The extracellular matrix of the immature and mature skeleton is key to the development and function of the skeletal system. Notwithstanding its importance, it has been technically challenging to obtain a comprehensive picture of the changes in skeletal composition throughout the development of bone and cartilage. In this study, we analyzed the extracellular protein composition of the zebrafish skeleton using a mass spectrometry-based approach, resulting in the identification of 262 extracellular proteins, including most of the bone and cartilage specific proteins previously reported in mammalian species. By comparing these extracellular proteins at larval, juvenile, and adult developmental stages, 123 proteins were found that differed significantly in abundance during development. Proteins with a reported function in bone formation increased in abundance during zebrafish development, while analysis of the cartilage matrix revealed major compositional changes during development. The protein list includes ligands and inhibitors of various signaling pathways implicated in skeletogenesis such as the Int/Wingless as well as the insulin-like growth factor signaling pathways. This first proteomic analysis of zebrafish skeletal development reveals that the zebrafish skeleton is comparable with the skeleton of other vertebrate species including mammals. In addition, our study reveals 6 novel proteins that have never been related to vertebrate skeletogenesis and shows a surprisingly large number of differences in the cartilage and bone proteome between the head, axis and caudal fin regions. Our study provides the first systematic assessment of bone and cartilage protein composition in an entire vertebrate at different stages of development.

Summary Zebrafish embryos were exposed to concentration ranges of selected thyroid-active model compounds in order to

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