Vertebral column regionalisation in Chinook salmon, Oncorhynchus tshawytscha

Effect of heterozygosity, ploidy and incubation temperature on post-cranial axial skeletal meristics and deformities in Atlantic salmon (Salmo salar)

The teleostean post-cranial axial skeleton is a highly specialized structure for an aquatic mode of life. However, there is limited knowledge regarding parental contributions, early-life environmental impacts on its meristic variation and if reduced heterozygosity challenges its development. To address this, the present study used isogenic homozygous and heterozygous lines of Atlantic salmon (Salmo salar) combined with ploidy manipulation (triploidization) to manipulate parental contributions, and incubation temperature (4 vs. 8°C) as an early-life variable, and reared the fish to ~150 g for a detailed radiological examination. Genetically identical fish incubated at 4°C, but not 8°C, segregated into two size modes (upper/lower), which differed in dorsal and tail fin lepidotrich counts as well as anal-fin pterygiophore counts. Incubation temperature did not impact on vertebrae counts, whereas 8°C incubation produced more supraneurals than 4°C incubation. After 8°C incubation, homozygous diploids (100% maternal chromosomes) and heterozygous triploids (67% maternal chromosomes) developed lower total vertebrae and dorsal- and anal-fin pterygiophore counts than heterozygous diploids (50% maternal chromosomes). For tail fin lepidotrichs, the same groups showed the following pattern: diploid heterozygous > triploid heterozygous > diploid homozygous. Homozygous diploids developed a high level of complete fusions in the vertebral column. The result of the present study indicates that the ability to enter different growth modes is dependent on embryo incubation temperature and may be controlled by epigenetic mechanisms. Further, the results show a strong maternal dosage effect on tail fin lepidotrich counts, whereas for other post-cranial skeletal parts, the presence of extra maternal chromosomes seems to overrule the paternal contribution. The findings may reflect evolutionary adaptations for the shaping of offspring phenotypes. Such mechanisms would impact on important fitness-related traits, such as swimming ability and fecundity, which are relevant for conservation and evolutionary biology and ecological and aquaculture sciences. Vertebral deformities developing in homozygous fish seem to be supported by active repair mechanisms, which may reflect an organism's ability to reduce the cost of inbreeding.

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.

Repatterning of mammalian backbone regionalization in cetaceans

Cetacean reinvasion of the aquatic realm is an iconic ecological transition that led to drastic modifications of the mammalian body plan, especially in the axial skeleton. Relative to the vertebral column of other mammals that is subdivided into numerous anatomical regions, regional boundaries of the cetacean backbone appear obscured. Whether the traditional mammalian regions are present in cetaceans but hard to detect due to anatomical homogenization or if regions have been entirely repatterned remains unresolved. Here we combine a segmented linear regression approach with spectral clustering to quantitatively investigate the number, position, and homology of vertebral regions across 62 species from all major cetacean clades. We propose the Nested Regions hypothesis under which the cetacean backbone is composed of six homologous modules subdivided into six to nine post-cervical regions, with the degree of regionalization dependent on vertebral count and ecology. Compared to terrestrial mammals, the cetacean backbone is less regionalized in the precaudal segment but more regionalized in the caudal segment, indicating repatterning of the vertebral column associated with the transition from limb-powered to axial-driven locomotion.

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.

Osteological features of some clupeid fishes (Teleostei: Clupeiformes) of Iran

This paper presents the conclusions of a comparative analysis of six osteological features: the Structure of the vertebral column, the morphology of the predorsal bones, the vertebral column regionalization, the pterygiophore interdigitation with neural spines of dorsal fin, the pterygiophores interdigitation of with the haemal spines of the anal fin, and the intermuscular bones (IMB) and hypomerals (HM) of 12 clupeid species of the families Alosidae, Dorosomatidae, Dussumieridae and Ehiravidae. Conceivable taxonomically beneficial osteological features are nominated and utilized to discrete the clupeid species explored. Formulae for the structure of the vertebral column, the dorsal- and anal-fin pterygiophores' interdigitation with the neural and haemal spines of the vertebrae are established. These morphological descriptive traits disclose a morphotype that may be related to the mode of swimming of the species searched. The morphological study of the vertebral column of the species in question permits the division of this bony structure into six morphologically different regions. This regionalization is more intricate than the classical division in abdominal and caudal parts only.

