Apatite biomaterials have potential not only as cell vehicles for engineering bone tissue but also as regulators of calcium (Ca) concentration in situ for controlling osteoblast functions, for example, osteogenic differentiation and fate management of hematopoietic stem cells (HSCs). To design apatite materials having optimal chemical properties for the latter purpose, more detailed investigations into what effect Ca concentrations have on osteoblast functions is crucial. In this study, osteoblasts were cultured at different Ca concentrations, and the temporal alterations in osteogenic differentiation and HSC niche-related protein (angiopoietin-1, 2 [Ang1, 2]) expression were investigated. The different Ca concentrations (1.8-50mmol/L) in the cell culture medium had no effect on the proliferation of osteoblasts, but did on the cell morphology. The higher Ca concentrations (<6mmol/L) enhanced the mineralization as well as Ang1 expression. In addition, Ang1 expression in osteoblasts showed higher correlation with expression of connexin43, the major marker of cell-cell interactions, whereas Ang2 related to integrin β1, the major marker of cell-matrix interactions. Thus, the local Ca concentration regulates cell morphology through the cell-cell or cell-matrix interactions, leading to the alteration of Ang1 expression in osteoblasts. Since these changes triggered by Ca are concerned with the osteogenic differentiation or reproduction of HSCs niche microenvironment, the results obtained in this study might be useful for designing apatite materials with optimal chemical properties.