A microphysiological model of bone development and regeneration.

Endochondral ossification (EO) is an essential biological process than underpins how human bones develop, grow, and heal in the event of a fracture. So much is unknown about this process, thus clinical manifestations of dysregulated EO cannot be adequately treated. This can be partially attributed to the absence of predictivemodels of musculoskeletal tissue development and healing, which are integral to the development and preclinical evaluation of novel therapeutics. Microphysiological systems, or organ-on-chip devices, are advancedmodels designed for improved biological relevance compared to traditionalculture models. Here we develop a microphysiological model of vascular invasion into developing/regenerating bone, thereby mimicking the process of EO. This is achieved by integrating endothelial cells and organoids mimicking different stages of endochondral bone development within a microfluidic chip. This microphysiological model is able to recreate key events in EO, such as the changing angiogenic profile of a maturing cartilage analogue, and vascular induced expression of the pluripotent transcription factors SOX2 and OCT4 in the cartilage analogue. This system represents an advancedplatform to further EO research, and may also serve as a modular unit to monitor drug responses on such processes as part of a multi-organ system.