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Primary osteogenic cells have been known to reside within the CD45-CD31-Ter119-Sca-1- cell fraction, particularly in the CD51+ subpopulation. However, detailed determination of the frequency of osteogenic cells within this Sca-1- cell population remains yet to be determined. In addition, it is not clear that other cell surface markers can be used to further sub-fractionate this Sca-1-CD51+ osteogenic cell population and to define their developmental stages. In this report, both Sca-1-CD24med and Sca-1- CD24-/lo cells have been shown to be two small subsets of the Sca-1-CD51+ cell fraction. These two cell fractions show subtle difference in the expression level of osteogenic marker genes such as Osx and Opn, and in vitro proliferate rate. All these observations suggest that they may be at different developmental stages of osteogenesis.
The Sca-1-CD24med cell fraction is enriched for the more mature osteolineage cells than the Sca-1-CD24-/lo counterpart. In contrast, most of the Sca-1-CD24hi and Sca-1+CD24-/lo cells do not contain CFU-ALP nor express osteogenic gene markers. The high proliferation ability and osteo-adipogenic differentiation potentials confirm that the Sca-1+CD24-/lo cells are the multipotential mesenchymal stromal cells. The determination of individual stromal cell subpopulations will lead to a better understanding in the hierarchical organization of these osteolineage cells.