Transcriptome Analysis Elucidates the Potential Key Genes Involved in Rib Development in bmp6-Deficient Silver Carp (Hypophthalmichthys molitrix)

Bone morphogenetic protein 6 (BMP-6) is a constituent of the TGF-β superfamily, known for its ability to stimulate bone and cartilage formation. The investigation of bmp6's involvement in the formation of intermuscular bones in fish has garnered significant attention in recent years. The rib cage is an important skeletal structure that plays a protective function for internal organs in fish. However, there has been limited research conducted on the effects of the bmp6 gene on rib development. Silver carp is one of four major fish in China, favoured for its affordability and tender muscle. Nevertheless, the presence of numerous intermuscular bones in silver carp significantly hinders the advancement of its palatability and suitability for processing. This study showcases the effective utilisation of CRISPR/Cas9 technology for the purpose of disrupting the bmp6 gene in silver carp, leading to the creation of chimeras in the P0 generation, marking the first instance of such an achievement. The chimeras exhibited complete viability, normal appearance, and partial intermuscular bones loss, with approximately 30% of them displaying rib bifurcation or bending. Subsequently, a transcriptome analysis on ribs of P0 chimeras and wild-type silver carp was conducted, leading to the identification of 934 genes exhibiting differential expression, of which 483 were found to be up-regulated and 451 were found to be down-regulated. The results of the KEGG analysis revealed that the "NF-kappa B signalling pathway", "Hippo signalling pathway", "osteoclast differentiation", and "haematopoietic cell lineage" exhibited enrichment and displayed a significant correlation with bone development. The up-regulated genes such as tnfα, fos, and ctgf in pathways may facilitate the proliferation and differentiation of osteoclasts, whereas the down-regulation of genes such as tgfb2 and tgfbr1 in pathways may hinder the formation and specialisation of osteoblasts, ultimately resulting in rib abnormalities. This study presents novel findings on the impact of bmp6 gene deletion on the rib development of silver carp, while simultaneously investigating the previously unexplored molecular mechanisms underlying rib defects in fish.

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.

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.

hox gene expression predicts tetrapod-like axial regionalization in the skate, Leucoraja erinacea

The axial skeleton of tetrapods is organized into distinct anteroposterior regions of the vertebral column (cervical, trunk, sacral, and caudal), and transitions between these regions are determined by colinear anterior expression boundaries of Hox5/6, -9, -10, and -11 paralogy group genes within embryonic paraxial mesoderm. Fishes, conversely, exhibit little in the way of discrete axial regionalization, and this has led to scenarios of an origin of Hox-mediated axial skeletal complexity with the evolutionary transition to land in tetrapods. Here, combining geometric morphometric analysis of vertebral column morphology with cell lineage tracing of hox gene expression boundaries in developing embryos, we recover evidence of at least five distinct regions in the vertebral skeleton of a cartilaginous fish, the little skate (Leucoraja erinacea). We find that skate embryos exhibit tetrapod-like anteroposterior nesting of hox gene expression in their paraxial mesoderm, and we show that anterior expression boundaries of hox5/6, hox9, hox10, and hox11 paralogy group genes predict regional transitions in the differentiated skate axial skeleton. Our findings suggest that hox-based axial skeletal regionalization did not originate with tetrapods but rather has a much deeper evolutionary history than was previously appreciated.

