Paracrine effects of living human bone particles on the osteogenic differentiation of mesenchymal stem cells.

Bone autografting remains the clinical model of choice for resolving problematic fractures. The precise mechanisms through which the autograft promotes bone healing are unknown. The present study examined the hypothesis that cells within the autograft secrete osteogenic factors promoting the differentiation of mesenchymal stem cells (MSCs) into osteoblasts. Particles of human bone ("chips") were recovered at the time of joint replacement surgery and placed in culture. Then, conditioned media were added to cultures of human, adipose-derived MSCs under both basal and osteogenic conditions. Contrary to expectation, medium conditioned by bone chips reduced the expression of alkaline phosphatase and strongly inhibited mineral deposition by MSCs cultured in osteogenic medium. Real time PCR revealed the inhibition of collagen type I alpha 1 chain (Col1A1) and osteopontin (OPN) expression. These data indicated that the factors secreted by bone chips inhibited the osteogenic differentiation of MSCs. However, in late cultures, bone morphogenetic protein-2 (BMP-2) expression was stimulated, suggesting the possibility of a delayed, secondary osteogenic effect.