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Rigid connective tissue of
Bones serve as the body's supporting framework, provide muscle-attachment points for movement, protect the internal organs, house the blood-cell formation system (red bone marrow ), and hold about 99% of the calcium vital to many body processes. Bone consists of a matrix of crystals of calcium, chiefly the phosphate and carbonate, embedded among collagen fibres, providing strength and elasticity, and bone cells (less than 5% of its volume). An external layer of compact bone surrounds a central area of spongy bone, except at the marrow cavity. Bone does not grow by cell division; instead, different types of bone cells generate bone matrix, break it down, and maintain it. Bone is remodeled by this process, which strengthens it in areas under greatest stress, permits healing of fracture s, and helps regulate calcium levels in body fluid (see calcium deficiency ). The process also causes underutilized bone, as in an immobilized limb, to atrophy. Bone disorders include rheumatoid arthritis , osteoarthritis , rickets , osteoporosis , and tumour s. Bone can fracture suddenly or over time, as in stress fractures.
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[c mediumvioletred] (as used in expressions)
anemia of bone marrow failure
{{link=bone china">bone china
bone marrow
Paget disease of bone