Chapter 12 The locomotor system/Fig 12.03.jpg

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Osteoclast precursors (monocyte/macrophage lineage cells) are stimulated to become osteoclasts by receptor activator of nuclear factor κB ligand (RANKL). This molecule is upregulated on the surface of marrow stromal cells by vitamin D (Vit D), parathyroid hormone (PTH) and other factors. Overactivity of RANK L is normally prevented by the soluble protein osteoprotegerin (OPG). In the process of removing bone, osteoclasts produce cytokines such as transforming growth factor β (TGFβ) which both stimulate osteoblasts to produce new bone and inhibit osteoclasts. Unwanted osteoclasts undergo apoptosis under the influence of oestrogens and bisphosphonates.