Immunomagnetic isolation of osteoprogenitors from human bone marrow stroma.

We report here the efficient and specific isolation of human bone marrow osteoprogenitors using magnetic beads coated with a mouse mAb (STRO-1) that recognizes a stromal cell surface protein. According to their pattern of differentiation, osteoprogenitors accounted for 100% of the STRO-1-positive cell population. Upon long term culture, osteoprogenitors differentiated down the osteoblastic lineage as evidenced by their in vitro morphology, their increased expression of alkaline phosphatase, their production of osteocalcin, and the deposition of a mineralized matrix. Upon differentiation, the cells first expressed alkaline phosphatase on their surface before they began proliferating as phenotypically recognizable osteoblasts. This observation conflicts with previous studies that have characterized human osteoprogenitors as highly proliferative cells that form colonies before differentiating into osteoblasts. The ability to isolate and cultivate pure populations of primary human osteoprogenitors will substantially advance investigations on osteogenesis and bone remodeling.