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

Effects and phenotypic consequences of transient thyrotoxicosis and hypothyroidism at different stages of zebrafish Danio rerio (Teleostei; Cyprinidae) skeleton development

The effects and consequences of changes in thyroid hormones (THs) level are among the actively studied topics in teleost developmental and evolutionary biology. In most of the experimental models used, the altered hormonal status (either hypo- or hyperthyroidism) is a stable characteristic of the developing organism, and the observed phenotypic outcomes are the cumulative consequences of multiple TH-induced developmental changes. Meanwhile, the influence of the transient fluctuations of TH content on skeleton development has been much less studied. Here, we present experimental data on the developmental effects and phenotypic consequences of transient, pharmacologically induced thyrotoxicosis and hypothyroidism at different stages of ossified skeleton patterning in zebrafish. According to the results, the skeleton structures differed in TH sensitivity. Some showed a notable shift in the developmental timing and rate, while other demonstrated little or no response to changes in TH content. The developmental stages also differed in TH sensitivity. We identified a relatively short developmental period, during which changes in TH level significantly increased the developmental instability and plasticity, leading to phenotypic consequences comparable to those in fish with a persistent hypo- or hyperthyroidism. These findings allow this period to be considered as a critical developmental window.

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.

From zero to ossified: Larval skeletal ontogeny of the Neotropical Cichlid fish Cichlasoma dimerus

The identification of skeletal elements, the analysis of their developmental sequence, and the time of their appearance during larval development are essential to broaden the knowledge of each fish species and to recognize skeletal abnormalities that may affect further fish performance. Therefore, this study aimed to provide a general description of the development of the entire skeleton highlighting its variability in Cichlasoma dimerus. Larvae of C. dimersus were stained with alcian blue and alizarin red from hatching to 25 days posthatching. Skeletogenesis began with the endoskeletal disk and some cartilage structures from the caudal fin and the splachnocranium, while the first bony structure observed was the cleithrum. When larvae reached the free-swimming and exogenous feeding stage, mostly bones from the jaws, the branchial arches, and the opercle series evidenced some degree of ossification, suggesting that the ossification sequence of C. dimerus adjusts to physiological demands such as feeding and ventilation. The caudal region was the most variable regarding meristic counts and evidenced higher incidence of bone deformities. In conclusion, this work provides an overview of C. dimerus skeletogenesis and lays the groundwork for further studies on diverse topics, like developmental plasticity, rearing conditions, or phylogenetic relationships.

Distinguishing the Effects of Water Volumes versus Stocking Densities on the Skeletal Quality during the Pre-Ongrowing Phase of Gilthead Seabream (Sparus aurata)

Gilthead seabream (Sparus aurata) production is a highly valued aquaculture industry in Europe. The presence of skeletal deformities in farmed gilthead seabream represents a major bottleneck for the industry leading to economic losses, negative impacts on the consumers' perception of aquaculture, and animal welfare issues for the fish. Although past work has primarily focused on the hatchery phase to reduce the incidence of skeletal anomalies, this work targets the successive pre-ongrowing phase in which more severe anomalies affecting the external shape often arise. This work aimed to test the effects of: (i) larger and smaller tank volumes, stocked at the same density; and (ii) higher and lower stocking densities maintained in the same water volume, on the skeleton of gilthead seabream fingerlings reared for ~63 days at a pilot scale. Experimental rearing was conducted with gilthead seabream juveniles (~6.7 ± 2.5 g), which were selected as 'non-deformed' based on external inspection, stocked at three different densities (Low Density (LD): 5 kg/m³; Medium Density (MD): 10 kg/m³; High Density (HD): 20 kg/m³) in both 500 L and 1000 L tanks. Gilthead seabream were sampled for growth performance and radiographed to assess the skeletal elements at the beginning and end of the experimental trial. Results revealed that (i) LD fish were significantly longer than HD fish, although there were no differences in final weights, regardless of the water volume; (ii) an increase in the prevalence of seabream exhibiting cranial and vertebral axis anomalies was found to be associated with increased density. These results suggest that farmers can significantly reduce the presence of some cranial and axis anomalies affecting pre-ongrown gilthead seabream by reducing the stocking density.

