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.

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.

Heterochrony and parallel evolution of echinoderm, hemichordate and cephalochordate internal bars

Deuterostomes comprise three phyla with radically different body plans. Phylogenetic bracketing of the living deuterostome clades suggests the latest common ancestor of echinoderms, hemichordates and chordates was a bilaterally symmetrical worm with pharyngeal openings, with these characters lost in echinoderms. Early fossil echinoderms with pharyngeal openings have been described, but their interpretation is highly controversial. Here, we critically evaluate the evidence for pharyngeal structures (gill bars) in the extinct stylophoran echinoderms Lagynocystis pyramidalis and Jaekelocarpus oklahomensis using virtual models based on high-resolution X-ray tomography scans of three-dimensionally preserved fossil specimens. Multivariate analyses of the size, spacing and arrangement of the internal bars in these fossils indicate they are substantially more similar to gill bars in modern enteropneust hemichordates and cephalochordates than to other internal bar-like structures in fossil blastozoan echinoderms. The close similarity between the internal bars of the stylophorans L. pyramidalis and J. oklahomensis and the gill bars of extant chordates and hemichordates is strong evidence for their homology. Differences between these internal bars and bar-like elements of the respiratory systems in blastozoans suggest these structures might have arisen through parallel evolution across deuterostomes, perhaps underpinned by a common developmental genetic mechanism.

Compression Fractures and Partial Phenotype Rescue With a Low Phosphorus Diet in the Chihuahua Zebrafish Osteogenesis Imperfecta Model

Osteogenesis imperfecta (OI) is a group of heritable disorders affecting bone and other connective tissues. Dominant OI forms are mainly caused by mutations in collagen type I. Patients suffer from skeletal deformities, fractures of long bones and vertebral compression fractures from early childhood onward. Altered collagen structure and excess mineralisation are the main causes for the bone phenotype. The Chihuahua (Chi/+) zebrafish has become an important model for OI. Given that reduced dietary phosphorus (P) intake reduces the bone mineral content and promotes bone matrix formation in teleosts, including zebrafish, we tested whether a low dietary P (LP) intake mitigates the OI phenotype in the Chi/+ model. To answer this question, we characterised the Chi/+ vertebral column phenotype at a morphological, cellular and subcellular level. We present the first description of vertebral compression fractures in Chi/+ and assess the effects of LP diet on the Chi/+ phenotype (Chi/+_LP). Compared to untreated Chi/+, two months of LP dietary treatment decreases vertebral deformities in the abdominal region and reduces shape variation of caudal vertebral bodies to a condition more similar to wild type (WT). At the histological level, the osteoid layer, covering the bone at the vertebral body endplates in WT zebrafish, is absent in Chi/+, but it is partially restored with the LP diet. Whole mount-stained specimens and histological sections show various stages of vertebral compression fractures in Chi/+ and Chi/+_LP animals. Both Chi/+ and Chi/+_LP show abundant osteoclast activity compared to WT. Finally, the ultrastructure analysis of WT, Chi/+ and Chi/+_LP shows Chi/+ and Chi/+_LP osteoblasts with enlarged endoplasmic reticulum cisternae and a high protein content, consistent with intracellular retention of mutated collagen. Nevertheless, the secreted collagen in Chi/+_LP appears better organised concerning fibre periodicity compared to Chi/+. Our findings suggest that a reduced mineral content of Chi/+ bone could explain the lower frequency of vertebral column deformities and the restored shape of the vertebral bodies in Chi/+_LP animals. This, together with the improved quality of the bone extracellular matrix, suggests that two months of reduced dietary P intake can alleviate the severe bone phenotype in Chi/+ zebrafish.

3D assessment of intervertebral disc degeneration in zebrafish identifies changes in bone density that prime disc disease

Back pain is a common condition with a high social impact and represents a global health burden. Intervertebral disc disease (IVDD) is one of the major causes of back pain; no therapeutics are currently available to reverse this disease. The impact of bone mineral density (BMD) on IVDD has been controversial, with some studies suggesting osteoporosis as causative for IVDD and others suggesting it as protective for IVDD. Functional studies to evaluate the influence of genetic components of BMD in IVDD could highlight opportunities for drug development and repurposing. By taking a holistic 3D approach, we established an aging zebrafish model for spontaneous IVDD. Increased BMD in aging, detected by automated computational analysis, is caused by bone deformities at the endplates. However, aged zebrafish spines showed changes in bone morphology, microstructure, mineral heterogeneity, and increased fragility that resembled osteoporosis. Elements of the discs recapitulated IVDD symptoms found in humans: the intervertebral ligament (equivalent to the annulus fibrosus) showed disorganized collagen fibers and herniation, while the disc center (nucleus pulposus equivalent) showed dehydration and cellular abnormalities. We manipulated BMD in young zebrafish by mutating sp7 and cathepsin K, leading to low and high BMD, respectively. Remarkably, we detected IVDD in both groups, demonstrating that low BMD does not protect against IVDD, and we found a strong correlation between high BMD and IVDD. Deep learning was applied to high-resolution synchrotron µCT image data to analyze osteocyte 3D lacunar distribution and morphology, revealing a role of sp7 in controlling the osteocyte lacunar 3D profile. Our findings suggest potential avenues through which bone quality can be targeted to identify beneficial therapeutics for IVDD.

