Vertebral column and associated elements in dipnoans and comparison with other fishes: development and homology

A new method for regionalization of the vertebral column in salmonids based on radiographic hallmarks

Current procedures to establish vertebral column regionalization (e.g., histology) in fish are time consuming and difficult to apply. The aim of this study was to develop a more rapid and accurate radiology-based method for Atlantic salmon (Salmo salar). A detailed analysis of 90 animals (4 kg) led to the establishment of region-specific radiographic hallmarks. To elucidate its transferability to other salmonid species, radiography was carried out in brown trout (Salmo trutta), Arctic char (Salvelinus alpinus), rainbow trout (Oncorhynchus mykiss), pink salmon (Oncorhynchus gorbuscha), and Chinook salmon (Oncorhynchus tshawytscha). This method was also evaluated for whole ungutted fish. The vertebral column of Atlantic salmon can be subdivided into five regions (R1-R5) based on anatomy: postcranial (R1, V1, and V2), abdominal (R2, V3-V26), transitional (R3, V27-V36), caudal (R4, V37-V53), and ural (R5, V54-V59). The following specific radiographic hallmarks allow the identification of regions: (i) lack of ribs in R1, (ii) modified parapophysis of the first vertebra of R3, (iii) prominent hemal spine of the first vertebra of R4, and (iv) the separated hemal spine of the most cranial pre-ural vertebra of R5. These hallmarks were all transferable to the other salmonid species assessed. The results include a further description of various region-specific characteristics in Atlantic salmon. The method was found applicable for sedated/whole ungutted fish, verifying it as quick and easy compared to other regionalization methods. The regions defined by radiology in this study agree with the vertebral column regions recently defined for Chinook salmon (O. tshawytscha). Thus, and considering the results of this study on various salmonid species, the currently developed regionalization protocol can be generally used for salmonids.

Morphological examination of abdominal vertebral bodies from grass carp using high-resolution micro-CT scans

The vertebral column, a defining trait of all vertebrates, is organized as a concatenated chain of vertebrae, and therefore its support to the body depends on individual vertebral morphology. Consequently, studying the morphology of the vertebral centrum is of anatomical and clinical importance. Grass carp (GC) is a member of the infraclass Teleostei (teleost fish), which accounts for the majority of all vertebrate species; thus, its vertebral anatomical structure can help us understand vertebrate development and vertebral morphology. In this study, we have investigated the morphology and symmetry of the grass carp vertebral centrum using high-resolution micro-CT scans. To this end, three abdominal vertebrae (V9, V10, & V11) from eight grass carp were micro-CT scanned and then segmented using Dragonfly (ORS Inc.). Grass carp vertebral centrum conformed to the basic teleost pattern and demonstrated an amphicoelous shape (biconcave hourglass). The centrum's cranial endplate was smaller, less circular, and shallower compared to the caudal endplate. While the vertebral centrum demonstrated bilateral symmetry along the sagittal plane (left/right), the centrum focus was shifted dorsally and cranially, breaking dorsoventral and craniocaudal symmetry. The sum of these findings implies that the caudal aspect of grass carp vertebral centrum is bigger and more robust. Currently, we have no information whether this is due to nature, for example, differences in gene expression, or nurture, for example, environmental effect. As the vertebral parapophyses and spinous processes are slanted caudally, the direction of muscle action during swimming may create a gradient of stresses from cranial to caudal, resulting in a more robust caudal aspect of the vertebral centrum. Expanding our study to include additional quadrupedal and bipedal (i.e., human) vertebrae, as well as testing if these morphological aspects of the vertebrae are indeed plastic and can be affected by environmental factors (i.e., temperature or other stressors) may help answer this question.

Lateral bone ridge expansion and internal tissue replacement for vertebral body growth in Pacific bluefin tuna Thunnus orientalis

Vertebral growth is an essential developmental process to support the expansion of the vertebrate body. In teleosts, the lateral side of the vertebral bodies develops to form different structures among species in the late stages of vertebral growth, although lateral structures are not apparent in the early stages. Lateral structures are one of the structural features that determine the diversity of teleost vertebrae. However, explanations for the formation of lateral structures are conflicting because few reports have investigated the growth of teleost vertebral bodies. To clarify the growth process, we analyzed the morphological changes in the vertebral body of Pacific bluefin tuna Thunnus orientalis at different developmental stages using micro-computed tomography (CT) scans. The micro-CT scans showed that the vertebral centrum formed a plate-like ridge on the lateral side along the cranial-caudal direction and extended laterally with increasing thickness. Simultaneously, the proximal region of the lateral ridges became porous as the vertebrae grew to form bone marrow cavities. Furthermore, we used histological observations to describe the relationship between these morphological changes and osteoblast and osteoclast activities. Osteoblasts accumulated on the distal edges of the lateral ridges, whereas osteoclasts were distributed in the bone marrow cavities. These observations suggest that bone resorption occurs proximally to form bone marrow cavities in addition to bone synthesis at the edges of the lateral ridges. The bone marrow cavities were occupied by blood vessels, extracellular matrix, and adipocytes, and the internal tissue composition changed to increase the area of adipose tissue. Because the ratio of bone volume decreases in large vertebrae, bone formation and resorption are regulated to separate the external cortical and internal trabecular bones to support the vertebrae. This study is the first to report the formation of lateral structures and can be applied to similar lateral structures in the vertebrae of other teleost species.

Deformity or variation? Phenotypic diversity in the zebrafish vertebral column

Vertebral bodies are composed of two types of metameric elements, centra and arches, each of which is considered as a developmental module. Most parts of the teleost vertebral column have a one-to-one relationship between centra and arches, although, in all teleosts, this one-to-one relationship is lost in the caudal fin endoskeleton. Deviation from the one-to-one relationship occurs in most vertebrates, related to changes in the number of vertebral centra or to a change in the number of arches. In zebrafish, deviations also occur predominantly in the caudal region of the vertebral column. In-depth phenotypic analysis of wild-type zebrafish was performed using whole-mount stained samples, histological analyses and synchrotron radiation X-ray tomographic microscopy 3D reconstructions. Three deviant centra phenotypes were observed: (i) fusion of two vertebral centra, (ii) wedge-shaped hemivertebrae and (iii) centra with reduced length. Neural and haemal arches and their spines displayed bilateral and unilateral variations that resemble vertebral column phenotypes of stem-ward actinopterygians or other gnathostomes as well as pathological conditions in extant species. Whether it is possible to distinguish variations from pathological alterations and whether alterations resemble ancestral conditions is discussed in the context of centra and arch variations in other vertebrate groups and basal actinopterygian species.

Elevated Water CO2 Can Prevent Dietary-Induced Osteomalacia in Post-Smolt Atlantic Salmon (Salmo salar, L.)