Anatomical and pathological characteristics of ribs in the Atlantic salmon (Salmo salar L.) and its relevance to soft tissue changes

Studies on the anatomical and pathological characteristics of ribs in farmed Atlantic salmon (Salmo salar L.) are warranted due to their possible association with red and melanized focal changes (RFC and MFC) in the fillet, a major quality and animal welfare concern. In this work, we provide an anatomical description of ribs based on radiographical and histological analyses. We also address various pathological rib changes and their association to RFC and MFC. In total, 129 fish were investigated; captured wild (n = 10) and hatchery reared (n = 119) Atlantic salmon (3.5-6.1 kg). The fish were selected based on the macroscopic presence of RFC, MFC or no changes (controls). Radiographic results revealed costal abnormalities in all fish groups. By histological investigations of the variations herein, our results provide new insight into the anatomical characteristics including vascularization within the ribs; a potential site for haemorrhage following costal fractures. Costal fractures were detected by radiology in 40 of 129 samples (RFC: 38.4%, MFC: 47.2%, controls: 9.5 %). A statistically significant association was found between costal fractures and red (p = 0.007) and melanized changes (p = 0.000). However, red and melanized changes were also observed in samples with no costal fractures (n = 45), indicating that also other factors influence the development of RFC/MFC.

A Baseline for Skeletal Investigations in Medaka (Oryzias latipes): The Effects of Rearing Density on the Postcranial Phenotype

Oryzias latipes is increasingly used as a model in biomedical skeletal research. The standard approach is to generate genetic variants with particular skeletal phenotypes which resemble skeletal diseases in humans. The proper diagnosis of skeletal variation is key for this type of research. However, even laboratory rearing conditions can alter skeletal phenotypes. The subject of this study is the link between skeletal phenotypes and rearing conditions. Thus, wildtype medaka were reared from hatching to an early juvenile stage at low (LD: 5 individuals/L), medium (MD: 15 individuals/L), and high (HD: 45 individuals/L) densities. The objectives of the study are: (I) provide a comprehensive overview of the postcranial skeletal elements in medaka; (II) evaluate the effects of rearing density on specific meristic counts and on the variability in type and incidence of skeletal anomalies; (III) define the best laboratory settings to obtain a skeletal reference for a sound evaluation of future experimental conditions; (IV) contribute to elucidating the structural and cellular changes related to the onset of skeletal anomalies. The results from this study reveal that rearing densities greater than 5 medaka/L reduce the animals' growth. This reduction is related to decreased mineralization of dermal (fin rays) and perichondral (fin supporting elements) bone. Furthermore, high density increases anomalies affecting the caudal fin endoskeleton and dermal rays, and the preural vertebral centra. A series of static observations on Alizarin red S whole mount-stained preural fusions provide insights into the etiology of centra fusion. The fusion of preural centra involves the ectopic formation of bony bridges over the intact intervertebral ligament. An apparent consequence is the degradation of the intervertebral ligaments and the remodeling and reshaping of the fused vertebral centra into a biconoid-shaped centrum. From this study it can be concluded that it is paramount to take into account the rearing conditions, natural variability, skeletal phenotypic plasticity, and the genetic background along with species-specific peculiarities when screening for skeletal phenotypes of mutant or wildtype medaka.

Larval Development in Tropical Gar (Atractosteus tropicus) Is Dependent on the Embryonic Thermal Regime: Ecological Implications under a Climate Change Context

In ectotherm species, environmental temperature plays a key role in development, growth, and survival. Thus, determining how temperature affects fish populations is of utmost importance to accurately predict the risk of climate change over fisheries and aquaculture, critical to warrant nutrition and food security in the coming years. Here, the potential effects of abnormal thermal regimes (24, 28 and 32 °C; TR24, TR28, and TR32, respectively) exclusively applied during embryogenesis in tropical gar (Atractosteus tropicus) has been explored to decipher the potential consequences on hatching and growth from fertilization to 16 days post-fertilization (dpf), while effects on skeletal development and body morphology were explored at fertilization and 16 dpf. Egg incubation at higher temperatures induced an early hatching and mouth opening. A higher hatching rate was obtained in eggs incubated at 28 °C when compared to those at 24 °C. No differences were found in fish survival at 16 dpf, with values ranging from 84.89 to 88.86%, but increased wet body weight and standard length were found in larvae from TR24 and TR32 groups. Thermal regime during embryogenesis also altered the rate at which the skeletal development occurs. Larvae from the TR32 group showed an advanced skeletal development, with a higher development of cartilaginous structures at hatching but reduced at 16 dpf when compared with the TR24 and TR28 groups. Furthermore, this advanced skeletal development seemed to determine the fish body morphology. Based on biometric measures, a principal component analysis showed how along development, larvae from each thermal regime were clustered together, but with each population remaining clearly separated from each other. The current study shows how changes in temperature may induce craniofacial and morphological alterations in fish during early stages and contribute to understanding the possible effects of global warming in early development of fish and its ecological implications.

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.

