusion is a normal process, mechanically required to support the caudal fin. To reveal whether the product of fusion is on an evolutionary or a developmental scale, we analyze the mode of formation of vertebral bodies, identify transitory rudiments, and characterize vestiges that indicate previous fusion events. Based on a series of closely spaced ontogenetic stages of cleared and stained zebrafish, parasagittal sections, and detection methods for elastin and mineral, we conclude that the formation of the urostyle involves four fusion events. Although fusion of preural 1 (PU1(+) ) with ural 1 (U1) and fusion within ural 2 (U2(+) ) are no longer traceable during centrum formation (phylogenetic fusion), fusion between the compound centrum [PU1(+) +U1] and U2(+) (ontogenetic fusion) occurs after individualization of the centra. This slow process is the last fusion and perhaps the latest fusion during the evolution of the zebrafish caudal fin endoskeleton. Newly described characters, such as a mineralized subdivision within U2(+) , together with the reinterpretation of known features in an evolutionary-developmental context, strongly suggest that the zebrafish caudal fin endoskeleton is made from more fused vertebral bodies than previously assumed. In addition, these fusion events occur at different developmental levels depending on their evolutionary status, allowing the dissection of fusion processes that have taken place over different evolutionary times.

Cavities in the compact bone in tetrapods and fish and their effect on mechanical properties

Bone includes cavities in various length scales, from nanoporosities occurring between the collagen fibrils and the mineral crystals all the way to macrocavities like the medullary cavity. In particular, bone is permeated by a vast number of channels (the lacunar-canalicular system), that reduce the stiffness and, more importantly, the strength of the bone that they permeate. These consequences are presumably a price worth paying for the ability of the lacunar-canalicular system to detect changes in the strain environment within the bone material and, when deleterious, to trigger processes like modeling or remodeling which 'rectify' it. Here we review the size and density of the various types of cavities in bone, and discuss their effect on the mechanical properties of cortical bone. In this respect the bones of advanced teleost fish species (probably the majority of all vertebrate species) are an unsolved conundrum because they lack bone cells (and therefore lacunae and canaliculi) in their skeleton. Yet, despite being acellular, some of these fish can undergo considerable remodeling in at least some parts of their skeleton. We address, but do not solve this mystery.

The enigmas of bone without osteocytes

One of the hallmarks of tetrapod bone is the presence of numerous cells (osteocytes) within the matrix. Osteocytes are vital components of tetrapod bone, orchestrating the processes of bone building, reshaping and repairing (modeling and remodeling), and probably also participating in calcium-phosphorus homeostasis via both the local process of osteocytic osteolysis, and systemic effect on the kidneys. Given these critical roles of osteocytes, it is thought-provoking that the entire skeleton of many fishes consists of bone material that does not contain osteocytes. This raises the intriguing question of how the skeleton of these animals accomplishes the various essential functions attributed to osteocytes in other vertebrates, and raises the possibility that in acellular bone some of these functions are either accomplished by non-osteocytic routes or not necessary at all. In this review, we outline evidence for and against the fact that primary functions normally ascribed to osteocytes, such as mechanosensation, regulation of osteoblast/clast activity and mineral metabolism, also occur in fish bone devoid of these cells, and therefore must be carried out through alternative and perhaps ancient pathways. To enable meaningful comparisons with mammalian bone, we suggest thorough, phylogenetic examinations of regulatory pathways, studies of structure and mechanical properties and surveys of the presence/absence of bone cells in fishes. Insights gained into the micro-/nanolevel structure and architecture of fish bone, its mechanical properties and its physiology in health and disease will contribute to the discipline of fish skeletal biology, but may also help answer questions of basic bone biology.

