Bone Remodeling

Bone remodeling (Figure 2) continues throughout adult life and is necessary for the maintenance of normal bone structure and bone quality. Maintenance of bone mass requires that bone formation and resorption should be balanced. Bone remodeling occurs in focal or discrete packets by groups of cells known as bone multicellular units (BMUs).7 In this process, both bone formation and resorption occur at the same place so that there is no change in the shape of the

Osteobla

Osteoclasts

Bone

New bone

Figure 2 Cancellous bone remodeling. (a) Normal bone is remodeled by teams of osteoclasts and osteoblasts, forming a bone multicellular unit that progresses across the bone surface with osteoclasts eroding a pit, which is subsequently refilled by osteoblasts. (b) In osteoporosis, bone removed by each cycle is greater than that replaced, so that the trabeculae become thinner and can be perforated and lost. (c) Antiresorptive treatment inhibits osteoclasts so that numbers and size of pits are reduced, resulting in a coupled reduction in bone formation. Bone volume is increased as existing holes are filled and then maintained through the reduction in remodeling cycles. (d) Anabolic factors like teriparatide induce osteoblasts to form more bone during each remodeling cycle. Bone volume increases until osteoclast activity increases, apparently to compensate for the increased bone formation. Teriparatide action depends on the presence of remodeling cycles, as concurrent treatment with an antiresorptive reduces its effects.

bone. After a certain amount of bone is removed as a result of osteoclastic resorption and the osteoclasts have moved away from the site, a reversal phase takes place, in which a cement line is laid down. Osteoblasts then synthesize matrix, which becomes mineralized. The BMU remodeling sequence normally takes about 3 months to produce a bone structural unit (BSU), the unit of bone tissue formed by one BMU. The constant remodeling is important in three ways. Firstly, bone, like other structural materials that undergo repetitive cyclical loadings, is subject to fatigue. After a number of loading cycles, tiny cracks may form, which, if not repaired in a timely fashion, will accumulate and eventually lead to structural failures. Bone remodeling will replace bone containing cracks and prevent structural failure. Secondly, bone remodeling is required to adapt bone material properties to the mechanical demands that are placed on the skeleton. Finally, bone remodeling plays a critical role in regulating calcium homeostasis, which is critical for life. Bone is a major reservoir for calcium. When serum calcium becomes low and there is insufficient dietary calcium, calcium will be released from bone by osteoclasts to meet the demand.

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