Expansion of land-based systems in fish farms elevate the content of metabolic carbon dioxide (CO₂) in the water. High CO₂ is suggested to increase the bone mineral content in Atlantic salmon (Salmo salar, L.). Conversely, low dietary phosphorus (P) halts bone mineralization. This study examines if high CO₂ can counteract reduced bone mineralization imposed by low dietary P intake. Atlantic salmon post-seawater transfer (initial weight 207.03 g) were fed diets containing 6.3 g/kg (0.5P), 9.0 g/kg (1P), or 26.8 g/kg (3P) total P for 13 weeks. Atlantic salmon from all dietary P groups were reared in seawater which was not injected with CO₂ and contained a regular CO₂ level (5 mg/L) or in seawater with injected CO₂ thus raising the level to 20 mg/L. Atlantic salmon were analyzed for blood chemistry, bone mineral content, vertebral centra deformities, mechanical properties, bone matrix alterations, expression of bone mineralization, and P metabolism-related genes. High CO₂ and high P reduced Atlantic salmon growth and feed intake. High CO₂ increased bone mineralization when dietary P was low. Atlantic salmon fed with a low P diet downregulated the fgf23 expression in bone cells indicating an increased renal phosphate reabsorption. The current results suggest that reduced dietary P could be sufficient to maintain bone mineralization under conditions of elevated CO₂. This opens up a possibility for lowering the dietary P content under certain farming conditions.

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

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

Big insight from the little skate: Leucoraja erinacea as a developmental model system

The vast majority of extant vertebrate diversity lies within the bony and cartilaginous fish lineages of jawed vertebrates. There is a long history of elegant experimental investigation of development in bony vertebrate model systems (e.g., mouse, chick, frog and zebrafish). However, studies on the development of cartilaginous fishes (sharks, skates and rays) have, until recently, been largely descriptive, owing to the challenges of embryonic manipulation and culture in this group. This, in turn, has hindered understanding of the evolution of developmental mechanisms within cartilaginous fishes and, more broadly, within jawed vertebrates. The little skate (Leucoraja erinacea) is an oviparous cartilaginous fish and has emerged as a powerful and experimentally tractable developmental model system. Here, we discuss the collection, husbandry and management of little skate brood stock and eggs, and we present an overview of key stages of skate embryonic development. We also discuss methods for the manipulation and culture of skate embryos and illustrate the range of tools and approaches available for studying this system. Finally, we summarize a selection of recent studies on skate development that highlight the utility of this system for inferring ancestral anatomical and developmental conditions for jawed vertebrates, as well as unique aspects of cartilaginous fish biology.

Phenotypic regionalization of the vertebral column in the thorny skate Amblyraja radiata: Stability and variation

Regionalization of the vertebral column occurred early during vertebrate evolution and has been extensively investigated in mammals. However, less data are available on vertebral regions of crown gnathostomes. This is particularly true for batoids (skates, sawfishes, guitarfishes, and rays) whose vertebral column has long been considered to be composed of the same two regions as in teleost fishes despite the presence of a synarcual. However, the numerous vertebral units in chondrichthyans may display a more complex regionalization pattern than previously assumed and the intraspecific variation of such pattern deserves a thorough investigation. In this study, we use micro-computed tomography (µCT) scans of vertebral columns of a growth series of thorny skates Amblyraja radiata to provide the first fine-scale morphological description of vertebral units in a batoids species. We further investigate axial regionalization using a replicable clustering analysis on presence/absence of vertebral elements to decipher the regionalization of the vertebral column of A. radiata. We identify four vertebral regions in this species. The two anteriormost regions, named synarcual and thoracic, may undergo strong developmental or functional constraints because they display stable patterns of shapes and numbers of vertebral units across all growth stages. The third region, named hemal transitional, is characterized by high inter-individual morphological variation and displays a transition between the monospondylous (one centrum per somite) to diplospondylous (two centra per somite) conditions. The posteriormost region, named caudal, is subdivided into three sub-regions with shapes changing gradually along the anteroposterior axis. These regionalized patterns are discussed in light of ecological habits of skates.

Turning the Curve Into Straight: Phenogenetics of the Spine Morphology and Coordinate Maintenance in the Zebrafish

The vertebral column, or spine, provides mechanical support and determines body axis posture and motion. The most common malformation altering spine morphology and function is adolescent idiopathic scoliosis (AIS), a three-dimensional spinal deformity that affects approximately 4% of the population worldwide. Due to AIS genetic heterogenicity and the lack of suitable animal models for its study, the etiology of this condition remains unclear, thus limiting treatment options. We here review current advances in zebrafish phenogenetics concerning AIS-like models and highlight the recently discovered biological processes leading to spine malformations. First, we focus on gene functions and phenotypes controlling critical aspects of postembryonic aspects that prime in spine architecture development and straightening. Second, we summarize how primary cilia assembly and biomechanical stimulus transduction, cerebrospinal fluid components and flow driven by motile cilia have been implicated in the pathogenesis of AIS-like phenotypes. Third, we highlight the inflammatory responses associated with scoliosis. We finally discuss recent innovations and methodologies for morphometrically characterize and analyze the zebrafish spine. Ongoing phenotyping projects are expected to identify novel and unprecedented postembryonic gene functions controlling spine morphology and mutant models of AIS. Importantly, imaging and gene editing technologies are allowing deep phenotyping studies in the zebrafish, opening new experimental paradigms in the morphometric and three-dimensional assessment of spinal malformations. In the future, fully elucidating the phenogenetic underpinnings of AIS etiology in zebrafish and humans will undoubtedly lead to innovative pharmacological treatments against spinal deformities.

Mineralized Cartilage and Bone-Like Tissues in Chondrichthyans Offer Potential Insights Into the Evolution and Development of Mineralized Tissues in the Vertebrate Endoskeleton

The impregnation of biominerals into the extracellular matrix of living organisms, a process termed biomineralization, gives rise to diverse mineralized (or calcified) tissues in vertebrates. Preservation of mineralized tissues in the fossil record has provided insights into the evolutionary history of vertebrates and their skeletons. However, current understanding of the vertebrate skeleton and of the processes underlying its formation is biased towards biomedical models such as the tetrapods mouse and chick. Chondrichthyans (sharks, skates, rays, and chimaeras) and osteichthyans are the only vertebrate groups with extant (living) representatives that have a mineralized skeleton, but the basal phylogenetic position of chondrichthyans could potentially offer unique insights into skeletal evolution. For example, bone is a vertebrate novelty, but the internal supporting skeleton (endoskeleton) of extant chondrichthyans is commonly described as lacking bone. The molecular and developmental basis for this assertion is yet to be tested. Subperichondral tissues in the endoskeleton of some chondrichthyans display mineralization patterns and histological and molecular features of bone, thereby challenging the notion that extant chondrichthyans lack endoskeletal bone. Additionally, the chondrichthyan endoskeleton demonstrates some unique features and others that are potentially homologous with other vertebrates, including a polygonal mineralization pattern, a trabecular mineralization pattern, and an unconstricted perichordal sheath. Because of the basal phylogenetic position of chondrichthyans among all other extant vertebrates with a mineralized skeleton, developmental and molecular studies of chondrichthyans are critical to flesh out the evolution of vertebrate skeletal tissues, but only a handful of such studies have been carried out to date. This review discusses morphological and molecular features of chondrichthyan endoskeletal tissues and cell types, ultimately emphasizing how comparative embryology and transcriptomics can reveal homology of mineralized skeletal tissues (and their cell types) between chondrichthyans and other vertebrates.

