Isolation of human osteoclasts formed in vitro: hormonal effects on the bone-resorbing activity of human osteoclasts.

Osteoclasts are multinucleated cells that carry out bone resorption. Analysis of the direct effect of hormones on the bone-resorbing activity of human osteoclasts has been limited by difficulties in isolating these cells from the human skeleton. In this study, human osteoclasts formed from cultures of peripheral blood mononuclear precursors (PBMCs) on a Type-I collagen gel were isolated by collagenase treatment for investigating their resorptive activity. PBMCs were cultured in the presence of M-CSF, soluble RANKL, dexamethasone, and 1,25(OH)2D3. The isolated multinucleated cells expressed the osteoclast markers, TRAP, VNR, cathepsin K, calcitonin receptors and were capable of extensive lacunar resorption. Calcitonin inhibited the motility and resorptive activity of osteoclasts. RANKL significantly stimulated osteoclast resorption, but 1,25(OH)2D3, PTH, and OPG did not. These findings indicate that calcitonin and RANKL act directly on human osteoclasts to inhibit and stimulate osteoclast bone-resorbing activity, respectively, and that PTH, 1,25(OH)2D3, and OPG are more likely to influence osteoclast activity indirectly. This technique of human osteoclast isolation should permit the effects of cellular and hormonal/humoral factors on the bone-resorbing activity of mature human osteoclasts to be assessed independently of any effect such factors have on osteoclast formation. It should also make it possible to examine directly the resorptive activity and other characteristics of osteoclasts in specific bone disorders such as Paget's disease.