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5 Bone (continued) Many metabolic and genetic diseases can affect the amount and quality of bone. Metabolic diseases, including diabetes, kidney disease, anorexia nervosa, and vitamin D–dependent rickets, may cause osteopenias (reductions in bone volume and bone structural quality). In addition, immunosuppressive therapy in organ transplant patients can lead to reduced bone mass. Tumors of bone and other sites can also lead to bone loss. Bone tumors include those that arise in bone and those that metastasize to bone from primary tumors in other locations (for example, the breast and prostate). In par- ticular, tumors can produce substances that cause the activation of osteoclastic bone resorption, which can result in the destruction of bone and elevated levels of calcium in the blood. Two examples of genetically based diseases of bone are osteopetrosis and osteogenesis imperfecta. In the latter, mutations in the gene for type I collagen result in the production of reduced amounts of collagen or altered collagen molecules by osteoblasts, with a reduced capacity to serve as a support for bone mineral. Osteopetrosis is a rare set of conditions characterized by increased bone density due to a defect in the number or activity of osteoclasts. Other common diseases of the skeleton are diseases of the joints. Rheumatoid arthritis is an autoimmune disease characterized by inflammation of the joints and subsequent destruction of the cartilage and bone in the joints. The causes of osteoarthritis are less clear, but this too is characterized by the gradual destruction of the articular cartilage. Fig. 6: Remodeling of a long bone: (left) a growing bone and (right) an adult bone. Bone is a dynamic tissue and is constantly being remodeled by the actions of osteoclasts and osteo- blasts. (Copyright © McGraw Hill) + ward ' s science 5100 West Henrietta Road • PO Box 92912 • Rochester, New York 14692-9012 • p: 800 962-2660 • wardsci.com This article was originally published by McGraw Hill's AccessScience. Click here to view and find more articles like this.

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