Lrp5 Mutant and Crispant Zebrafish Faithfully Model Human Osteoporosis, Establishing the Zebrafish as a Platform for CRISPR-Based Functional Screening of Osteoporosis Candidate Genes

Genomewide association studies (GWAS) have improved our understanding of the genetic architecture of common complex diseases such as osteoporosis. Nevertheless, to attribute functional skeletal contributions of candidate genes to osteoporosis-related traits, there is a need for efficient and cost-effective in vivo functional testing. This can be achieved through CRISPR-based reverse genetic screens, where phenotyping is traditionally performed in stable germline knockout (KO) mutants. Recently it was shown that first-generation (F0) mosaic mutant zebrafish (so-called crispants) recapitulate the phenotype of germline KOs. To demonstrate feasibility of functional validation of osteoporosis candidate genes through crispant screening, we compared a crispant to a stable KO zebrafish model for the lrp5 gene. In humans, recessive loss-of-function mutations in LRP5, a co-receptor in the Wnt signaling pathway, cause osteoporosis-pseudoglioma syndrome. In addition, several GWAS studies identified LRP5 as a major risk locus for osteoporosis-related phenotypes. In this study, we showed that early stage lrp5 KO larvae display decreased notochord mineralization and malformations of the head cartilage. Quantitative micro-computed tomography (micro-CT) scanning and mass-spectrometry element analysis of the adult skeleton revealed decreased vertebral bone volume and bone mineralization, hallmark features of osteoporosis. Furthermore, regenerating fin tissue displayed reduced Wnt signaling activity in lrp5 KO adults. We next compared lrp5 mutants with crispants. Next-generation sequencing analysis of adult crispant tissue revealed a mean out-of-frame mutation rate of 76%, resulting in strongly reduced levels of Lrp5 protein. These crispants generally showed a milder but nonetheless highly comparable skeletal phenotype and a similarly reduced Wnt pathway response compared with lrp5 KO mutants. In conclusion, we show through faithful modeling of LRP5-related primary osteoporosis that crispant screening in zebrafish is a promising approach for rapid functional screening of osteoporosis candidate genes. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Three-dimensional topology optimization model to simulate the external shapes of bone

Elucidation of the mechanism by which the shape of bones is formed is essential for understanding vertebrate development. Bones support the body of vertebrates by withstanding external loads, such as those imposed by gravity and muscle tension. Many studies have reported that bone formation varies in response to external loads. An increased external load induces bone synthesis, whereas a decreased external load induces bone resorption. This relationship led to the hypothesis that bone shape adapts to external load. In fact, by simulating this relationship through topology optimization, the internal trabecular structure of bones can be successfully reproduced, thereby facilitating the study of bone diseases. In contrast, there have been few attempts to simulate the external structure of bones, which determines vertebrate morphology. However, the external shape of bones may be reproduced through topology optimization because cells of the same type form both the internal and external structures of bones. Here, we constructed a three-dimensional topology optimization model to attempt the reproduction of the external shape of teleost vertebrae. In teleosts, the internal structure of the vertebral bodies is invariable, exhibiting an hourglass shape, whereas the lateral structure supporting the internal structure differs among species. Based on the anatomical observations, we applied different external loads to the hourglass-shaped part. The simulations produced a variety of three-dimensional structures, some of which exhibited several structural features similar to those of actual teleost vertebrae. In addition, by adjusting the geometric parameters, such as the width of the hourglass shape, we reproduced the variation in the teleost vertebrae shapes. These results suggest that a simulation using topology optimization can successfully reproduce the external shapes of teleost vertebrae. By applying our topology optimization model to various bones of vertebrates, we can understand how the external shape of bones adapts to external loads.

In this paper, we developed a computational method to investigate the relationship between three-dimensional bone shape and external loads imposed on bones. Many studies report that bone formation varies in response to external loads. An increased external load induces bone synthesis, whereas a decreased external load induces bone resorption. This relationship led to the hypothesis that the shape of bones adapts to external load. However, it remains unclear whether this hypothesis can explain the shape of bones. Here, we constructed a three-dimensional mathematical model that imitates the cellular activities of bone formation to attempt the reproduction of the shape of teleost vertebrae. In teleosts, the shape of the vertebrae differs among the species. We set the multiple types of external load conditions in the simulations and compared the simulation results with different teleost vertebrae. The produced structures that can resist the deformation of the surrounding tissues exhibited multiple structural features similar to the vertebrae of several teleost species. This result shows that the formation of bone shape can be explained by the adaptation to external load.

Early amphibians evolved distinct vertebrae for habitat invasions

Living tetrapods owe their existence to a critical moment 360-340 million years ago when their ancestors walked on land. Vertebrae are central to locomotion, yet systematic testing of correlations between vertebral form and terrestriality and subsequent reinvasions of aquatic habitats is lacking, obscuring our understanding of movement capabilities in early tetrapods. Here, we quantified vertebral shape across a diverse group of Paleozoic amphibians (Temnospondyli) encompassing different habitats and nearly the full range of early tetrapod vertebral shapes. We demonstrate that temnospondyls were likely ancestrally terrestrial and had several early reinvasions of aquatic habitats. We find a greater diversity in temnospondyl vertebrae than previously known. We also overturn long-held hypotheses centered on weight-bearing, showing that neural arch features, including muscle attachment, were plastic across the water-land divide and do not provide a clear signal of habitat preferences. In contrast, intercentra traits were critical, with temnospondyls repeatedly converging on distinct forms in terrestrial and aquatic taxa, with little overlap between. Through our geometric morphometric study, we have been able to document associations between vertebral shape and environmental preferences in Paleozoic tetrapods and to reveal morphological constraints imposed by vertebrae to locomotion, independent of ancestry.

Plasticity of the skeleton and skeletal deformities in zebrafish (Danio rerio) linked to rearing density

The teleost zebrafish (Danio rerio), an established model for human skeletal diseases, is reared under controlled conditions with defined parameters for temperature and photoperiod. Studies aimed at defining the proper rearing density have been performed with regard to behavioural and physiological stress response, sex ratio and reproduction. Studies concerning the effect of rearing density on the skeletal phenotype are lacking. This study analyses the response of the skeleton to different rearing densities and describes the skeletal deformities. Wild-type zebrafish were reared up to 30 dpf (days post-fertilization) in a common environment. From 30 to 90 dpf, animals were reared at three different densities: high density (HD), 32 fish l^-1 ; medium density (MD), 8 fish l^-1 and low density (LD), 2 fish l^-1 . Animals at 30 and 90 dpf were collected and whole-mount stained with Alizarin red S to visualize mineralized tissues. The entire skeleton was analysed for meristic counts and 172 types of deformities. The results showed that the rearing density significantly influenced the specimens' average standard length, which decreased with the increase in the rearing density. Differences in meristic counts among the three groups were not observed. Rearing density-independent malformations affected the ribs, neural arches and the spines of the abdominal region, as well as vertebrae of the caudal complex. The HD group showed the highest number of deformities per specimen, the highest number of observed types of deformities and, together with the MD group, the highest frequency of specimens affected by severe deformities. In particular, the HD group showed deformities affecting arches, spines and vertebral centra in the caudal region of the vertebral column. This study provides evidence of an effect of the rearing density on the development of different skeletal phenotypes.

