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)
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