Exercise induced mechano-sensing and substance P mediated bone modeling in Atlantic salmon

Mechanical stress plays a vital role in maintaining bone architecture. The process by which osteogenic cells convert the mechanical signal into a biochemical response governing bone modeling is not clear, however. In this study, we investigated how Atlantic salmon (Salmo salar) vertebra responds to exercise-induced mechanical loading. Bone formation in the vertebrae was favored through increased expression of genes involved in osteoid production. Fourier transform infrared spectroscopy (FT-IR) showed that bone matrix secreted both before and during sustained swimming had different properties after increased load compared to control, suggesting that both new and old bones are affected. Concomitantly, both osteoblasts and osteocytes in exercised salmon showed increased expression of the receptor nk-1 and its ligand substance P (SP), both known to be involved in osteogenesis. Moreover, in situ hybridization disclosed SP mRNA in osteoblasts and osteocytes, supporting an autocrine function. The functional role of SP was investigated in vitro using osteoblasts depleted for SP. The cells showed severely reduced transcription of genes involved in mineralization, demonstrating a regulatory role for SP in salmon osteoblasts. Investigation of α-tubulin stained osteocytes revealed cilia-like structures. Together with SP, cilia may link mechanical responses to osteogenic processes in the absence of a canaliculi network. Our results imply that salmon vertebral bone responds to mechanical load through a highly interconnected and complex signal and detection system, with SP as a key factor for initializing mechanically-induced bone formation in bone lacking the canaliculi system.

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

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

Vertebrate extracellular calcium-sensing receptor evolution: selection in relation to life history and habitat

Ionic calcium (Ca(2+)) supports essential functions within physiological systems, and consequently its concentration is homeostatically regulated within narrow bounds in the body fluids of animals through endocrine effects at ion-transporting osmoregulatory tissues. In vertebrates, extracellular Ca(2+) is detected at the cell surface by the extracellular calcium-sensing receptor (CaSR), a member of the G protein-coupled receptor (GPCR) superfamily. Interestingly, the taxonomic distribution of CaSRs is restricted to vertebrates, with some CaSR-like receptors apparently present in non-vertebrate chordates. Since bone is a known Ca(2+) storage site and is characteristically restricted to the vertebrate lineage, we hypothesized a functional association of CaSR with vertebrate skeleton that may have an ancient origin. Protein sequence alignment and phylogenetic analysis of vertebrate CaSRs and related GPCRs of the glutamate receptor-like family expose similarities and indel differences among these receptors, and reveal the evolutionary history of CaSRs. Evolutionary selection was tested statistically by evaluating the relationship between non-synonymous (replacement, dN) versus synonymous (silent, dS) amino acid substitution rates (as dN/dS) of protein-coding DNA sequences among branches of the estimated protein phylogeny. On a background of strong purifying selection (dN/dS<1) in the CaSR phylogeny, statistical evidence for adaptive evolution (dN/dS>1) was detected on some branches to major clades in the CaSR phylogeny, especially to the tetrapod vertebrate CaSRs and chordate CaSR-like branches. Testing also revealed overall purifying selection at the codon level. At some sites relaxation from strong purifying selection was seen, but evidence for adaptive evolution was not detected for individual sites. The results suggest purifying selection of CaSRs, and of adaptive evolution among some major vertebrate clades, reflecting clade specific differences in natural history and organismal biology, including skeletal involvement in calcium homeostasis.

Divergent responsiveness of the dentary and vertebral bone to a selective estrogen-receptor modulator (SERM) in the teleost Sparus auratus

In teleosts the regulation of skeletal homeostasis and turnover by estrogen is poorly understood. For this reason raloxifene, a selective estrogen-receptor modulator (SERM), was administered to sea bream (Sparus auratus) and its effect on plasma calcium balance and transcript expression in dentary (dermal bone) and vertebra (perichondral bone) was studied. The concentration of total calcium or phosphorus in plasma was unchanged by raloxifene treatment for 6days. The activity of alkaline phosphatase (ALP) in dentary bone of raloxifene treated fish was significantly (p<0.05) higher than control fish but it was not changed in vertebral bone. Transcripts for estrogen receptor (ER) α were in very low abundance in the sea bream dentary and vertebra and were unchanged by raloxifene treatment. In contrast, raloxifene caused significant (p<0.05) up-regulation of the duplicate ERβ transcripts in the dentary but did not affect specific transcripts for osteoclast (TRAP), osteoblast (ALP, Runx2, osteonectin) or cartilage (IGF1, CILP2, FN1a). In the vertebra ERβb was not changed by raloxifene but ERβa was significantly (p<0.05) down-regulated as was the skeletal specific transcripts, TRAP, ALP, CILP2, FN1a. In summary, ERβs regulate estrogen sensitivity of the skeleton in sea bream, which responds in a non uniform manner. In common with mammals raloxifene appears to have an anti-resorptive role (in sea bream vertebra), but also an osteoblast stimulatory role, inducing ALP activity in the dentary of sea bream. Overall, the results indicate bone specific responsiveness to raloxifene in the sea bream. Further work will be required to understand the basis of bone responsiveness and the role of E(2) and ERs in teleost bone homeostasis.