New insights into benzo[⍺]pyrene osteotoxicity in zebrafish

Persistent and ubiquitous organic pollutants, such as the polycyclic aromatic hydrocarbon benzo[⍺]pyrene (BaP), represent a major threat to aquatic organisms and human health. Beside some well-documented adverse effects on the development and reproduction of aquatic organisms, BaP was recently shown to affect fish bone formation and skeletal development through mechanisms that remain poorly understood. In this work, zebrafish bone-related in vivo assays were used to evaluate the osteotoxic effects of BaP during bone development and regeneration. Acute exposure of zebrafish larvae to BaP from 3 to 6 days post-fertilization (dpf) induced a dose-dependent reduction of the opercular bone size and a depletion of osteocalcin-positive cells, indicating an effect on osteoblast maturation. Chronic exposure of zebrafish larvae to BaP from 3 to 30 dpf affected the development of the axial skeleton and increased the incidence and severity of skeletal deformities. In young adults, BaP affected the mineralization of newly formed fin rays and scales, and impaired fin ray patterning and scale shape, through mechanisms that involve an imbalanced bone remodeling. Gene expression analyses indicated that BaP induced the activation of xenobiotic and metabolic pathways, while negatively impacting extracellular matrix formation and organization. Interestingly, BaP exposure positively regulated inflammation markers in larvae and increased the recruitment of neutrophils. A direct interaction between neutrophils and bone extracellular matrix or bone forming cells was observed in vivo, suggesting a role for neutrophils in the mechanisms underlying BaP osteotoxicity. Our work provides novel data on the cellular and molecular players involved in BaP osteotoxicity and brings new insights into a possible role for neutrophils in inflammatory bone reduction.

Performance of Co-Housed Neon Tetras (Paracheirodon innesi) and Glowlight Rasboras (Trigonostigma hengeli) Fed Commercial Flakes and Lyophilized Natural Food

Little to no research has been conducted thus far regarding aquarium fish nutrition. In order to ensure the welfare of house-kept ornamentals, such studies should take into account that there are distinct biological differences occurring between different fish species/taxa, especially in regard to the structure of their digestive organs. Accordingly, a 12-week trial was executed to assess the effects of two commercial flakes and a mix of lyophilized natural food on the condition of co-reared neon tetras, Paracheirodon innesi (Characidae), and glowlight rasboras, Trigonostigma hengeli (Danionidae). The four feeding groups were as follows: (T)-Tetra flakes; (O)-Omega flakes; (TO)-Tetra + Omega; (TOL)-Tetra + Omega + Lyophilizate (twice a week). There were no differences in final body weight (FBW) between the feeding groups of either species, but in the case of neon tetras, FBW increased significantly from the initial value only for the T group. However, histological observations and measurements of digestive organs (livers, intestines) showed pronounced differences between the two species. The supplementation with natural food in group TOL caused lipoid hepatic degeneration only in the rasboras. The healthiest histological structure of livers and longest intestinal folds were found in group T of the tetras and group TO of the rasboras. Whole-mount staining for bone and cartilage did not reveal any significant deformities or differences in terms of bone mineralization. In conclusion, it was outlined that concurrent feeding of co-housed, anatomically diverse ornamental fish species is a highly ambiguous task, because the nutritional strategy applied for a community tank may yield radically divergent effects, most of which may remain unnoticed when depending only on external body observations and measurements. Most emphatically, this was highlighted in regard to the dietary supplementation with natural food-although no significant effects were observed in neon tetras, severe lipoid liver degeneration occurred in glowlight rasboras.

Balancing between Artemia and microdiet usage for normal skeletal development in zebrafish (Danio rerio)

Targeting in zebrafish fast growth, high survival rates and improved reproductive performance has led over the last years in variable feeding regimes between different facilities. Despite its significance on fish function and welfare, normal skeletal development has rarely been evaluated in establishing the best feeding practices for zebrafish. The aim of this study was to establish a protocol for normal skeletal development, growth and survival of zebrafish larvae through live feed-to-microdiet transition at an appropriate rate. Four feeding regimes including feeding exclusively on Artemia nauplii (A) or dry microdiet (D), and feeding on both Artemia and microdiet at two different transition rates (slow (B) or fast (C)) were applied from 5 to 24 dpf (days post-fertilization). Results demonstrated a significant effect of feeding regimes on the incidence of skeletal abnormalities (gill cover, fins and vertebral column, p < .05) in zebrafish larvae. The A and B experimental groups presented the highest (88 ± 3 and 84 ± 17%, respectively), but the C and D the lowest (18 ± 14 and 11 ± 2%, respectively), rates of normal fish (fish without any abnormality). Similarly, growth rate was comparatively elevated in A and B groups. No significant differences were observed in fish survival between A, B and C groups. However, D group presented a significantly lower survival rate. To our knowledge, this is the first study to show that the live feed-to-microdiet transition rate influences larval growth, survival and abnormality rates in a non-homogenous pattern.