A quantitative analysis of gilthead seabream (Sparus aurata) juvenile dentition as a tool to assess the effect of diet

Gilthead seabream (Sparus aurata Linnaeus, 1758 (Perciformes, Sparidae)) is an important aquaculture species in the Mediterranean Sea basin. Yet, quantitative data on its dentition under standard farming conditions are currently lacking. Furthermore, it is unknown if the dentition can adapt to food of different sizes. Here, we describe the lower jaw dentition of juvenile S. aurata fed a standard pellet size (4 mm) and present a detailed analysis of 11 representative teeth. Overall, the number of teeth showed large individual variation, but it was not significantly related to fish length. Considerable left–right differences were observed, without clear side dominance. We also assessed the influence of feeding S. aurata a smaller (2 mm) or larger (6 mm) pellet size. Four months of feeding with different pellet sizes did not cause detectable differences in total tooth number on the dentaries at the time of harvest, nor in size of the teeth assumed to be most relevant in food processing. If and how different pellet sizes may nevertheless affect digestion, and eventually fish health, is subject for further studies.

Bones of teleost fish demonstrate high fracture strain

The endoskeleton of teleosts (bony fish) includes a vertebral spine with articulating rib bones (RBs) similar to humans and further encompasses mineralized tissues that are not found in mammals, including intermuscular bones (IBs). RBs form through endochondral ossification and protect the inner organs, and IBs form through intramembranous ossification within the myosepta and play a role in force transmission and propulsion during locomotion. Based on previous findings suggesting that IBs show a much higher ability for fracture strain compared to mammalian bones, this study aims to investigate whether this ability is general to teleost bones or specific to IBs. We analyzed RBs and IBs of 25 North Atlantic Herring fish. RBs were analyzed using micro-mechanical tensile testing and micro-computed tomography, and both RB and IB were additionally analyzed with Raman spectroscopy. Based on our previous results from IB, we found that RBs are more elastically deformable (on average, 50% higher yield strain and 115% higher elastic work) and stronger (55% higher fracture stress) than values reported for IBs. However, these differences were neither associated with a higher Young's modulus nor a higher degree of mineralization in RBs. Astonishingly, RBs and IBs showed similar fracture strains (12-15% on average, reaching up to 20%), reflecting a much higher ability for tensile deformation than reported for mammalian bone, and further highlighting the biomimetic potential of teleost fish bones for inspiring innovative biomaterials.

Skeletal Anomalies in Senegalese Sole (Solea senegalensis, Kaup) Fed with Different Commercial Enriched Artemia: A Study in Postlarvae and Juveniles

Simple Summary

Enrichment products for Artemia spp. metanauplii are commonly used to enhance the nutritional quality of this live prey offered to fish during conventional larval feeding. However, there are few reports on the influence of such enrichments on the development of skeletal anomalies in Senegalese sole, a major problem for this flatfish aquaculture. This study evaluated the frequency of vertebral anomalies in postlarvae and juvenile Senegalese sole fed with Artemia spp. metanauplii enriched with four commercial products (EA, EB, EC, and ED) in a fish farm. The results show a high percentage of individuals with skeletal anomalies in every dietary group. Some types of anomalies were very frequent in all diet-age groups, indicating the presence of a common trend or mainstay of vertebral deformities. Despite some variations in the frequency of anomalies among diets, it was not possible to establish a clear effect of the enrichment products on the development of vertebral deformities at both rearing stages, probably for the “masking effect” of other rearing conditions. The multivariate statistical technique, as the correspondence analysis, indicated a different anomaly pattern among ages, where bone adaptative responses may be implied.

Abstract

The high incidence of skeletal anomalies in Senegalese sole (Solea senegalensis) still constitutes a bottleneck constraining its production. There are diverse commercially available products for the enrichment of live preys, but few reports of their influence on skeletogenesis in Senegalese sole. This study evaluated the presence of vertebral anomalies in postlarvae and juvenile Senegalese sole fed with Artemia spp. metanauplii enriched with four commercial products (EA, EB, EC, and ED) in a fish farm. The most frequent alterations consisted of deformations of the neural/haemal arches and spines and fusions and deformations of hypurals, epural, or parhypural. The correspondence analysis ordered fish from each age in separated semiaxis, indicating the presence of different anomaly patterns for the two sampled stages. The results showed only very light changes in the frequency of vertebral abnormalities among tested enrichment products, i.e., individuals from EC and EA lots displayed less vertebral body anomalies and/or vertebral column deviations at 31 and 105 days after hatching, respectively. The existence of a large shared malformation pattern in all the experimental groups leads to impute to the rearing conditions as the main driving factor of the onset of such group of anomalies, probably masking some dietary effect.