Handed foraging behavior in scale-eating cichlid fish: its potential role in shaping morphological asymmetry

Scale-eating cichlid fish, Perissodus microlepis, from Lake Tanganyika display handed (lateralized) foraging behavior, where an asymmetric ‘left’ mouth morph preferentially feeds on the scales of the right side of its victim fish and a ‘right’ morph bites the scales of the left side. This species has therefore become a textbook example of the astonishing degree of ecological specialization and negative frequency-dependent selection. We investigated the strength of handedness of foraging behavior as well as its interaction with morphological mouth laterality in P. microlepis. In wild-caught adult fish we found that mouth laterality is, as expected, a strong predictor of their preferred attack orientation. Also laboratory-reared juvenile fish exhibited a strong laterality in behavioral preference to feed on scales, even at an early age, although the initial level of mouth asymmetry appeared to be small. This suggests that pronounced mouth asymmetry is not a prerequisite for handed foraging behavior in juvenile scale-eating cichlid fish and might suggest that behavioral preference to attack a particular side of the prey plays a role in facilitating morphological asymmetry of this species.

Emerging Functions of RANKL in Lymphoid Tissues

The tumor necrosis factor superfamily (TNFSF) members play pivotal roles in embryonic development of lymphoid tissue and their homeostasis. RANKL (Receptor activator of NF-κB ligand, also called TRANCE, TNFSF11) is recognized as an important player in bone homeostasis and lymphoid tissue formation. In its absence bone mass control is deregulated and lymph nodes fail to develop. While its function in bone is well described, there is still little functional insight into the action of RANKL in lymphoid tissue development and homeostasis. Here we provide an overview of the known functions of RANKL, its signaling receptor RANK and its decoy receptor OPG from the perspective of lymphoid tissue development and immune activation in the mouse. Expressed by the hematopoietic lymphoid tissue inducing (LTi) cells and the mesenchymal lymphoid tissue organizer (LTo) cells, RANKL was shown to stimulate Lymphotoxin (LT) expression and to be implicated in LTi cell accumulation. Our recent finding that RANKL also triggers proliferation of adult lymph node stroma suggests that RANKL may furthermore directly activate LTo cells. Beyond bone, the RANKL-RANK-OPG triad plays important roles in immunobiology that are waiting to be unraveled.

Comparison of structural, architectural and mechanical aspects of cellular and acellular bone in two teleost fish

The histological diversity of the skeletal tissues of fishes is impressive compared with that of other vertebrate groups, yet our understanding of the functional consequences of this diversity is limited. In particular, although it has been known since the mid-1800s that a large number of fish species possess acellular bones, the mechanical advantages and consequences of this structural characteristic – and therefore the nature of the evolution of this feature – remain unclear. Although several studies have examined the material properties of fish bone, these have used a variety of techniques and there have been no direct contrasts of acellular and cellular bone. We report on a comparison of the structural and mechanical properties of the ribs and opercula between two freshwater fish – the common carp Cyprinus carpio (a fish with cellular bone) and the tilapia Oreochromis aureus (a fish with acellular bone). We used light microscopy to show that the bones in both fish species exhibit poor blood supply and possess discrete tissue zones, with visible layering suggesting differences in the underlying collagen architecture. We performed identical micromechanical testing protocols on samples of the two bone types to determine the mechanical properties of the bone material of opercula and ribs. Our data support the consensus of literature values, indicating that Young’s moduli of cellular and acellular bones are in the same range, and lower than Young’s moduli of the bones of mammals and birds. Despite these similarities in mechanical properties between the bone tissues of the fish species tested here, cellular bone had significantly lower mineral content than acellular bone; furthermore, the percentage ash content and bone mineral density values (derived from micro-CT scans) show that the bone of these fishes is less mineralized than amniote bone. Although we cannot generalize from our data to the numerous remaining teleost species, the results presented here suggest that while cellular and acellular fish bone may perform similarly from a mechanical standpoint, there are previously unappreciated differences in the structure and composition of these bone types.

