Osteoblast interactions within a biomimetic apatite microenvironment

Numerous reports have shown that accelerated apatites can mediate osteoblastic differentiation in vitro and bone formation in vivo. However, how cells interact within the apatite microenvironment remains largely unclear, despite the vast literature available today. In response, this study evaluates the in vitro interactions of a well-characterized osteoblast cell line (MC3T3-E1) with the apatite microenvironment. Specifically, cell attachment, spreading, and viability were evaluated in the presence and absence of serum proteins. Proteins were found to be critical in the mediation of cell-apatite interactions, as adherence of MC3T3-E1 cells to apatite surfaces without protein coatings resulted in significant levels of cell death within 24 h in serum-free media. In the absence of protein-apatite interaction, cell viability could be "rescued" upon treatment of MC3T3-E1 cells with inhibitors to phosphate (PO(4) (3-)) transport, suggesting that PO(4) (3-) uptake may play a role in viability. In contrast, rescue was not observed upon treatment with calcium (Ca(2+)) channel inhibitors. Interestingly, a rapid "pull-down" of extracellular Ca(2+) and PO(4) (3-) ions onto the apatite surface could be measured upon the incubation of apatites with α-MEM, suggesting that cells may be subject to changing levels of Ca(2+) and PO(4) (3-) within their microenvironment. Therefore, the biomimetic apatite surface may significantly alter the microenvironment of adherent osteoblasts and, as such, be capable of affecting both cell survival and differentiation.