Bone remodeling

Bone remodeling (bone metabolism) is the process of removal of old bone tissue and formation of a new one.

Functions of bone remodeling

– Renewal of bone tissue to prevent accumulation of old bone with multiple micro-damages.

– Repair of small injuries : In response to small fractures, the damaged tissue is removed and new bone matrix is formed to replace it.

– Adjustment of bone architecture to meet changing mechanical needs: new bone matrix is deposited where needed, old bone tissue is removed elsewhere.

– Role in maintenance of calcium homeostasis of blood plasma.

Bone remodeling process

Bone remodeling involves the removal of bone tissue by osteoclasts followed by the formation of new bone matrix deposited and mineralized by osteoblasts (Fig. 1).

Fig. 1: Bone remodeling cycle: resorption, reversal and formation. See text for details. Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.

Bone resorption : removal of old bone tissue by osteoclasts. Osteoclasts are bone-dissolving cells. They derive from hematopoietic stem cells and are product of fusion of several cell precursors. Because of this, osteoclasts are unusually large and multinuclear. Osteoclast resembles an octopus crawling on the bone surface due to the presence of a ruffled border – multiple infoldings of plasma membrane – served to increase its surface area. They also have a foamy appearance as they contain lots of lysosomes.

Reversal: Mononuclear cells (monocytes, macrophages) clean up the debris on bone surface.

New bone formation : Recruitment of pre-osteoblasts to the surface, these mature to become osteoblasts. Osteoblasts are bone – forming cells. They synthesize the organic matter of bone matrix (osteoid). Osteoid mineralized and becomes new bone.

Osteocytes are former osteoblasts that have been trapped in the bone matrix they deposited. They can no longer synthesize bone matrix and instead serve as mechanical sensors. When they detect a strain in a bone, they communicate with the osteoblasts on the bone surface. These latter would deposit bone matrix where needed in response.

Bone modeling versus Bone remodeling

Bone modeling is the process in which bones change their overall shape to adapt to physiological and mechanical changes. In bone modeling, the two sub-processes of bone resorption and bone formation are less coordinated, i.e. bone resorption may happen without subsequent new bone formation and vice versa: new bone formation may happen without old bone being removed. Bone modeling is more frequent in growing children while bone remodeling is more frequent in adults.

Disorders of bone metabolism

Bone resorption and formation must be in balance to maintain healthy bone metabolism. When bone resorption overtakes new bone formation, bone loss – osteoporosis – may result (Fig. 2). Osteoporosis (or porous bone) is very common in older adults, especially in post-menopause women. This condition usually affects all the bones in the body.

Fig. 2: Bone loss in osteoporosis (right panel) compared to normal bone tissue (left). Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.

Another disorder of bone remodeling is Paget’s disease of bone (osteitis deformans). This condition is characterized by larger and denser but weaker bones. Paget’s disease typically is localized to just a few bones. The pelvis, lower spine and long bones of the legs are the most commonly affected.