Effects of Holding Density on the Welfare of Zebrafish: A Systematic Review

The zebrafish is becoming an increasingly popular research animal around the world. Its welfare is affected by an array of environmental factors, such as food access and water quality. Holding density is an important welfare aspect, not least due to its interaction with other housing conditions. Despite the extensive use of zebrafish in research, little is known of how densities affect its welfare. In this systematic review, we have performed a large literature search, compiled, and evaluated all publications regarding zebrafish holding density. We have analyzed how density effects growth, reproduction, and stress response, including behavior, water quality, and pathogenic outbreaks in young and adult fish. Our review shows that the holding densities tested vary largely depending on the research focus, for example, body growth or behavior. In fact, research indicates that future recommendations on holding density could depend on which welfare aspects are considered. Overall, there is a need for more studies investigating the interactive effects of density on welfare indicators, such as reproduction coupled with stress response. We stress the necessity of including holding density in universal housing guidelines and reporting information on holding conditions of larvae and adults when publishing zebrafish work.

Monsters with a shortened vertebral column: A population phenomenon in radiating fish Labeobarbus (Cyprinidae)

The phenomenon of a massive vertebral deformity was recorded in the radiating Labeobarbus assemblage from the middle reaches of the Genale River (south-eastern Ethiopia, East Africa). Within this sympatric assemblage, five trophic morphs—generalized, lipped, piscivorous and two scraping feeders—were reported between 1993 and 2019. In 2009, a new morph with prevalence of ~10% was discovered. The new morph, termed ‘short’, had an abnormally shortened vertebral column and a significantly deeper body. This type of deformity is common in farmed Atlantic salmon and other artificially reared fish, but is rare in nature. In the Genale Labeobarbus assemblage, the deformity was present exclusively within the generalized and lipped morphs. The short morph had between seven and 36 deformed (compressed and/or fused) vertebrae. Their body depth was positively correlated with number of deformed vertebrae. In another collection in 2019, the short morph was still present at a frequency of 11%. Various environmental and genetic factors could contribute to the development of this deformity in the Genale Labeobarbus, but based on the available data, it is impossible to confidently identify the key factor(s). Whether the result of genetics, the environment, or both, this deep-bodied phenotype is assumed to be an anti-predator adaptation, as there is evidence of its selective advantage in the generalized morph. The Genale monstrosity is the first reported case of a massive deformity of the vertebral column in a natural population of African fishes.

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.

Axial Skeletal Malformations in Genetically Modified Xenopus laevis and Xenopus tropicalis

Skeletal malformations in captive-bred, adult Xenopus spp., have not previously been reported. Here we describe 10 sexually mature, genetically modified laboratory frogs (6 Xenopus laevis and 4 Xenopus tropicalis) with axial skeletal abnormalities. The young adult frogs were described by veterinary staff as presenting with "hunchbacks," but were otherwise considered to be in good health. All affected frogs were genetically engineered using various techniques: transcription activator-like effector nucleases (TALEN) editing using thyroid hormone receptor α TALEN mRNA, restriction enzyme-mediated integration methods involving insertion of the inducible transgene pCAR/TRDN, or via I-SceI meganuclease transgenesis using either pDRTREdpTR-HS4 or pDPCrtTA-TREG-HS4 plasmid sequences. Radiographic findings (6 frogs) and gross necropsy (10 frogs) revealed vertebral column malformations and sacroiliac deformities that resulted in moderate to severe kyphosis and kyphoscoliosis. These findings were confirmed and additional skeletal abnormalities were identified using computed tomography to create a 3D reconstruction of 4 frogs. Additional findings visible on the 3D reconstructions included incomplete vertebral segmentation, malformed transverse processes, and a short and/or curved urostyle. Histopathologic findings included misshapen intervertebral joints with nonconforming articular surfaces, narrowed joint cavities, flattened or irregularly-formed articular cartilage, irregular maturation lines and nonpolarized chondrocytes, excess fibrocartilage, and evidence of irregular bone resorption and growth. While the specific etiology of the vertebral skeletal abnormalities remains unclear, possibilities include: 1) egg/oocyte physical manipulation (dejellying, microinjection, fertilization, etc.), 2) induction and expression of the transgenes, 3) inactivation (knockout) of existing genes by insertional mutagenesis, or 4) a combination of the above. Furthermore, the possibility of undetected changes in the macro or microenvironment, or a feature of the genetic background of the affected frogs cannot be ruled out.

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.