Osteoporosis and Osteopetrosis

Abstract

Osteoporosis is the most prevalent among bone diseases. It is manifested by low bone mass and architectural abnormalities that lead to bone fragility and elevated fracture risk. Mechanistically, osteoporosis occurs as a result of decreased bone formation (by osteoblasts) and increased bone resorption (by osteoclasts). Thus, a tip in the balance favouring bone resorption results in osteoporosis e.g. porous bone. A wide range of factors contribute to the development of osteoporosis including genetic predisposition, ageing, reduced physical activity, diet, hormonal changes and more. Osteopetrosis, however, reflects the state of dense yet fragile bones due to dysfunctional or lack of osteoclasts.

Keywords: osteoporosis; osteopetrosis; calcium; vitamin D; osteoclast; BMD

Figure 1.

Mechanisms of cytokine regulation of osteoclasts. Under basal conditions, factors such as receptor activator of NF‐κB ligand (RANKL), osteoprotegerin (OPG) and M‐CSF are secreted by stromal and osteoblast cells and support differentiation of macrophage (also known as osteoclast precursors; OCPs) into bone resorbing osteoclasts. Other cytokines, such as TNF, IL‐1 and IL‐6, are also secreted by stromal and osteoblast cells, and modulate the process of osteoclastogenesis. Conditions of oestrogen deficiency lead to activation of T lymphocytes and to secretion of high levels of RANKL and TNF by these cells. This process exacerbates osteoclastogenesis leading to enhanced bone loss.

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Abu‐Amer, Yousef, Darwech, Isra, and Otero, Jesse(Sep 2007) Osteoporosis and Osteopetrosis. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0006017]