Zebrafish are teleosts (bony fish) that share with mammals a common ancestor belonging to the phylum Osteichthyes, from which their endoskeletal systems have been inherited. Indeed, teleosts and mammals have numerous genetically conserved features in terms of skeletal elements, ossification mechanisms, and bone matrix components in common. Yet differences related to bone morphology and function need to be considered when investigating zebrafish in skeletal research. In this review, we focus on zebrafish skeletal architecture with emphasis on the morphology of the vertebral column and associated anatomical structures. We provide an overview of the different ossification types and osseous cells in zebrafish and describe bone matrix composition at the microscopic tissue level with a focus on assessing mineralization. Processes of bone formation also strongly depend on loading in zebrafish, as we elaborate here. Furthermore, we illustrate the high regenerative capacity of zebrafish bones and present some of the technological advantages of using zebrafish as a model. We highlight zebrafish axial and fin skeleton patterning mechanisms, metabolic bone disease such as after immunosuppressive glucocorticoid treatment, as well as osteogenesis imperfecta (OI) and osteopetrosis research in zebrafish. We conclude with a view of why larval zebrafish xenografts are a powerful tool to study bone metastasis. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Osteoporosis is metabolic bone disease caused by an altered balance between bone anabolism and catabolism. This dysregulated balance is responsible for fragile bones that fracture easily after minor falls. With an aging population, the incidence is rising and as yet pharmaceutical options to restore this imbalance is limited, especially stimulating osteoblast bone-building activity. Excitingly, output from large genetic studies on people with high bone mass (HBM) cases and genome wide association studies (GWAS) on the population, yielded new insights into pathways containing osteo-anabolic players that have potential for drug target development. However, a bottleneck in development of new treatments targeting these putative osteo-anabolic genes is the lack of animal models for rapid and affordable testing to generate functional data and that simultaneously can be used as a compound testing platform. Zebrafish, a small teleost fish, are increasingly used in functional genomics and drug screening assays which resulted in new treatments in the clinic for other diseases. In this review we outline the zebrafish as a powerful model for osteoporosis research to validate potential therapeutic candidates, describe the tools and assays that can be used to study bone homeostasis, and affordable (semi-)high-throughput compound testing.

Fibroblast growth factors (Fgfs) have long been implicated in regulating vertebrate skeletal muscle differentiation, but their precise role(s) in vivo remain unclear. Here, we show that Fgf8 signalling in the somite is required for myod expression and terminal differentiation of a subset of fast muscle cells in the zebrafish lateral somite. In the absence of Fgf8, lateral somite cells transiently express myf5 but fail to make muscle and remain in a dermomyotome-like state characterised by pax3 and meox expression. Slow muscle fibres form and commence normal migration in the absence of Fgf8, but fail to traverse the expanded undifferentiated lateral somite. The Fgf8-independent residual population of medial fast muscle fibres is not Hedgehog dependent. However, Fgf8-independent medial fast muscle precursors are lacking in floatinghead mutants, suggesting that they require another ventral midline-derived signal. We conclude that Fgf8 drives terminal differentiation of a specific population of lateral muscle precursor cells within the early somite.

Hedgehog (Hh) signalling has been implicated in the development of osteoblasts and osteoclasts whose balanced activities are critical for proper bone formation. As many mouse mutants in the Hh pathway are embryonic lethal, questions on the exact effects of Hh signalling on osteogenesis remain. Using zebrafish, we show that there are two populations of endochondral osteoblasts with differential sensitivity to Hh signalling. One, formed outside the cartilage structure, requires low levels of Hh signalling and fails to differentiate in Indian hedgehog mutants. The other derives from chondrocytes and requires higher levels of Hh signalling to form. This latter population develops significantly earlier in mutants with increased Hh signalling, leading to premature endochondral ossification, and also fails to differentiate in Indian hedgehog mutants, resulting in severely delayed endochondral ossification. Additionally, we demonstrate that the timing of first osteoclast activity positively correlates to Hh levels in both endochondral and dermal bone.