Architectural and ultrastructural features of tessellated calcified cartilage in modern and extinct chondrichthyan fishes

Tessellated calcified cartilage (TCC) is a distinctive kind of biomineralized perichondral tissue found in many modern and extinct chondrichthyans (sharks, rays, chimaeroids and their extinct allies). Customarily, this feature has been treated somewhat superficially in phylogenetic analyses, often as a single "defining" character of a chondrichthyan clade. TCC is actually a complex hard tissue with numerous distinctive attributes, but its use as a character complex for phylogenetic analysis has not yet been optimized. This study attempts to improve this situation by presenting new terminology for certain aspects of tesseral architecture, including single-monolayered, multiple-monolayered, polylayered and voussoir tesserae; new histological data, including thin sections of TCC in several Palaeozoic taxa, and new proposals for ways in which various characters and states (many of which are defined here for the first time) could be applied in future phylogenetic analyses of chondrichthyan fishes. It can be concluded that many, but not all, of the unique attributes of modern TCC evolved by the Early Devonian (ca. 400 before present (bp)). The globular calcified cartilage reported in Silurian sinacanthids and the so-called subtessellated perichondral biomineralization (with irregular and ill-defined geometries of a layer or layers of calcified cartilage blocks) of certain extinct "acanthodians" (e.g., Climatius, Ischnacanthus, Cheiracanthus) could represent evolutionary precursors of TCC, which seems to characterize only part of the chondrichthyan total group. It is hypothesized that heavily biomineralized "layer-cake" TCC in certain Palaeozoic chondrichthyans perhaps served a dual physiological role, as a phosphate sink and in providing increased skeletal density in very large (>7 m) Devonian-Permian marine sharks such as ctenacanths and as an adaptation to calcium-deficient environments among Permo-Carboniferous non-marine sharks such as xenacanths. By contrast, the equivalent tissue in modern elasmobranchs probably serves only to reinforce regions of cartilage (mostly in the jaws) subjected to high loading. It is also noted that much of the variation observed in tesseral architecture (including localized remodelling), ultrastructure and histology in modern and extinct chondrichthyans is confined to the perichondrally facing cap zone (where Type-1 collagen matrix predominates in modern TCC), whereas the main body of the tessera (where Type-2 collagen matrix predominates) exhibits comparatively little evidence of remodelling and histological or structural variation.

The gas bladder of Pantodon buchholzi: Structure and relationships with the vertebrae

We report here on the histological and structural characteristics of the gas bladder, the vertebral morphology, and the bladder-vertebra relationships of the butterfly fish, Pantodon buchholzi. The bladder opens at the boundary between the pharynx and the esophagus by a middle slit. A pneumatic duct is absent. The bladder shows a dorsolateral wall that adapts to the anfractuosities of the coelomic cavity and a ventral wall in contact with the abdominal organs. The vertebral bodies are formed by an hourglass shaped autocentrum, and by an arcocentrum reduced to several longitudinal ridges. The transverse processes adopt the structure of a cage whose walls are formed by bone trabeculae of variable size and distribution pattern. The dorsolateral wall of the bladder is a membrane that covers the kidney, adapts to the irregular shape of the vertebrae, and invades the transverse processes at several points before extending laterally. However, invasion of the vertebral bodies, the presence of a labyrinth, or the formation of respiratory parenchyma were not observed. The luminal surface of this wall is a thin respiratory barrier containing a single epithelial cell type. In addition, the wall contains numerous eosinophils that may be implicated in immune defense. The bladder ventral wall is a membrane rich in collagen, vessels, smooth muscle, and nerves that lacks a respiratory barrier. Its luminal surface contains ciliated and nonciliated cells. The two cell types appear implicated in surfactant production.

Salamander-like tail regeneration in the West African lungfish

Salamanders, frog tadpoles and diverse lizards have the remarkable ability to regenerate tails. Palaeontological data suggest that this capacity is plesiomorphic, yet when the developmental and genetic architecture of tail regeneration arose is poorly understood. Here, we show morphological and molecular hallmarks of tetrapod tail regeneration in the West African lungfish Protopterus annectens, a living representative of the sister group of tetrapods. As in salamanders, lungfish tail regeneration occurs via the formation of a proliferative blastema and restores original structures, including muscle, skeleton and spinal cord. In contrast with lizards and similar to salamanders and frogs, lungfish regenerate spinal cord neurons and reconstitute dorsoventral patterning of the tail. Similar to salamander and frog tadpoles, Shh is required for lungfish tail regeneration. Through RNA-seq analysis of uninjured and regenerating tail blastema, we show that the genetic programme deployed during lungfish tail regeneration maintains extensive overlap with that of tetrapods, with the upregulation of genes and signalling pathways previously implicated in amphibian and lizard tail regeneration. Furthermore, the lungfish tail blastema showed marked upregulation of genes encoding post-transcriptional RNA processing components and transposon-derived genes. Our results show that the developmental processes and genetic programme of tetrapod tail regeneration were present at least near the base of the sarcopterygian clade and establish the lungfish as a valuable research system for regenerative biology.

More Bone with Less Minerals? The Effects of Dietary Phosphorus on the Post-Cranial Skeleton in Zebrafish

Dietary phosphorus (P) is essential for bone mineralisation in vertebrates. P deficiency can cause growth retardation, osteomalacia and bone deformities, both in teleosts and in mammals. Conversely, excess P supply can trigger soft tissue calcification and bone hypermineralisation. This study uses a wide range of complementary techniques (X-rays, histology, TEM, synchrotron X-ray tomographic microscopy, nanoindentation) to describe in detail the effects of dietary P on the zebrafish skeleton, after two months of administering three different diets: 0.5% (low P, LP), 1.0% (regular P, RP), and 1.5% (high P, HP) total P content. LP zebrafish display growth retardation and hypomineralised bones, albeit without deformities. LP zebrafish increase production of non-mineralised bone matrix, and osteoblasts have enlarged endoplasmic reticulum cisternae, indicative for increased collagen synthesis. The HP diet promotes growth, high mineralisation, and stiffness but causes vertebral centra fusions. Structure and arrangement of bone matrix collagen fibres are not influenced by dietary P in all three groups. In conclusion, low dietary P content stimulates the formation of non-mineralised bone without inducing malformations. This indicates that bone formation and mineralisation are uncoupled. In contrast, high dietary P content promotes mineralisation and vertebral body fusions. This new zebrafish model is a useful tool to understand the mechanisms underlying osteomalacia and abnormal mineralisation, due to underlying variations in dietary P levels.

Notochordal Signals Establish Phylogenetic Identity of the Teleost Spine

The spine is a defining feature of the vertebrate body plan. However, broad differences in vertebral structures and morphogenetic strategies occur across vertebrate groups, clouding the homology between their developmental programs. Analysis of a zebrafish mutant, spondo, whose spine is dysmorphic, prompted us to reconstruct paleontological evidence, highlighting specific transitions during teleost spine evolution. Interestingly, the spondo mutant recapitulates characteristics present in basal fishes, not found in extant teleosts. Further analysis of the mutation implicated the teleost-specific notochord protein, Calymmin, as a key regulator of spine patterning in zebrafish. The mutation in cmn results in loss of notochord sheath segmentation, altering osteoblast migration to the developing spine, and increasing sensitivity to somitogenesis defects associated with congenital scoliosis in amniotes. These data suggest that signals from the notochord define the evolutionary identity of the spine and demonstrate how simple shifts in development can revert traits canalized for about 250 million years.

