Bone turnover

Bone is a dynamic tissue, which undergoes continuous remodelling throughout life. This process requires deposition of new bone, a process which is controlled by the bone-forming osteoblasts, as well as the degradation of bone, exerted by the bone-resorbing osteoclasts. Osteoblast differentiate from mesenchymal precursors and are metabolically active cells, which produce bone matrix proteins such as collagen and osteocalcin, as well as alkaline phosphatase. These proteins can be used to measure osteoblast function in humans. Osteoclasts are haematopoietic cells of the monocyte linage degrading the bone by enzymes cleaving collagen such as metalloproteinases and cathepsins. Collagen cleavage products are indicators for bone resorption in the body. Osteoblast-osteoclast actions are controlled by a fine network of bone cells, osteocytes, which are responsible for sensing bone damage and directing bone remodelling to the right anatomical locations. Finally, mineralization of the newly formed bone is governed by vitamin D and parathyroid hormones, the key regulators of calcium homeostasis in our body.