At present, it is well known that populations of human bone marrow stromal cells (HBMSCs) can differentiate into osteoblasts and produce bone. However, the amount of cells with osteogenic potential that is ultimately obtained will still be dependent on both patient physiological status and culture system. In addition, to use a cell therapy approach in orthopedics, large cell numbers will be required and, as a result, knowledge of the factors affecting the growth kinetics of these cells is needed. In the present study we investigated the effect of dexamethasone stimulation on the in vivo osteogenic potential of HBMSCs. After a proliferation step, the cells were seeded and cultured on porous calcium phosphate scaffolds for 1 week, and then subcutaneously implanted in nude mice for 6 weeks, in order to evaluate their in vivo bone-forming ability. Furthermore, the effect of donor age on the proliferation rate of the cultures and their ability to induce in vivo bone formation was studied. In 67% of the assayed patients (8 of 12), the presence of dexamethasone in culture was not required to obtain in vivo bone tissue formation. However, in cultures without bone-forming ability or with a low degree of osteogenesis, dexamethasone increased the bone-forming capacity of the cells. During cellular proliferation, a significant age-related decrease was observed in the growth rate of cells from donors older than 50 years as compared with younger donors. With regard to the effect of donor age on in vivo bone formation, HBMSCs from several donors in all age groups proved to possess in vivo osteogenic potential, indicating that the use of cell therapy in the repair of bone defects can be applicable irrespective of patient age. However, the increase in donor age significantly decreased the frequency of cases in which bone formation was observed.

The use of cell therapies in bone reconstruction has been the subject of extensive research. It is known that human bone marrow stromal cell (HBMSC) cultures contain a population of progenitor cells capable of differentiation towards the osteogenic lineage. In the present study, the correlation between the in vitro osteogenic potential of HBMSC cultures and their capacity to form bone in vivo was investigated. HBMSC cultures were established from 14 different donors. Fourth passage cells were examined for the expression of alkaline phosphatase (ALP), procollagen I (PCI) and osteopontin (OP), through flow cytometry and the effect of the osteogenic differentiation factor dexamethasone (Dex) on this expression was evaluated. In addition, the capacity of the cultures to induce in vivo bone formation was analysed by culturing the cells on porous hydroxyapatite (HA) scaffolds followed by subcutaneous implantation of these constructs in nude mice. Results showed expression of PCI, OP and ALP in all cultures, irrespective of the presence of Dex in the culture medium. Dex failed to have a significant effect on the expression of PCI and OP but it induced a consistent increase in the relative amount of cells expressing ALP. Nevertheless, although in vitro testing clearly indicated osteogenic potential in all cultures, HBMSC from six of the 14 tested donors did not form bone in vivo. The results, therefore, demonstrate that neither the expression of PCI, OP and ALP nor the absolute increase in Dex-stimulated ALP expression can as yet be used as predictive markers for in vivo bone formation by HBMSC. However, preliminary data indicate that not the absolute, but the relative increase in the percentage of ALP expressing cells caused by Dex stimulation may be related to the ability of the HBMSC to form bone.

Bone tissue engineering has the potential to provide us with an autologous bone substitute. Despite extensive research to optimize the technique, little is known about the survival and function of the cells after implantation. To monitor the cells, in vivo labeling is the method of choice. In this study we investigated the use of the fluorescent membrane marker chloromethyl-benzamidodialkylcarbocyanine (CM-Dil) to label cells used in bone tissue engineering. When applying label concentrations up to 50 microM, cells could be labeled efficiently without negative effects on cell vitality, proliferation, or bone-forming capacity. Porous hydroxyapatite scaffolds were seeded with labeled cells, and up to 6 weeks after implantation in nude mice cells could be traced inside tissue-engineered bone. However, contrary to other reports concerning intramembranous labels, transfer of the label from labeled to unlabeled cells was detected. Transfer occurred both in vitro and in vivo between vital cells and between dead and living cells. To determine when in vivo label transfer happened, devitalized, labeled constructs were implanted for various time periods in nude mice. The presence of vital labeled cells inside these constructs, when evaluated at different implantation periods, indicated transfer of the label. Transfer occurred at 7 days postimplantation when 40 microM label was applied, whereas 10 microM labeled constructs showed transfer 10 days after implantation. These findings indicate that CM-Dil label is useful for in vivo tracing of cells for follow-up periods up to 10 days. This makes the label particularly useful for cell survival studies in tissue-engineered implants.

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