From Devo to Evo: patterning, fusion and evolution of the zebrafish terminal vertebra

Background

With more than 30,000 species, teleosts comprise about half of today’s living vertebrates, enriched with a wide set of adaptations to all aquatic systems. Their evolution was marked by modifications of their tail, that involved major rearrangements of the metameric organization of the axial skeleton. The most posterior or ural caudal skeleton, primitively included more than 10 vertebrae and, through a series of fusions and losses, became reduced to a single vertebra in modern ostariophysans, one of the largest clades of teleosts. The ontogeny of the ostariophysan Danio rerio recapitulates this process by forming two or three separate vertebrae that become a single vertebra in adults. We characterize the developmental sequence of this change by describing the processes of patterning, fusion and differential growth on each of the constitutive elements that sculpt the adult terminal vertebra.

Results

The ontogenetic changes of the terminal vertebra were characterized, highlighting their shared and derived characters in comparison with other teleosts. In zebrafish, there is: i) a loss of the preural centrum 1, ii) the formation of an hourglass-shaped autocentrum only in the anterior but not the posterior border of the compound centrum, iii) the formation of a vestigial posterior centrum that does not form an autocentrum and becomes incorporated beneath the compound centrum during development, and iv) the elongated dorso-posterior process of the compound centrum or pleurostyle appears as an independent element posterior to the compound centrum, before fusing to the ural neural arches and the anterior portion of the compound centrum.

Conclusions

The unique features of the formation of the terminal vertebra in Danio rerio reflect the remarkable changes that occurred during the evolution of teleosts, with potential shared derived characteristics for some of the major lineages of modern teleosts. A new ontogenetic model is proposed to illustrate the development of the terminal vertebra, and the phylogenetic implications for the evolution of caudal skeleton consolidation in ostariophysans are discussed.

Skeletal Mineralization in Association with Type X Collagen Expression Is an Ancestral Feature for Jawed Vertebrates

In order to characterize the molecular bases of mineralizing cell evolution, we targeted type X collagen, a nonfibrillar network forming collagen encoded by the Col10a1 gene. It is involved in the process of endochondral ossification in ray-finned fishes and tetrapods (Osteichthyes), but until now unknown in cartilaginous fishes (Chondrichthyes). We show that holocephalans and elasmobranchs have respectively five and six tandemly duplicated Col10a1 gene copies that display conserved genomic synteny with osteichthyan Col10a1 genes. All Col10a1 genes in the catshark Scyliorhinus canicula are expressed in ameloblasts and/or odontoblasts of teeth and scales, during the stages of extracellular matrix protein secretion and mineralization. Only one duplicate is expressed in the endoskeletal (vertebral) mineralizing tissues. We also show that the expression of type X collagen is present in teeth of two osteichthyans, the zebrafish Danio rerio and the western clawed frog Xenopus tropicalis, indicating an ancestral jawed vertebrate involvement of type X collagen in odontode formation. Our findings push the origin of Col10a1 gene prior to the divergence of osteichthyans and chondrichthyans, and demonstrate its ancestral association with mineralization of both the odontode skeleton and the endoskeleton.

Zebrafish: A Resourceful Vertebrate Model to Investigate Skeletal Disorders

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

Zebrafish and medaka as models for biomedical research of bone diseases

The identification of disease-causing mutations has in recent years progressed immensely due to whole genome sequencing approaches using patient material. The task accordingly is shifting from gene identification to functional analysis of putative disease-causing genes, preferably in an in vivo setting which also allows testing of drug candidates or biotherapeutics in whole animal disease models. In this review, we highlight the advances made in the field of bone diseases using small laboratory fish, focusing on zebrafish and medaka. We particularly highlight those human conditions where teleost models are available.

Comparative morphological examination of vertebral bodies of teleost fish using high-resolution micro-CT scans

Vertebral bodies of teleost fish are formed by the sclerotomal bone covering the chordacentrum. The internal part of the sclerotomal bone is composed of an amphicoelous hourglass shaped autocentrum, which is common in most fish species. In contrast, the external shape of the sclerotomal bone varies extensively among species. There are multiple hypotheses regarding the composition and formation of the external structure. However, as they are based on studies of few extant or extinct species, their applicability to other species remains to be clarified. To understand the morphology, formation, and composition of vertebral bodies in teleosts, we performed a comparative analysis using micro‐CT scans of 32 species from 10 orders of Teleostei and investigated the detailed morphology of the sclerotomal bone, especially its plate‐like ridge and trabeculae. We discovered two structural characteristics that are shared among most of the examined species. One was the sheet‐like trabeculae that extend radially from the center of the vertebral body with a constant thickness. The other was the presence of hollow spaces on the internal parts of the lateral ridge and trabeculae. The combination of different arrangements of sheet‐like trabeculae and internal hollow spaces formed different shapes of the lateral structure of the vertebral body. The properties of these two characteristics suggest that the external part of the sclerotomal bone grows outward by deposition at the bone tip, and that, concurrently, bone absorption occurs in the internal part of the sclerotomal bone. The vertebral arches were also formed by the sheet‐like trabeculae, indicating that both, the vertebral body and the arches, are formed by the same component. The micro‐CT scanning data were uploaded to a public database so they can be used for future studies on fish vertebrae.

Embryonic origin of the gnathostome vertebral skeleton

The vertebral column is a key component of the jawed vertebrate (gnathostome) body plan, but the primitive embryonic origin of this skeleton remains unclear. In tetrapods, all vertebral components (neural arches, haemal arches and centra) derive from paraxial mesoderm (somites). However, in teleost fishes, vertebrae have a dual embryonic origin, with arches derived from somites, but centra formed, in part, by secretion of bone matrix from the notochord. Here, we test the embryonic origin of the vertebral skeleton in a cartilaginous fish (the skate, Leucoraja erinacea) which serves as an outgroup to tetrapods and teleosts. We demonstrate, by cell lineage tracing, that both arches and centra are somite-derived. We find no evidence of cellular or matrix contribution from the notochord to the skate vertebral skeleton. These findings indicate that the earliest gnathostome vertebral skeleton was exclusively of somitic origin, with a notochord contribution arising secondarily in teleosts.

Evolution of caudal fin ray development and caudal fin hypural diastema complex in spotted gar, teleosts, and other neopterygian fishes

BACKGROUND

The caudal fin of actinopterygians transitioned from a heterocercal dorsoventrally asymmetrical fin to a homocercal externally symmetrical fin in teleosts through poorly understood evolutionary developmental mechanisms. We studied the caudal skeleton of major living actinopterygian lineages, including polypteriformes, acipenseriformes, Holostei (gars and bowfin), and teleosts, compared with reports of extinct neopterygians and basal teleosteans. We focused on the hypural diastema complex, which includes (1) a gap between hypurals 2 and 3, that (2) separates two plates of connective tissue at (3) the branching of caudal vasculature; these features had been considered as a shared, derived trait of teleosts, a synapomorphy.

RESULTS

These studies revealed that gars and teleosts share all three features of the hypural diastema complex. Absence of a complex with these features from bowfin, fossil Holostei, and stem Teleostei argues in favor of repetitive, independent emergence in several neopterygian and basal Teleostei lineages, or less likely, many independent losses. We further observed that, in gars and teleosts, the earliest developing lepidotrichia align with the horizontal adult body axis, thus participating in external symmetry.

