Bone form

Bone metabolism comprises the processes of bone formation and bone resorption, the key actions by which skeletal mass, structure and quality are accrued and maintained throughout life. In the mature skeleton, anabolic and catabolic actions are mostly balanced due to the tight regulation of the activity of bone forming ( osteoblast) and bone resorbing ( osteoclast) cells through circulating osteotropic hormones and locally active cytokines. 

Osteoblasts are the primary cells responsible for bone formation. They are derived from mesenchymal (stromal) cells that first differentiate into pre-osteoblasts and then into mature, bone matrix producing osteoblasts. Inactivated or resting osteoblasts become lining cells and thus a reservoir for bone forming cells to be activated at the next remodelling cycle. Osteoblasts trapped and embedded in the mineralised matrix are called osteocyts, and are important for many properties of living bone. 

Figure 7.2. Schematics of bone anatomy (a) the structure of a long bone demonstrating the distribution of the two different tissue structures, cortical and cancellous bone, and (b) the cells present in bone osteoblasts, bone-forming cells found on surfaces osteocytes, bone cells embedded in the mineralised matrix and osteoclasts, bone-removing cells.

Figure 7.5. Feedback diagram for skeletal mechanical regulation. When forces are applied to a whole bone, the stimulus that results is sensed by the bone cells in the tissue. The sensor cells then signal bone-forming and -removing cells to change the geometry and material properties of the bone.

Strontium ranelate is an oral agent possessing bone-forming and antiresorptive properties. Some data suggest significant reductions in vertebral fractures.46 However, the benefit in nonvertebral fractures is unclear. This agent has not yet been approved for use by the FDA. 

Sodium fluoride is an agent with bone-forming properties. Its use has fallen out of favor after data suggested an increased risk of fracture despite increases in bone mineral density. It is not currently available commercially in the United States. 

The family of connective-tissue cells includes fibroblasts, chondrocytes (cartilage cells), and osteoblasts (bone-forming cells). They are specialized to secrete extracellular proteins, particularly collagens, and mineral substances, which they use to build up the extracellular matrix (see p. 346). By contrast, osteoclasts dissolve bone matter again by secreting and collagenases (see p. 342). 

Bone - Tetracycline forms a stable calcium complex in any bone-forming tissue. Decreased fibula growth rate occurred in premature infants given 25 mg/kg oral tetracycline every 6 hours. 

Abstract Compact bone is a well-organised, multi-level porous structure. Strain-derived fluid flow likely steers the activity of cells within the bone matrix, which in turn orchestrate the concerted activity of bone resorbing and bone forming cells at the surface. We present a model of the strain-driven bone remodelling proces, which could explain the mechanically optimised structure of compact bone. 

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