Bone marrow stromal cells (BMSCs) are a promising cell source for bone tissue-engineering application due to their osteogenic differentiation potential. Effective growth and osteoblastic differentiation of BMSCs are required in promoting successful bone formation. In this study, the effect of collagen I nanospheres on the proliferation and osteogenic differentiation of rat BMSCs in vitro was investigated by the addition of collagen I nanospheres directly into the culture medium. The mitochondrial activity of rat BMSCs was measured by colorimetric assay: results showed that collagen I nanospheres promoted rat BMSC proliferation. Osteogenic differentiation of rat BMSCs was qualified via
measurements of alkaline phosphatase (ALP) activity and matrix mineralization. Our results demonstrated that there was a striking difference in ALP staining and von Kossa staining. Collagen I nanospheres induced an increase in the number of ALP-positive cells. Bone nodule formation occurred under cells treated with or without collagen I nanospheres, but the extent of mineralization was increased when collagen I nanospheres were present in cultures. These data support that collagen I nanospheres might be suitable for the cultivation of BMSCs for clinical applications. Moreover, the observations suggest that collagen I nanospheres might have a great potential for bone tissue engineering.