Role of extracellular microenvironment on regenerative bone tissue capacity

In the present study, I aimed to investigate how acidic extracellular pH can affect MSCs behavior in terms of proliferation, osteogenic potential, activation of autophagy, and release of inflammatory molecules, and I looked for potential osteo- and chondro-progenitor cell populations activated during the repair of bone injury in the periosteum of a mouse model in vivo. I cultured human MSCs in acidic medium, and I observed a decrease in their proliferation and mineralization potential, and an increase in the release of inflammatory cytokines. The survival mechanism of autophagy was found to be activated by MSCs in both acidic and physiological conditions. These observations have allowed to further characterize human MSCs biology in a non-physiological context, typical of patients with an altered acid-base balance. An altered extracellular microenvironment is also typical of the early stages of the fracture healing process, and the identification of cells participating in bone repair is necessary to eventually improve therapeutic approaches of regenerative medicine. Through the use of a transgenic mouse model of lineage tracing, I observed that αSMA+ periosteal cells identify osteo– and chondro-progenitors activated after fracture, although only a low percentage of them represents long-term progenitors. Furthermore, other markers previously characterized in the bone marrow are suitable for identification of progenitor cell populations in the periosteum. Further studies are needed, especially aimed to better investigate different surface markers combinations, and the ability of these progenitor cells to form bone in vivo. Through the entire work, I highlighted the importance of the extracellular microenvironment in the modulation of MSCs behavior, and I contributed to the identification of osteo- and chondro-progenitors involved in bone repair, aiming to improve approaches of regenerative medicine, especially in patients in which the healing process is impaired by pathological conditions.