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.

Crtap and p3h1 knock out zebrafish support defective collagen chaperoning as the cause of their osteogenesis imperfecta phenotype.

Prolyl 3-hydroxylation is a rare collagen type I post translational modification in fibrillar collagens. The primary 3Hyp substrate sites in type I collagen are targeted by an endoplasmic reticulum (ER) complex composed by cartilage associated protein (CRTAP), prolyl 3-hydroxylase 1 (P3H1) and prolyl cis/trans isomerase B, whose mutations cause recessive forms of osteogenesis imperfecta with impaired levels of α1(I)3Hyp986. The absence of collagen type I 3Hyp in wild type zebrafish provides the unique opportunity to clarify the role of the complex in vertebrate. Zebrafish knock outs for crtap and p3h1 were generated by CRISPR/Cas9. Mutant fish have the typical OI patients' reduced size, body disproportion and altered mineralization. Vertebral body fusions, deformities and fractures are accompanied to reduced size, thickness and bone volume. Intracellularly, collagen type I is overmodified, and partially retained causing enlarged ER cisternae. In the extracellular matrix the abnormal collagen type I assembles in disorganized fibers characterized by altered diameter. The data support the defective chaperone role of the 3-hydroxylation complex as the primary cause of the skeletal phenotype.

Microstructure, mineral and mechanical properties of teleost intermuscular bones.

There is an increasing interest in understanding teleost bone biomechanics in several scientific communities, for instance as interesting biomaterials with specific structure-function relationships. Intermuscular bones of teleost fish have previously been described to play a role in the mechanical force transmission between muscle and bone, but their biomechanical properties are not yet fully described. Here, we have investigated intermuscular bones (IBs) of the North Atlantic Herring with regard to their structure and micro-architecture, mineral-related properties, and micro-mechanical tensile properties. A total of 115 IBs from 18 fish were investigated. One cohort of IBs, containing 20 bones from 2 smaller fish and 23 bones of 3 larger fish, was used for mechanical testing, wide-angle X-ray scattering, and scanning electron microscopy. Another cohort, containing 36 bones from 7 smaller fish and 36 bones from 6 larger fish, was used for microCT. Results show some astonishing properties of the IBs: (i) IBs present higher ductility, lower Young's modulus but similar strength and TMD (Tissue Mineral Density) compared to mammalian bone, and (ii) IBs from small fish were 49% higher in Young's modulus than fish bones from larger fish while their TMD was not statistically different and crystal length was 8% higher in large fish bones. Our results revealed that teleost IB presents a hybrid nature of soft and hard tissue that differs from other bone types, which might be associated with their evolution from mineralized tendons. This study provides new data regarding teleost fish bone biomechanical and micro-structural properties.