CONCLUSIONS

These results suggest that the hypural diastema complex in teleosts and gars represents a homoplasy among neopterygians and that it emerged repeatedly by parallel evolution due to shared inherited underlying genetic and developmental programs (latent homology). Because the hypural diastema complex exists in gars with heterocercal tails, this complex is independent of homocercality. Developmental Dynamics 247:832-853, 2018. © 2018 Wiley Periodicals, Inc.

Patterning the spine

The patterning of the spine of a zebrafish is controlled by the notochord, a rod-like structure that supports and instructs the developing embryo.

Direct activation of chordoblasts by retinoic acid is required for segmented centra mineralization during zebrafish spine development

Zebrafish mutants with increased retinoic acid (RA) signaling due to the loss of the RA-inactivating enzyme Cyp26b1 develop a hyper-mineralized spine with gradually fusing vertebral body precursors (centra). However, the underlying cellular mechanisms remain incompletely understood. Here, we show that cells of the notochord epithelium named chordoblasts are sensitive to RA signaling. Chordoblasts are uniformly distributed along the anteroposterior axis and initially generate the continuous collagenous notochord sheath. However, subsequently and iteratively, subsets of these cells undergo further RA-dependent differentiation steps, acquire a stellate-like shape, downregulate expression of the collagen gene col2a1a, switch on cyp26b1 expression and trigger metameric sheath mineralization. This mineralization fails to appear upon chordoblast-specific cell ablation or RA signal transduction blockade. Together, our data reveal that, despite their different developmental origins, the activities and regulation of chordoblasts are very similar to those of osteoblasts, including their RA-induced transition from osteoid-producing cells to osteoid-mineralizing ones. Furthermore, our data point to a requirement for locally controlled RA activity within the chordoblast layer in order to generate the segmented vertebral column.

Segmentation of the zebrafish axial skeleton relies on notochord sheath cells and not on the segmentation clock

Segmentation of the axial skeleton in amniotes depends on the segmentation clock, which patterns the paraxial mesoderm and the sclerotome. While the segmentation clock clearly operates in teleosts, the role of the sclerotome in establishing the axial skeleton is unclear. We severely disrupt zebrafish paraxial segmentation, yet observe a largely normal segmentation process of the chordacentra. We demonstrate that axial entpd5+ notochord sheath cells are responsible for chordacentrum mineralization, and serve as a marker for axial segmentation. While autonomous within the notochord sheath, entpd5 expression and centrum formation show some plasticity and can respond to myotome pattern. These observations reveal for the first time the dynamics of notochord segmentation in a teleost, and are consistent with an autonomous patterning mechanism that is influenced, but not determined by adjacent paraxial mesoderm. This behavior is not consistent with a clock-type mechanism in the notochord.

The external phenotype-skeleton link in post-hatch farmed Chinook salmon (Oncorhynchus tshawytscha)

Skeletal deformities in farmed fish are a recurrent problem. External malformations are easily recognized, but there is little information on how external malformations relate to malformations of the axial skeleton: the external phenotype-skeleton link. Here, this link is studied in post-hatch to first-feed life stages of Chinook salmon (Oncorhynchus tshawytscha) raised at 4, 8 and 12°C. Specimens were whole-mount-stained for cartilage and bone, and analysed by histology. In all temperature groups, externally normal specimens can have internal malformations, predominantly fused vertebral centra. Conversely, externally malformed fish usually display internal malformations. Externally curled animals typically have malformed haemal and neural arches. External malformations affecting a single region (tail malformation and bent neck) relate to malformed notochords and early fusion of fused vertebral centra. The frequencies of internal malformations in both externally normal and malformed specimens show a U-shaped response, with lowest frequency in 8°C specimens. The fused vertebral centra that occur in externally normal specimens represent a malformation that can be contained and could be carried through into harvest size animals. This study highlights the relationship between external phenotype and axial skeleton and may help to set the framework for the early identification of skeletal malformations on fish farms.

Neopterygian phylogeny: the merger assay

The phylogenetic relationships of the recently described genus †Ticinolepis from the Middle Triassic of the Monte San Giorgio are explored through cladistic analyses of the so far largest morphological dataset for fossil actinopterygians, including representatives of the crown-neopterygian clades Halecomorphi, Ginglymodi and Teleostei, and merging the characters from previously published systematic studies together with newly proposed characters. †Ticinolepis is retrieved as the most basal Ginglymodi and our results support the monophyly of Teleostei and Holostei, as well as Halecomorphi and Ginglymodi within the latter clade. The patterns of relationships within these clades mostly agree with those of previous studies, although a few important differences require future research. According to our results, ionoscopiforms are not monophyletic, caturids are not amiiforms and leptolepids and luisiellids form a monophyletic clade. Our phylogenetic hypothesis confirms the rapid radiation of the holostean clades Halecomorphi and Ginglymodi during the Early and Middle Triassic and the radiation of pholidophoriform teleosts during the Late Triassic. Crown-group Halecomorphi have an enormous ghost lineage throughout half of the Mesozoic, but ginglymodians and teleosts show a second radiation during the Early Jurassic. The crown-groups of Halecomorphi, Ginglymodi and Teleostei originated within parallel events of radiation during the Late Jurassic.

Increased mechanical loading through controlled swimming exercise induces bone formation and mineralization in adult zebrafish

Exercise promotes gain in bone mass through adaptive responses of the vertebrate skeleton. This mechanism counteracts age- and disease-related skeletal degradation, but remains to be fully understood. In life sciences, zebrafish emerged as a vertebrate model that can provide new insights into the complex mechanisms governing bone quality. To test the hypothesis that musculoskeletal exercise induces bone adaptation in adult zebrafish and to characterize bone reorganization, animals were subjected to increased physical exercise for four weeks in a swim tunnel experiment. Cellular, structural and compositional changes of loaded vertebrae were quantified using integrated high-resolution analyses. Exercise triggered rapid bone adaptation with substantial increases in bone-forming osteoblasts, bone volume and mineralization. Clearly, modeling processes in zebrafish bone resemble processes in human bone. This study highlights how exercise experiments in adult zebrafish foster in-depth insight into aging-related bone diseases and can thus catalyze the search for appropriate prevention and new treatment options.

Bone without minerals and its secondary mineralization in Atlantic salmon (Salmo salar): the recovery from phosphorus deficiency

Calcium and phosphorus (P) are the main bone minerals and P-deficiency causes hypomineralized bones (osteomalacia) and malformations. This study uses a P-deficient salmon model to falsify three hypotheses. First, an extended period of dietary P-deficiency does not cause pathologies other than osteomalacia. Second, secondary mineralization of non-mineralized bone is possible. Third, secondary mineralization can restore the bones' mineral composition and mechanical properties.

Post-smolt Atlantic salmon (Salmo salar) received for seven weeks diets with regular P-content (RP), or with a 50% lowered P-content (LP). For additional nine weeks RP animals continued on the regular diet (RP-RP). LP animals continued on the LP-diet (LP-LP), on a regular P diet (LP-RP), or on a high P diet (LP-HP).

After 16 weeks, animals in all groups maintained a non-deformed vertebral column. LP-LP animals continued bone formation albeit without mineralization. Nine weeks of RP diet largely restored the mineral content and mechanical properties of vertebral bodies. Mineralization resumed deep inside the bone and away from osteoblasts. The history of P-deficiency was traceable in LP-RP and LP-HP animals as a ring of low-mineralized bone in the vertebral body endplates but no tissue alterations occurred that foreshadow vertebral body compression or fusion. Large quantities of non-mineralized salmon bone have the capacity to re-mineralize. If 16 weeks of P-deficiency as a single factor is not causal for typical vertebral body malformations other factors remain to be identified. This example of functional bone without minerals may explain why some teleost species can afford to have an extremely low mineralized skeleton.

