Osteoporosis—Bone Remodeling and Animal Models

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Osteoporosis is a very common skeletal disorder characterized by reduced bone mass and altered trabecular microarchitecture that leads to bone fragility and fractures. Such disease is due to alterations of the remodeling process that occurs in the basic multicellular units that are transitory cellular complexes including an osteoclastic phase (osteoclast activation and resorption of microscopic portions of bone), a reversion phase (osteoclast replacement by so-called postosteoclastic cells), and an osteoblastic phase (osteoblastic reconstruction of the resorbed bone matrix till the initial volume is regained). Bone remodeling is regulated by a number of systemic and local factors; among the former, besides physical activity and mechanical stresses, a primary role is played by hormones such as parathyroid hormone, vitamin D metabolites, estrogens, calcitonin, and glucocorticoids; among the latter, several growth factors (macrophage colony-stimulating factor, transforming growth factor β, platelet-derived growth factor, fibroblast growth factor 1, bone morphogenetic protein, and insulin-like growth factor 1), as well as the osteoprotegerin-receptor activator of nuclear factor-κ B ligand system and the sclerostin, play a primary function. The remodeling phases can be evaluated by static and dynamic histomorphometry. Their abnormalities may lead to several osteopathies, the most common of which is osteoporosis (above all senile and postmenopausal), a rather elusive disease chiefly due to its slow development. The use of animal models in its study is emphasized.