1,25-Dihydroxyvitamin D3 causes an increase in the number of osteoclastlike cells in cat bone marrow cultures.

The direct effect of 1,25(OH)2D3 upon osteoclast formation from precursor cells is still unknown. In the present experiments we have tested the effects of 1,25(OH)2D3 on the generation of osteoclastlike cells in cat bone marrow cultures. These cultures contain proliferating nonattached mononuclear cells and precursor cells that subsequently attach to the culture flask surface and then fuse to form multinucleated osteoclastlike cells. After 7 days of culture we separated the nonattached precursor cells from the attached cells and studied the effects of 1,25(OH)2D3 (10(-10) M - 10(-8) M) on multinucleated cell formation in these two cell populations. In cultures derived from the nonattached precursor cells, 7 days of treatment with 1,25(OH)2D3 (10(-8) M) resulted in a 180% increase in the number of attached mononuclear cells and a 90% increase in the number of nuclei contained within multinucleated cells. These effects were dose-dependent. 1,25(OH)2D3 did not have a consistent effect on the number of nonattached precursor cells. In cultures derived from attached cells, 7 days of treatment with 1,25(OH)2D3 (10(-8) M) induced a 50% increase in the number of mononuclear attached cells and a 40% increase in the number of nuclei within polykaryons. The most likely explanation for these results is that 1,25(OH)2D3 promotes the differentiation and subsequent adhesion of nonattached precursor cells, stimulates proliferation of attached mononuclear precursor cells, and possibly stimulates fusion of these attached precursor cells.