Development of an osteoblast/osteoclast co-culture derived by human bone marrow stromal cells and human monocytes for biomaterials testing.

The communication of bone-forming osteoblasts and bone-resorbing osteoclasts is a fundamental requirement for balanced bone remodelling. For biomaterial research, development of in vitro models is necessary to investigate this communication. In the present study human bone marrow stromal cells and human monocytes were cultivated in order to differentiate into osteoblasts and osteoclasts, respectively. Finally, a cultivation regime was identified which firstly induces the differentiation of the human bone marrow stromal cells followed by the induction of osteoclastogenesis through the osteoblasts formed--without the external addition of the factors RANKL and M-CSF. As a feedback on osteoblasts enhanced gene expression of BSP II was detected for modifications which facilitated the formation of large multinuclear osteoclasts. Phenotype characterization was performed by biochemical methods (DNA, LDH, ALP, TRAP 5b), gene expression analysis (ALP, BSP II, RANKL, IL-6, VTNR, CTSK, TRAP, OSCAR, CALCR) as well as light microscopy, confocal laser scanning microscopy, and scanning electron microscopy. After establishing this model on polystyrene, similar positive results were obtained for cultivation on a relevant bone substitution material--a composite xerogel of silica, collagen, and calcium phosphate.