Study on the morphological and metabolic changes of femur in laying hens with hypophosphatemia

Layer fatigue syndrome caused by the lack of calcium and phosphorus can cause fracture in laying hens. The effect of phosphorus deficiency on the femur of laying hens with layer fatigue syndrome has not been studied. In this study, sixty 22-week-old Roman white layers were randomly divided into control group (group C) and low phosphorus group (group P), 30 individuals in each group. The available phosphorus content of group P was 0.18%. At the age of 26, 30 and 34 weeks, the production performance, biomechanical index, protein expression, histopathological change of femur and serological index were detected. The results showed that the laying rate, egg quality and body weight of laying hens, bone density, cortical bone thickness, rigidity, flexural modulus, flexural rigidity, the maximum load of femur and expression of osteocalcin (OCN), receptor activator of nuclear factor kappa-Β (RANK) and receptor activator of nuclear factor kappa-Β ligand (RANKL) decreased of group P. The number of osteocytes was decreased, and the voids was increased. However, cell lacunae were not obvious. The levels of phosphorus, calcium and OCN were increased, and the content of estradiol (E₂), OPG and calcitonin (CT) were decreased in serum. In conclusion, the low phosphorus diet can induce layer fatigue syndrome and affect the content of OPG and E₂ in serum and the expression of OCN, OPG, RANK and RANKL in femur protein, which leads to the imbalance of bone homeostasis, the thinning of femur cortex bone and the decrease of bone density.

Comparative transcriptome analyses and identification of candidate genes involved in vertebral abnormality of bighead carp Hypophthalmichthys nobilis

Body deformity occurs both in wild and farmed fishes, which is one of the most challenging problems for aquaculture industry. In most cases, such body deformities are linked to skeletal deformities. Currently, very limited information is available on skeletal deformities of farmed fish species which may be caused by genetic factor. In this study, we performed muscle and vertebra transcriptome analyses in body deformity and normality of bighead carp Hypophthalmichthys nobilis (from one meiotic gynogenesis family) using RNA-Seq. A total of 43,923 and 44,416 unigenes were predicted in muscles and vertebrae, respectively. Based on these data, we further explored the gene expression profiles in gynogenetic normal and abnormal bighead carp. No differentially expressed gene (DEG) was found in transcriptome data of muscles. Totally, 20 key DEGs were identified in transcriptome data of vertebrae, such as low density lipoprotein-related protein 2 (lrp2), bone morphogenetic protein 2B (bmp2b) and collagen alpha-1(IV) (col4a1). 12 potential pathways were also identified in vertebra transcriptome data, which were mainly involved in development, growth, cytoskeleton and energy metabolism, such as MAPK signaling pathway, regulation of actin cytoskeleton and TGF-beta signaling pathway. Results of this study will be informative for the understanding of genetic mechanisms for body shape formation and also provide potential candidate genes for selection program involved in body shape and skeletal development in H. nobilis.

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.

Triploid Atlantic salmon Salmo salar have a higher dietary phosphorus requirement for bone mineralization during early development

The effect of a dietary phosphorus regime in freshwater on vertebra bone mineralization was assessed in diploid and triploid Atlantic salmon, Salmo salar. Fish were fed either a low phosphorus (LP) diet containing 10.5 g kg^-1 total phosphorus or a normal phosphorus (NP) diet containing 17.4 g kg^-1 total phosphorus from ∼3 to ∼65 g (day 126) in body weight. Two further groups were fed the NP diet from ∼3 g in body weight, but were then switched to the LP diet after 38 (∼10 g in body weight) or 77 (∼30 g in body weight) days. Growth, vertebral ash content (% ash) and radiologically detectable vertebra pathologies were assessed. Triploids were initially smaller than diploids, and again on day 77, but there was no ploidy effect on days 38 or 126. Vertebral ash content increased with increasing body size and those fish fed the NP diet had higher vertebral ash content than those groups fed the LP diet during the intervening time period, but this diet effect became less apparent as fish grew, with all groups having relatively equal vertebral ash content at termination. In general, triploids had lower vertebral ash content than diploids on day 38 and this was most evident in the group fed the LP diet. On day 77, those triploids fed the LP diet during the intervening time period had lower vertebral ash content than diploids. At termination on day 126, the triploids had the same vertebral ash content as diploids, irrespective of diet. There was a ploidy × diet interaction on vertebral deformities, with triploids having higher prevalences of fish with ≥1 deformed vertebra in all dietary groups except continuous NP. In conclusion, between days 0 and 77 (3-30 g body size), triploids required more dietary phosphorus than diploids in order to maintain similar vertebral ash content. A possible link between phosphorus feeding history and phosphorus demand is also discussed.

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.