The mechanical impact of col11a2 loss on joints; col11a2 mutant zebrafish show changes to joint development and function, which leads to early-onset osteoarthritis

Collagen is the major structural component of cartilage, and mutations in the genes encoding type XI collagen are associated with severe skeletal dysplasias (fibrochondrogenesis and Stickler syndrome) and early-onset osteoarthritis (OA). The impact of the lack of type XI collagen on cell behaviour and mechanical performance during skeleton development is unknown. We studied a zebrafish mutant for col11a2 and evaluated cartilage, bone development and mechanical properties to address this. We show that in col11a2 mutants, type II collagen is made but is prematurely degraded in maturing cartilage and ectopically expressed in the joint. These changes are correlated with increased stiffness of both bone and cartilage; quantified using atomic force microscopy. In the mutants, the skeletal rudiment terminal region in the jaw joint is broader and the interzone smaller. These differences in shape and material properties impact on joint function and mechanical performance, which we modelled using finite element analyses. Finally, we show that col11a2 heterozygous carriers reach adulthood but show signs of severe early-onset OA. Taken together, our data demonstrate a key role for type XI collagen in maintaining the properties of cartilage matrix; which when lost leads to alterations to cell behaviour that give rise to joint pathologies.This article is part of the Theo Murphy meeting issue 'Mechanics of development'.

Building Finite Element Models to Investigate Zebrafish Jaw Biomechanics

Skeletal morphogenesis occurs through tightly regulated cell behaviors during development; many cell types alter their behavior in response to mechanical strain. Skeletal joints are subjected to dynamic mechanical loading. Finite element analysis (FEA) is a computational method, frequently used in engineering that can predict how a material or structure will respond to mechanical input. By dividing a whole system (in this case the zebrafish jaw skeleton) into a mesh of smaller 'finite elements', FEA can be used to calculate the mechanical response of the structure to external loads. The results can be visualized in many ways including as a 'heat map' showing the position of maximum and minimum principal strains (a positive principal strain indicates tension while a negative indicates compression. The maximum and minimum refer the largest and smallest strain). These can be used to identify which regions of the jaw and therefore which cells are likely to be under particularly high tensional or compressional loads during jaw movement and can therefore be used to identify relationships between mechanical strain and cell behavior. This protocol describes the steps to generate Finite Element models from confocal image data on the musculoskeletal system, using the zebrafish lower jaw as a practical example. The protocol leads the reader through a series of steps: 1) staining of the musculoskeletal components, 2) imaging the musculoskeletal components, 3) building a 3 dimensional (3D) surface, 4) generating a mesh of Finite Elements, 5) solving the FEA and finally 6) validating the results by comparison to real displacements seen in movements of the fish jaw.

Mechanobiology of embryonic skeletal development: Insights from animal models

A range of clinical conditions in which fetal movement is reduced or prevented can have a severe effect on skeletal development. Animal models have been instrumental to our understanding of the interplay between mechanical forces and skeletal development, particularly the mouse and the chick model systems. In the chick, the most commonly used means of altering the mechanical environment is by pharmaceutical agents which induce paralysis, whereas genetically modified mice with nonfunctional or absent skeletal muscle offer a valuable tool for examining the interplay between muscle forces and skeletogenesis in mammals. This article reviews the body of research on animal models of bone or joint formation in vivo in the presence of an altered or abnormal mechanical environment. In both immobilized chicks and "muscleless limb" mice, a range of effects are seen, such as shorter rudiments with less bone formation, changes in rudiment and joint shape, and abnormal joint cavitation. However, although all bones and synovial joints are affected in immobilized chicks, some rudiments and joints are unaffected in muscleless mice. We propose that extrinsic mechanical forces from movements of the mother or littermates impact on skeletogenesis in mammals, whereas the chick embryo is reliant on intrinsic movement for mechanical stimulation. The insights gained from animal models into the mechanobiology of embryonic skeletal development could provide valuable cues to prospective tissue engineers of cartilage and bone and contribute to new or improved treatments to minimize the impact on skeletal development of reduced movement in utero.

3D representation of the developing chick knee joint: a novel approach integrating multiple components

The knee joint has a highly complex 3-dimensional (3D) morphology that is sculpted at the interface of the forming long bones as they are generated in the embryo. Although it is clear that regulatory genes guide joint formation, the mechanisms that are responsible for morphogenesis of the knee are poorly understood. Certainly the process involves integration across several tissues and physical/mechanical influences from neighbouring tissues are important. We describe the acquisition of shape in the chick knee joint in detail and show that by HH34 the joint already displays shape characteristics of the adult structure. Through imaging developing cartilage, tendons, ligaments and muscle across developmental stages from HH28-34 we have built 3D representations of the forming structure including the various components important in knee formation. We describe the timing of muscle and tendon development in parallel with the refinement of cartilage shape, showing when and where (tendon attachment points) muscle forces are applied to the cartilage elements. Shape begins to emerge as the tendons are forming (HH30-32) but is fully refined (HH34) in the presence of tendons. The resulting integrated 3D representations of the developing knee across time will serve as the foundation for computational analysis of the mechanical environment, and experimental approaches to investigating morphogenetic mechanisms.