Ossification and development of vertebrae in the Balkan crested newt Triturus ivanbureschi (Salamandridae, Caudata)

Vertebral morphology, development, and evolution have been investigated for many decades, especially in the recent evo-devo era. Nevertheless, comparative data on development and ossification modes within the major tetrapod groups are scarce and frequently suffer from the use of a simplistic approach, resulting in simplistic generalizations about the formation of tetrapod vertebrae. Here, we describe the development and ossification of trunk vertebrae in Triturus ivanbureschi (Salamandridae, Caudata) and compare the results with published data on other related taxa. In so doing, we focus on the modes of ossification and development of the centrum and neural arches by analysing three developmental stages defined by the degree of limb development: stages 47, 52, and 62 according to Glücksohn (1932). Our examination of histological sections through trunk vertebrae enabled us to identify three modes of ossification within single trunk vertebrae: (i) perichordal (direct ossification of the connective tissue surrounding the notochord); (ii) perichondrial (direct ossification of the perichondrium, consisting of cartilage-covering connective tissue), and (iii) endochondral (ossification within the preformed cartilage template). We also noted the presence of intravertebral or notochordal cartilage. Although our results indicate that this cartilage develops within the notochord surrounded by the continuous notochordal sheath, more detailed further studies could shed light on its origin and development.

Vertebral column regionalisation in Chinook salmon, Oncorhynchus tshawytscha

Teleost vertebral centra are often similar in size and shape, but vertebral-associated elements, i.e. neural arches, haemal arches and ribs, show regional differences. Here we examine how the presence, absence and specific anatomical and histological characters of vertebral centra-associated elements can be used to define vertebral column regions in juvenile Chinook salmon (Oncorhynchus tshawytscha). To investigate if the presence of regions within the vertebral column is independent of temperature, animals raised at 8 and 12 °C were studied at 1400 and 1530 degreedays, in the freshwater phase of the life cycle. Anatomy and composition of the skeletal tissues of the vertebral column were analysed using Alizarin red S whole-mount staining and histological sections. Six regions, termed I-VI, are recognised in the vertebral column of specimens of both temperature groups. Postcranial vertebrae (region I) carry neural arches and parapophyses but lack ribs. Abdominal vertebrae (region II) carry neural arches and ribs that articulate with parapophyses. Elastic- and fibrohyaline cartilage and Sharpey's fibres connect the bone of the parapophyses to the bone of the ribs. In the transitional region (III) vertebrae carry neural arches and parapophyses change stepwise into haemal arches. Ribs decrease in size, anterior to posterior. Vestigial ribs remain attached to the haemal arches with Sharpey's fibres. Caudal vertebrae (region IV) carry neural and haemal arches and spines. Basidorsals and basiventrals are small and surrounded by cancellous bone. Preural vertebrae (region V) carry neural and haemal arches with modified neural and haemal spines to support the caudal fin. Ural vertebrae (region VI) carry hypurals and epurals that represent modified haemal and neural arches and spines, respectively. The postcranial and transitional vertebrae and their respective characters are usually recognised, but should be considered as regions within the vertebral column of teleosts because of their distinctive morphological characters. While the number of vertebrae within each region can vary, each of the six regions is recognised in specimens of both temperature groups. This refined identification of regionalisation in the vertebral column of Chinook salmon can help to address evolutionary developmental and functional questions, and to support applied research into this farmed species.

Skeletal ontogeny of the vertebral column and of the fins in shi drum Umbrina cirrosa (Teleostei: Perciformes: Sciaenidae), a new candidate species for aquaculture

The osteological development of the vertebral column and fins in shi drum Umbrina cirrosa was studied in order to improve knowledge for its introduction in Mediterranean aquaculture. The osteological development was studied in 171 individuals, of total length (L_T ) from 2·7 to 30·2 mm that were reared under the mesocosm technique. Vertebral ontogeny starts at 3·4 and 4·0 mm L_T , with the formation of the first cartilaginous neural and haemal arches, and spines, respectively, and is completed with the full attainment of epicentrals (12·5 mm L_T ). The formation of vertebral centra occurs between 4·1 and 7·4 mm L_T . Pectoral supports are the first fin elements to develop (3·0 mm L_T ), followed by those of the caudal fin (3·8 mm L_T ), pelvic fin (3·9 mm L_T ) and finally by those of the dorsal and anal fins (4·5 mm L_T ). The caudal fin is the first to develop fin rays and attain the full count of principal fin rays (4·5-6·8 mm L_T ), but the last to be fully completed with the formation of procurrent fin rays (6·9-17·5 mm L_T ). The next fins starting to present rays are the dorsal (5·3 mm L_T ) and the pectoral fins (5·6 mm L_T ), while the anal and pelvic fins are the last (5·7 mm L_T ). Following the caudal principal fin rays (6·8 mm L_T ), the dorsal, anal (6·9 mm L_T ), pelvic (7·4 mm L_T ) and pectoral fins (9·8 mm L_T ) are the next with fully completed ray counts. Aggregation of qualitative changes, such as the appearance of cartilages, the beginning and the complement of the ossification process and the full complement of elements in U. cirrosa were measured as cumulative frequency counts. These measurements reveal three ontogenetic intervals: one very developmentally active period during early life stages (from 3 to 5·9 mm L_T ), a second slower developmental period (from 6·0 to 8·9 mm L_T ) and finally a period of ontogeny more focused on structure refinement up to metamorphosis and settlement (>9·0 mm L_T ).

The homology and function of the lung plates in extant and fossil coelacanths

The presence of a pulmonary organ that is entirely covered by true bone tissue and fills most of the abdominal cavity is hitherto unique to fossil actinistians. Although small hard plates have been recently reported in the lung of the extant coelacanth Latimeria chalumnae, the homology between these hard structures in fossil and extant forms remained to be demonstrated. Here, we resolve this question by reporting the presence of a similar histological pattern–true cellular bone with star-shaped osteocytes, and a globular mineralisation with radiating arrangement–in the lung plates of two fossil coelacanths (Swenzia latimerae and Axelrodichthys araripensis) and the plates that surround the lung of the most extensively studied extant coelacanth species, L. chalumnae. The point-for-point structural similarity of the plates in extant and fossil coelacanths supports their probable homology and, consequently, that of the organ they surround. Thus, this evidence questions the previous interpretations of the fatty organ as a component of the pulmonary complex of Latimeria.

