For many years bone research has been mainly performed in mice, chicken, cell culture systems, or human material from the clinic. In this thesis, we make use of the zebrafish (Danio rerio), a relatively new model system in this field. This small teleost offers possibilities which makes it a great complement to the mouse: forward genetic screens are possible in fish due to extra-uterine development and large brood size, and the recent generation of osteoblast-specific reporter lines allows visualization of osteoblasts in vivo. As key regulators of bone formation are highly conserved between mammals and teleosts, findings in fish likely apply to mammalian osteogenesis and tissue mineralization. Out of a forward genetic screen we performed, we identified 27 specific bone mutants which can be categorized in three different groups: (1) less or no bone, (2) ectopic over-ossification, and (3) more perichondral bone, but normal dermal bone. Two mutants out of the second class were analyzed in detail. The stocksteif mutant is characterized by over-ossification of the notochord resulting in fused vertebrae. stocksteif encodes cyp26b1, a retinoic acid metabolizing enzyme. We show that cyp26b1 is expressed in osteoblasts and that -upon retinoic acid treatment- osteoblasts enhance their activity. Consequently, the over-production of bone causes the fusion of the vertebrae. Thus, regulation of retinoic acid levels and the tight control of cyp26b1 activity are essential for regulation of skeletogenesis in zebrafish. The second mutant, dragonfish, is also characterized by over-ossification of the notochord and the subsequent fusion of the vertebrae. However, dragonfish functions in a pathway which is clearly distinct of that of retinoic acid. Additionally, ectopic mineralizations in the brain and neural tube can be observed in these mutants and, interestingly, at these spots the typical osteoblast-specific genes are not detected. Future research has to unravel the molecular lesion in these mutants and will provide novel insight into the regulation of bone development and disease
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