Osteoclasts are terminally differentiated cells which play a key role in bone resorption. Due to the low number of mammalian osteoclasts and the difficulty to isolate them from bone tissue, and from other cells, their characterization has been limited primarily to immunohistochemistry or anatomical and physiological measurements on single cells.
Several attempts have been made, with limited success, to identify, isolate and establish in culture, cells capable of differentiating into osteoclasts from either bone marrow (Billecocq et al., 1990 Proc. Nat. Acad. Sci. USA 87:6470-6474; Prallet et al., 1992, J. Bone Min. Res. 7:405-414; and Chambers et al., 1993, Proc. Nat. Acad. Sci. USA 90:5578-5582) or leukemic and promyelocytic cell lines (Yoneda et al., 1991, Endocrinology 129:683-689; Gattei et al., 1992 J. Cell Biol. 116:437-447). Recently, cells obtained from human giant cell tumors of bone were reported to form osteoclasts in culture (Grano et al., 1994 J. Bone Min. Res. 9:1013-1020; Grano, et al., 1994 Exp. Cell Res. 212:209-218).
To obtain enriched osteoclast preparations, Akatsu et al., 1992, J. Bone Miner. Res. 7:1297-1306; cultured the osteoblastic and bone marrow cells on collagen gel-coated dishes, and released the cells from the collagen matrix using collagenase. However, the yield of osteoclastic cells was quite low. Recently, Shioi et al., 1994, Calcif. Tissue Int. 55:387-394; using a similar co-culture system reported the enrichment of generated osteoclasts by treating the cultured cells with bacterial collagenase to remove the stromal supporting cells. By this procedure they obtained 60% pure tartrate resistant acid phosphate positive (TRAP.sup.+) cells, however the osteoclasts were not available except as attached cells to either dishes or bone slices.
Oursler et al., 1991, J. Bone Min. Res. 6:375-385 cultured avian osteoclasts and further purified the osteoclasts by density gradient centrifugation. This method yielded an enriched population of TRAP.sup.+ cells capable of bone resorption.
To date, however, the only mammalian osteoclasts obtained in high yields (300,000 per rabbit) and purity (98%) are rabbit osteoclasts (Tezuka et al., 1992, Biochem. Biophys. Res. Comm. 186: 911-917). These cells, however were attached to the plastic culture dishes and cannot be removed.