Simulation of the bone remodelling process by the co-culture of human monocytes and mesenchymal stem cells

Introduction: In the field of orthopaedics and traumatology, the improvement of implant materials plays a crucial role for a better treatment of bone defects. Newly developed biodegradable materials have the potential to provide enhanced mechanical properties and reduce the need for implant removal surgery. The initial evaluation of newly designed materials in vitro to predict the outcome in vivo is strongly desired. Therefore, to approach the in vivo situation we established co-culture conditions for primary human mesenchymal stem cells (hMSCs) and monocytes (MCs) to address the bone remodelling process. Our aim of this study was the evaluation of the osteoclastogenesis of monocytes in consequence to the interaction and cross talk with hMSCs without any differentiation factors by media supplementation. Materials and Methods: Human monocytes were isolated of buffy coats and were characterized by FACS analysis, histochemical staining and differentiation studies. For co-culture experiments, hMSCs were seeded either on tissue culture plates (TCPS) or on bovine cortical bone slices. Two set ups were considered: proliferative media and the induction of the osteogenic differentiation for 14 days. Subsequently, monocytes were seeded on top of the pre-cultured hMSCs and were evaluated after another 14 days for an osteoclastic phenotype by immunofluorescence staining and REM analysis. Results and Discussion / References: MCs were successfully differentiated to osteoclasts by media supplementation including RANKL and M-CSF. Co-culture of hMSCs and MCs led to the expression of an osteoclastic phenotype after 14 days. Cathepsin K, VNR and Actin/Vinculin were expressed and analyzed by immunofluorescence staining revealing multinucleated cells within the co-culture set up. The co-culture of hMSCs and hMCs lead to an osteoclastic phenotype after 14 days. The interaction of both cell types is therefore sufficient to generate osteoclast like cells. The bone remodelling process can be simulated in vitro by using monocytes and mesenchymal stem cells. However, the activity of the osteoclasts must be further investigated e.g. by the detection of resorption pits in order to test efficiently newly designed materials. The coculture could further be transferred into a three-dimensional setting to consider the influence of a multilayer culture. This work was supported by the Fraunhofer Internal Programs under Grant No. MAVO 824682 (DegraLast).