The aim of the study was to establish an in vitro fracture hematoma (FH) model, which mimics the in vivo situation of the human fracture gap in order to assess drug efficacy and effectiveness for the treatment of fracture healing disorders. Therefore, human peripheral blood and mesenchymal stromal cells (MSCs) were coagulated to produce in vitro FH models, incubated in osteogenic medium under normoxia/hypoxia, and analyzed for cell composition, gene expression and cytokine/chemokine secretion.

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... o evaluate the model, we studied the impact of dexamethasone (impairing fracture healing) and deferoxamine (promoting fracture healing). Under hypoxic conditions, MSCs represented the predominant cell population, while the frequencies of leukocytes decreased. Marker gene expression of osteogenesis, angiogenesis, inflammation, migration and hypoxic adaptation increased significantly over time and compared to normoxia while cytokine/chemokine secretion remained unchanged. Finally, dexamethasone favored the frequency of immune cells compared to MSCs, suppressed osteogenic and pro-angiogenic gene expression and enhanced the secretion of inflammatory cytokines. Conversely, deferoxamine favored the frequency of MSCs over that of immune cells and enhanced the expression of the osteogenic marker RUNX2 and markers of the hypoxic adaptation. In summary, we demonstrate that hypoxia is an important factor for in vitro modeling the initial phase of fracture healing, that both fracture-healing disrupting and promoting substances can influence the in vitro model comparable to the in vivo situation. Therefore, we conclude that our model is able to mimic in part the human FH and to reduce the number of animal experiments in early preclinical studies.