Primary trabecular bone formation in vitro by the OmGFP66 osteogenic cell line: Multiscale symmetry breaking and characterization in 3D

Osteocytes are abundant bone cells that serve as central regulators of skeletal homeostasis. Within mineralized bone tissue, osteocytes and their cell processes/dendrites maintain cell connectivity through a lacunocanalicular network morphologically positioned for executing myriad cell signaling pathways, including those related to mechanosensation, mineral ion homeostasis, and extracellular matrix mineralization. Given the complexity of osteocyte morphological transitions within mineralized bone, there are few robust in vitro models that reproduce the mineralized bone microenvironment. A recently developed mouse calvarial osteocyte cell line, OmGFP66, overcomes many of these limitations. Here we provide a comprehensive 3D multiscale characterization of mineralized, primary bone-like trabeculae formed by OmGFP66 cells in vitro, with a comparison to mouse calvariae. Submicron X-ray microcomputed tomography (μCT) was used to image and reveal quantitative features of thousands of discrete, variably shaped trabeculae formed by OmGFP66 cells with osteocyte lacunae having features quantitatively similar to those of neonatal mouse calvarial primary bone. Moreover, FIB-SEM and TEM analyses revealed the 3D ultrastructure of an extended lacunocanalicular network formed by the OmGFP66 cells within mineralized extracellular matrix and extending through an osteoid layer to osteoblasts at the surface, comparable to bone in vivo. At the nanoscale-to-microscale, again like bone, OmGFP66 trabeculae exhibit a 3D crossfibrillar mineral tessellation pattern. We also fit OmGFP66 trabecular morphology patterning and mineralization to the Gray-Scott model of oscillating reaction-diffusion patterns to describe symmetry breaking that initiates and facilitates mineralization through the combined dynamics of diffusing mineral ions and inhibitors. Together, these findings establish OmGFP66 cell cultures as a powerful in vitro bone model for studying osteocyte differentiation, matrix mineralization, and pattern formation.