A neck-like vertebral motion in fish

Tetrapods use their neck to move the head three-dimensionally, relative to the body and limbs. Fish lack this anatomical neck, yet during feeding many species elevate (dorsally rotate) the head relative to the body. Cranial elevation is hypothesized to result from the craniovertebral and cranial-most intervertebral joints acting as a neck, by dorsally rotating (extending). However, this has never been tested due to the difficulty of visualizing and measuring vertebral motion in vivo. I used X-ray reconstruction of moving morphology to measure three-dimensional vertebral kinematics in rainbow trout (Oncorhynchus mykiss) and Commerson's frogfish (Antennarius commerson) during feeding. Despite dramatically different morphologies, in both species dorsoventral rotations extended far beyond the craniovertebral and cranial intervertebral joints. Trout combine small (most less than 3°) dorsal rotations over up to a third of their intervertebral joints to elevate the neurocranium. Frogfish use extremely large (often 20-30°) rotations of the craniovertebral and first intervertebral joint, but smaller rotations occurred across two-thirds of the vertebral column during cranial elevation. Unlike tetrapods, fish rotate large regions of the vertebral column to rotate the head. This suggests both cranial and more caudal vertebrae should be considered to understand how non-tetrapods control motion at the head-body interface.

Vertebral deformities in interspecific diploid and triploid salmonid hybrids

Vertebral deformities in salmonid interspecific hybrids, some of which were triploidised, were assessed across three separate year classes during the freshwater life stage. Initially, eggs from a farmed Atlantic salmon Salmo salar were crossed with the sperm from a S. salar, arctic char Salvelinus alpinus or brown trout Salmo trutta. For S. salar × S. trutta, half the eggs were triploidised. In a second- and third-year class, the eggs from a farmed S. salar were crossed with the sperm from either a S. salar or a S. trutta, and half of each group was triploidised. In the two initial-year classes, all hybrids were larger than the S. salar controls, and triploid S. salar × S. trutta were larger than diploid counterparts. In the third-year class, the S. salar × S. trutta were smaller than the S. salar, in contrast to the initial 2 year classes, although the triploid hybrids were still larger than the diploids. In the third-year class, a high degree of spontaneous triploidy was also observed in the putative diploid groups (between 16 and 39%). Vertebral deformities were consistently higher in pressure-shocked triploids than diploids, irrespective of hybridisation, but there was no consistent effect of hybridisation among experiments. Although this study was not able to explain the contrasting results for vertebral deformities between year classes, triploid S. salar × S. trutta can demonstrate impressive freshwater growth that could be of interest for future farming programmes.

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.

Radiographic characterisation of spinal curvature development in farmed New Zealand Chinook salmon Oncorhynchus tshawytscha throughout seawater production

Spinal anomalies are a recognised source of downgrading in finfish aquaculture, but identifying their cause(s) is difficult and often requires extensive knowledge of the underlying pathology. Late-onset spinal curvatures (lordosis, kyphosis, scoliosis) can affect up to 40% of farmed New Zealand Chinook (king) salmon (Oncorhynchus tshawytscha) at harvest, but little is known about their pathogenesis. Curvature development was radiographically documented in two related cohorts of commercially-farmed Chinook salmon throughout seawater production to determine (1) the timing of radiographic onset and relationships between (2) the curvature types, (3) the spinal regions in which they develop and (4) their associations with co-existing vertebral body anomalies (vertebral compression, fusion and vertical shift). Onset of curvature varied between individuals, but initially occurred eight months post-seawater transfer. There were strong associations between the three curvature types and the four recognised spinal regions: lordosis was predominantly observed in regions (R)1 and R3, kyphosis in R2 and R4, manifesting as a distinct pattern of alternating lordosis and kyphosis from head to tail. This was subsequently accompanied by scoliosis, which primarily manifested in spinal regions R2 and R3, where most of the anaerobic musculature is concentrated. Co-existing vertebral body anomalies, of which vertebral compression and vertical shift were most common, appeared to arise either independent of curvature development or as secondary effects. Our results suggest that spinal curvature in farmed New Zealand Chinook salmon constitutes a late-onset, rapidly-developing lordosis-kyphosis-scoliosis (LKS) curvature complex with a possible neuromuscular origin.

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.

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.

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.