Osteoblast-Induced Collagen Alignment in a 3D in vitro Bone Model

The bone extracellular matrix consists of a highly organized collagen matrix that is mineralized by hydroxyapatite. Even though the structure and composition of bone have been studied extensively, the mechanisms underlying collagen matrix organization remain elusive. In this study, we developed a 3D cell culture system in which osteogenic cells deposit an oriented collagen matrix, which can be mineralized. Using live fluorescence imaging combined with volume electron microscopy, we visualized the organization of the cells and collagen in the cell culture. We showed that the osteogenic cells are organizing the collagen matrix during development. Based on the observation of tunnel-like structures surrounded by aligned collagen in the center of the culture, we propose that osteoblasts organize the deposited collagen during migration towards the periphery of the culture. Overall, we show that cell-matrix interactions are involved in collagen alignment during early-stage osteogenesis and that the matrix is organized by the osteoblasts even in the absence of osteoclast activity. ### Competing Interest Statement The authors have declared no competing interest.