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NORMAL SKELETON: CARTILAGE AND BONE

The skeleton is a dynamic organ composed of cartilage and bone. It functions as a rigid framework for the protection of soft tissue organs and as an attachment site for muscles and tendons for movement, enabling physical activity. In addition, it stores bone marrow and serves as a reservoir for calcium, phosphate, and other ions.

Osteoblasts are bone-forming cells derived from stromal mesenchymal stem cells that secrete collagen types I and III and noncollagenous proteins that together form osteoid or bone matrix into which calcium and phosphate are deposited. Osteoblasts have a life span of about 3 months. Osteoclasts are bone-resorbing cells that are derived from hematopoietic precursor cells that become multinucleated giant cells. These cells attach to bone and form a tightly sealed space (lacuna) between the cell’s basal ruffled membrane and the bone’s outer surface (Fig. 536-1). Osteoclasts reabsorb bone by pumping hydrogen and chloride ions into the lacunae that dissolve hydroxyapatite and enzymes (eg, cathepsins, matrix metalloproteinases) that degrade the protein matrix of bone. Acidification of the subosteoclastic space requires the action of carbonic anhydrase II and proton and chloride channels. The dissolved minerals and degraded proteins are absorbed by the osteoclast and extruded into the circulation. When local bone resorption is complete, osteoclasts leave and osteoblasts enter the site.

Figure 536-1

Mature osteoclasts form a ruffled border that enables the border to adhere to bone surface through αvβ3 integrin receptors. A subosteoclast lacuna is formed by the dissolution of bone mineral and the resorption of organic bone matrix by osteoclast secreted hydrochloric acid and cathepsin K, respectively. Subsequently, osteoblasts are attracted to this pit, and new bone is formed in the continuous process of bone remodeling.

Bone modeling occurs during intrauterine development and postnatal growth and determines the shape and size of over 200 bones. It is accomplished by osteoblasts and osteoclasts acting independently of one another, and it is not dependent on prior bone resorption. Bone remodeling is the process of reforming new bone in place of reabsorbed bone that occurs throughout life. It sequentially links the processes of bone resorption and formation. The bone remodeling unit is composed of both osteoclasts and osteoblasts and is a continuous process in which old bone is replaced by new bone. Bone remodeling occurs in both growing and mature bone either at random locations or at sites of mechanical stress. Osteoclasts initially arrive at a bone remodeling site to initiate bone resorption, which signals osteoblasts to converge at the site for bone formation.

The skeleton is comprised of flat (eg, cranium, scapula, pelvis) and long (eg, humerus, femur, phalanges) bones. The outer surface of bone is enclosed in periosteum (a fibrous network of osteoblasts that synthesizes peripheral compact bone); endosteum lines its inner surface. The central portion of long bones ...

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