Schematic presentation of the cell types involved in bone remodeling. Bone-forming osteoblasts (left side) derive from mesenchymal progenitor cells and are arranged in large groups of cells simultaneously producing the bone matrix. This matrix is first non-mineralized (osteoid), before hydroxyapatite crystals get incorporated into the collagen fibrils to form mineralized bone. Some osteoblasts become embedded and differentiate into osteocytes, thereby forming a cellular network within the mineralized bone matrix. Bone-resorbing osteoclasts (right side) are derived from hematopoietic progenitors by cellular fusion. They are large multinucleated cells migrating along the bone surface to resorb it by two major mechanisms, i.e. extracellular acidification and secretion of matrix-degrading enzymes. The most important regulators of osteoclastogenesis (Rankl and Opg) and bone formation (Lrp5 and Sclerostin/Scl) are described in the text.