Replicative aging and gene expression in long-term cultures of human bone marrow stromal cells

Bone marrow stromal cells (BMSCs) can be easily isolated from adult marrow and contain a population of pluripotent progenitors that can give rise to different mesenchymal lineages both in vitro and in vivo. These properties make BMSCs an attractive target for cell-based therapeutic strategies for a variety of disorders. However, because of their low frequency in vivo, to obtain a sufficient number of cells for tissue engineering a step of extensive in vitro expansion is required, which could significantly alter BMSC properties. Therefore, effective therapeutic use of BMSCs requires the design of appropriate approaches for in vitro cell expansion. In this study we have investigated the biological effects of in vitro expansion on BMSC proliferative ability and on their spontaneous differentiation. Telomerase activity and telomere shortening kinetics were evaluated together with variations in osteogenic, chondrogenic, and adipogenic gene expression throughout the BMSC life span. In culture BMSCs never displayed telomerase activity and during in vitro expansion telomere length decreased. Furthermore, gene expression patterns spontaneously varied during expansion, indicating a progressive commitment of the population toward the osteogenic lineage. In conclusion, BMSCs in culture undergo progressive replicative aging and osteogenic differentiation. These observations are relevant to their successful use in clinics and should be considered when designing novel therapeutic strategies.