Control Of Bone Turnover	

	The activity of osteoclasts and osteoblasts is under the control of systemic hormones and local factors. Systemic hormones that control osteoclastic activity are PTH, calcitonin, and 1,25-dihydroxyvitamin D. PTH and 1,25-dihydroxyvitamin D stimulate osteoclasts to resorb bone, whereas calcitonin inhibits the activity of osteoclasts. Local factors produced by bone cells and by immune cells in the marrow cavity are also capable of stimulating osteoclasts; these include cytokines such as interleukin-1 (IL-1), tumour necrosis factor (TNF), and interleukin-6 (IL-6). The activity of osteoblasts is also regulated by factors that stimulate the activity of osteoclasts.