Anatomical integration of the sacral-hindlimb unit coordinated by GDF11 underlies variation in hindlimb positioning in tetrapods

Elucidating how body parts from different primordia are integrated during development is essential for understanding the nature of morphological evolution. In tetrapod evolution, while the position of the hindlimb has diversified along with the vertebral formula, the mechanism responsible for this coordination has not been well understood. However, this synchronization suggests the presence of an evolutionarily conserved developmental mechanism that coordinates the positioning of the hindlimb skeleton derived from the lateral plate mesoderm with that of the sacral vertebrae derived from the somites. Here we show that GDF11 secreted from the posterior axial mesoderm is a key factor in the integration of sacral vertebrae and hindlimb positioning by inducing Hox gene expression in two different primordia. Manipulating the onset of GDF11 activity altered the position of the hindlimb in chicken embryos, indicating that the onset of Gdf11 expression is responsible for the coordinated positioning of the sacral vertebrae and hindlimbs. Through comparative analysis with other vertebrate embryos, we also show that each tetrapod species has a unique onset timing of Gdf11 expression, which is tightly correlated with the anteroposterior levels of the hindlimb bud. We conclude that the evolutionary diversity of hindlimb positioning resulted from heterochronic shifts in Gdf11 expression, which led to coordinated shifts in the sacral-hindlimb unit along the anteroposterior axis.

Identification of a new mineralized tissue in the notochord of reared Siberian sturgeon (Acipenser baerii)

In a study aiming to improve knowledge on the mineralization of the axial skeleton in reared Siberian sturgeon (Acipenser baerii Brandt, 1869), we discovered a new mineralized tissue within the notochord. To our knowledge, such a structure has never been reported in any vertebrate species with the exception of the pathological mineralization of the notochord remains in degenerative intervertebral disks of mammals. Here, we describe this enigmatic tissue using X-ray microtomography, histological analyses and solid state NMR-spectroscopy. We also performed a 1-year monitoring of the mineral content (MC) of the notochord in relation with seasonal variations of temperature. In all specimens studied from 2-year-old juveniles onwards, this mineralized structure was found within a particular region of the notochord called funiculus. This feature first appears in the abdominal region then extends posteriorly with ageing, while the notochord MC also increases. The mineral phase is mainly composed of amorphous calcium phosphate, a small amount of which changes into hydroxyapatite with ageing. The putative role of this structure is discussed as either a store of minerals available for the phosphocalcic metabolism, or a mechanical support in a species with a poorly mineralized axial skeleton. A pathological feature putatively related to rearing conditions is also discussed.

Phylogenetic classification of bony fishes

BACKGROUND

Fish classifications, as those of most other taxonomic groups, are being transformed drastically as new molecular phylogenies provide support for natural groups that were unanticipated by previous studies. A brief review of the main criteria used by ichthyologists to define their classifications during the last 50 years, however, reveals slow progress towards using an explicit phylogenetic framework. Instead, the trend has been to rely, in varying degrees, on deep-rooted anatomical concepts and authority, often mixing taxa with explicit phylogenetic support with arbitrary groupings. Two leading sources in ichthyology frequently used for fish classifications (JS Nelson's volumes of Fishes of the World and W. Eschmeyer's Catalog of Fishes) fail to adopt a global phylogenetic framework despite much recent progress made towards the resolution of the fish Tree of Life. The first explicit phylogenetic classification of bony fishes was published in 2013, based on a comprehensive molecular phylogeny ( www.deepfin.org ). We here update the first version of that classification by incorporating the most recent phylogenetic results.

RESULTS

The updated classification presented here is based on phylogenies inferred using molecular and genomic data for nearly 2000 fishes. A total of 72 orders (and 79 suborders) are recognized in this version, compared with 66 orders in version 1. The phylogeny resolves placement of 410 families, or ~80% of the total of 514 families of bony fishes currently recognized. The ordinal status of 30 percomorph families included in this study, however, remains uncertain (incertae sedis in the series Carangaria, Ovalentaria, or Eupercaria). Comments to support taxonomic decisions and comparisons with conflicting taxonomic groups proposed by others are presented. We also highlight cases were morphological support exist for the groups being classified.

CONCLUSIONS

This version of the phylogenetic classification of bony fishes is substantially improved, providing resolution for more taxa than previous versions, based on more densely sampled phylogenetic trees. The classification presented in this study represents, unlike any other, the most up-to-date hypothesis of the Tree of Life of fishes.

Development of short-snouted seahorse (Hippocampus hippocampus, L. 1758): osteological and morphological aspects

Information about early development after male release lags behind studies of juveniles and adult seahorses, and newborn seahorses, similar in shape to adults, are considered juveniles or fry. During early life, Hippocampus hippocampus present behavioural (shift in habitat, from planktonic to benthic) and morphological changes; for this reasons, the aims of this study are to define the stage of development of H. hippocampus after they are expelled from the male brood pouch and to establish direct or indirect development through an osteological analysis. The ossification process was studied in 120 individuals, from their release to 30 days after birth. To analyse the osteological development, Alcian Blue-Alizarin Red double staining technique for bone and cartilage was adapted to this species. At birth, H. hippocampus presents a mainly cartilaginous structure that ossifies in approximately 1 month. The bony armour composed of bony rings and plates develops in 10 days. The caudal fin, a structure absent in juveniles and adult seahorses, is present at birth and progressively disappears with age. The absence of adult osteological structure in newborns, like coronet, bony rings and plates, head spines and components allowing tail prehensile abilities, suggests a metamorphosis before the juvenile stage. During the indirect development, the metamorphic stage started inside brood pouch and followed outside and leads up to reconsider the status of H. hippocampus newborns.

Vertebral Development and Ossification in the Siberian Sturgeon (Acipenser Baerii), with New Insights on Bone Histology and Ultrastructure of Vertebral Elements and Scutes

In order to improve our knowledge on the vertebral development, structure and mineralization in Acipenseriformes, we undertook a study in a growth series of reared Siberian sturgeons (Acipenser baerii) using in toto clear and stain specimens, histological and ultrastructural observations, X-ray micro-tomography, and solid state NMR analyses. Scutes were also studied to compare the tissue structure and mineralization of endoskeletal and dermal skeletal elements. This study completes and clarifies previous investigations on vertebral development and architecture in sturgeons, and brings original data on the structure of (i) the perichondral bone that is progressively deposited around the vertebral elements during ontogeny, (ii) the typical cartilage composing these elements, and (iii) the scutes. In addition we provide data on the mineralization process, on the nature of the bone mineral phase, and on the growth dynamics of the vertebral elements. Anat Rec, 300:437-449, 2017. © 2016 Wiley Periodicals, Inc.

Small teleost fish provide new insights into human skeletal diseases

Small teleost fish such as zebrafish and medaka are increasingly studied as models for human skeletal diseases. Efficient new genome editing tools combined with advances in the analysis of skeletal phenotypes provide new insights into fundamental processes of skeletal development. The skeleton among vertebrates is a highly conserved organ system, but teleost fish and mammals have evolved unique traits or have lost particular skeletal elements in each lineage. Several unique features of the skeleton relate to the extremely small size of early fish embryos and the small size of adult fish used as models. A detailed analysis of the plethora of interesting skeletal phenotypes in zebrafish and medaka pushes available skeletal imaging techniques to their respective limits and promotes the development of new imaging techniques. Impressive numbers of zebrafish and medaka mutants with interesting skeletal phenotypes have been characterized, complemented by transgenic zebrafish and medaka lines. The advent of efficient genome editing tools, such as TALEN and CRISPR/Cas9, allows to introduce targeted deficiencies in genes of model teleosts to generate skeletal phenotypes that resemble human skeletal diseases. This review will also discuss other attractive aspects of the teleost skeleton. This includes the capacity for lifelong tooth replacement and for the regeneration of dermal skeletal elements, such as scales and fin rays, which further increases the value of zebrafish and medaka models for skeletal research.