Swim-training changes the spatio-temporal dynamics of skeletogenesis in zebrafish larvae (Danio rerio)

Fish larvae experience many environmental challenges during development such as variation in water velocity, food availability and predation. The rapid development of structures involved in feeding, respiration and swimming increases the chance of survival. It has been hypothesized that mechanical loading induced by muscle forces plays a role in prioritizing the development of these structures. Mechanical loading by muscle forces has been shown to affect larval and embryonic bone development in vertebrates, but these investigations were limited to the appendicular skeleton. To explore the role of mechanical load during chondrogenesis and osteogenesis of the cranial, axial and appendicular skeleton, we subjected zebrafish larvae to swim-training, which increases physical exercise levels and presumably also mechanical loads, from 5 until 14 days post fertilization. Here we show that an increased swimming activity accelerated growth, chondrogenesis and osteogenesis during larval development in zebrafish. Interestingly, swim-training accelerated both perichondral and intramembranous ossification. Furthermore, swim-training prioritized the formation of cartilage and bone structures in the head and tail region as well as the formation of elements in the anal and dorsal fins. This suggests that an increased swimming activity prioritized the development of structures which play an important role in swimming and thereby increasing the chance of survival in an environment where water velocity increases. Our study is the first to show that already during early zebrafish larval development, skeletal tissue in the cranial, axial and appendicular skeleton is competent to respond to swim-training due to increased water velocities. It demonstrates that changes in water flow conditions can result into significant spatio-temporal changes in skeletogenesis.

Matrilin-1 expression is increased in the vertebral column of Atlantic salmon (Salmo salar L.) individuals displaying spinal fusions

We have previously characterized the development of vertebral fusions induced by elevated water temperature in Atlantic salmon. Molecular markers of bone and cartilage development together with histology were used to understand the complex pathology and mechanism in the development of this spinal malformation. In this study, we wanted to use proteomics, a non-hypothetical approach to screen for possible new markers involved in the fusion process. Proteins extracted from non-deformed and fused vertebrae of Atlantic salmon were therefore compared by two-dimensional electrophoresis (2DE) and MALDI-TOF analysis. Data analysis of protein spots in the 2DE gels demonstrated matrilin-1, also named cartilage matrix protein, to be the most highly up-regulated protein in fused compared with non-deformed vertebrae. Furthermore, real-time PCR analysis showed strong up-regulation of matrilin-1 mRNA in fused vertebrae. Immunohistochemistry demonstrated induced matrilin-1 expression in trans-differentiating cells undergoing a metaplastic shift toward chondrocytes in fusing vertebrae, whereas abundant expression was demonstrated in cartilaginous tissue and chordocytes of both non-deformed and fused vertebrae. These results identifies matrilin-1 as a new interesting candidate in the fusion process, and ratify the use of proteomic as a valuable technique to screen for markers involved in vertebral pathogenesis.

Rankl-induced osteoclastogenesis leads to loss of mineralization in a medaka osteoporosis model

