Cell behavior of human mesenchymal stromal cells in response to silica/collagen based xerogels and calcium deficient culture conditions

Herein, we aim to elucidate osteogenic effects of two silica-based xerogels with different degrees of bioactivity on human bone-derived mesenchymal stromal cells by means of scanning electron microscopy, quantitative PCR enhanced osteogenic effects and the formation of an extracellular matrix which could be ascribed to the sample with lower bioactivity. Given the high levels of bioactivity, the cells revealed remarkable sensitivity to extremely low calcium levels of the media. Therefore, additional experiments were performed to elucidate cell behavior under calcium deficient conditions. The results refer to capacity of the bone-derived stromal cells to overcome calcium deficiency even though proliferation, migration and osteogenic differentiation capabilities were diminished. One reason for the differences of the cellular response (on tissue culture plates versus xerogels) to calcium deficiency seems to be the positive effect of silica. The silica could be detected intracellularly as shown by time of flight-secondary ion mass spectrometry after cultivation of primary cells for 21 days on the surfaces of the xerogels. Thus, the present findings refer to different osteogenic differentiation potentials of the xerogels according to the different degrees of bioactivity, and to the role of silica as a stimulator of osteogenesis. Finally, the observed pattern of connexin-based hemichannel gating supports the assumption that connexin 43 is a key factor for calcium-mediated osteogenesis in bone-derived mesenchymal stromal cells.