The effects of parathyroid hormone or 1,25-dihydroxyvitamin D3 on monocyte-osteoclast fusion.

3H-thymidine-labeled blood monocytes were cultured with osteoclasts in the presence or absence of parathyroid hormone or 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in order to evaluate (1) the percentage of monocytes capable of fusing with osteoclasts, (2) if parathyroid hormone or 1,25(OH)2D3 influences the contribution of blood monocytes to osteoclast nuclear turnover. We found that within 24 hours of culture, about 8% of blood monocytes fuse with osteoclasts regardless of the presence of parathyroid hormone (PTH) or 1,25(OH)2D3. On the other hand, formation of non-osteoclastic giant cells by fusion of monocytes is enhanced by 5 x 10(-9) M 1,25(OH)2D3 but only in the presence of the bone resorptive cells.