Osteoclasts are macrophage-related bone resorbing cells of hematopoietic origin. Factors that regulate osteoclastogenesis are of great interest for investigating the pathology and treatment of bone diseases such as osteoporosis. In mammals, receptor activator of NF-κB ligand (Rankl) is a regulator of osteoclast formation and activation: its misexpression causes osteoclast stimulation and osteoporotic bone loss. Here, we report an osteoporotic phenotype that is induced by overexpression of Rankl in the medaka model. We generated transgenic medaka lines that express GFP under control of the cathepsin K promoter in osteoclasts starting at 12 days post-fertilization (dpf), or Rankl together with CFP under control of a bi-directional heat-shock promoter. Using long-term confocal time-lapse imaging of double and triple transgenic larvae, we monitored in vivo formation and activation of osteoclasts, as well as their interaction with osteoblasts. Upon Rankl induction, GFP-positive osteoclasts are first observed in the intervertebral regions and then quickly migrate to the surface of mineralized neural and haemal arches, as well as to the centra of the vertebral bodies. These osteoclasts are TRAP (tartrate-resistant acid phosphatase) and cathepsin K positive, mononuclear and highly mobile with dynamically extending protrusions. They are exclusively found in tight contact with mineralized matrix. Rankl-induced osteoclast formation resulted in severe degradation of the mineralized matrix in vertebral bodies and arches. In conclusion, our in vivo imaging approach confirms a conserved role of Rankl in osteoclastogenesis in teleost fish and provides new insight into the cellular interactions during bone resorption in an animal model that is useful for genetic and chemical screening.

Comparative morphology of the osteocyte lacunocanalicular system in various vertebrates

Osteocytes are embedded in the bone matrix, and they communicate with adjacent osteocytes, osteoblasts, and osteoclasts through the osteocyte lacunocanalicular system. Osteocytes are believed to be essential for the maintenance of bone homeostasis because they regulate mechanical sensing and mineral metabolism in mammalian bones; however, osteocyte morphology in other vertebrates has not been well documented. We conducted a comparative study on the morphology of osteocytes and the lacunocanalicular system of the following vertebrates: two teleost fishes [medaka (Oryzias latipes), and zebrafish (Danio rerio)], three amphibians [African clawed frog (Xenopus laevis), black-spotted pond frog (Rana nigromaculata), and Japanese fire-bellied newt (Cynops pyrrhogaster)], two reptiles [four-toed tortoise (Testudo horsfieldii) and green iguana (Iguana iguana)], and two mammals (laboratory mouse C57BL6 and human). The distribution of the osteocyte lacunocanalicular system in all these animals was investigated using the modified silver staining and the fluorescein-conjugated phalloidin staining methods. Bones of medaka had few osteocytes (acellular bone). Bones of zebrafish contained osteocytes (cellular bone) but had a poorly developed osteocyte lacunocanalicular system. Bones of Xenopus laevis, a freshwater species, and of other amphibians, reptiles, and mammals contained numerous osteocytes and a well-developed lacunocanalicular system. The present study indicates that development of the osteocyte lacunocanalicular system differs between teleost fishes and land vertebrates, but this pattern is not directly related to aquatic habitat.

Osteoclasts in bone modeling, as revealed by in vivo imaging, are essential for organogenesis in fish

Bone modeling is the central system controlling the formation of bone including bone growth and shape in early development, in which bone is continuously resorbed by osteoclasts and formed by osteoblasts. However, this system has not been well documented, because it is difficult to trace osteoclasts and osteoblasts in vivo during development. Here we showed the important role of osteoclasts in organogenesis by establishing osteoclast-specific transgenic medaka lines and by using a zebrafish osteoclast-deficient line. Using in vivo imaging of osteoclasts in the transgenic medaka carrying an enhanced GFP (EGFP) or DsRed reporter gene driven by the medaka TRAP (Tartrate-Resistant Acid Phosphatase) or Cathepsin K promoter, respectively, we examined the maturation and migration of osteoclasts. Our results showed that mononuclear or multinucleated osteoclasts in the vertebral body were specifically localized at the inside of the neural and hemal arches, but not at the vertebral centrum. Furthermore, transmission electron microscopic (TEM) analyses revealed that osteoclasts were flat-shaped multinucleated cells, suggesting that osteoclasts initially differentiate from TRAP-positive mononuclear cells residing around bone. The zebrafish panther mutant lacks a functional c-fms (receptor for macrophage colony-stimulating factor) gene crucial for osteoclast proliferation and differentiation and thus has a low number of osteoclasts. Analysis of this mutant revealed deformities in both its neural and hemal arches, which resulted in abnormal development of the neural tube and blood vessels located inside these arches. Our results provide the first demonstration that bone resorption during bone modeling is essential for proper development of neural and vascular systems associated with fish vertebrae.