Osteoclasts are large (3000-250000 mm3) highly mobile multinucleate cells (10-20 nuclei), which contribute to bone remodeling and calcium home-ostasis by resorbing bone. The osteoclast's resorptive apparatus consists of a central 'ruffled border', a highly folded region of cell membrane across which acid and degradative enzymes are extruded, and a peripheral 'clear zone', which seals the osteoclasts onto the bone.

Osteoclasts dissolve bone mineral by secreting acid across their ruffled borders using proton-pumping ATPases. Acid production also requires carbonic anhydrase - an enzyme used for acid production by the stomach, which is absent in some forms of the lethal bone disease osteopetrosis. The organic components of bone are degraded by lysosomal enzymes; one of these, acid phosphatase, is used as a marker for osteoclast activity.

Origin of the osteoclast Osteoclasts are descended from blood-cell-forming interleukin-3-dependent stem cells located in the bone marrow and are therefore part of the same superfamily of cells as macrophages, lymphocytes and red blood cells. Partially differentiated mononuclear preosteoclasts migrate via the circulation to resorption sites, where they proliferate and acquire differentiated features (e.g., acid phosphatase and calcitonin receptors), before fusing into multinucleate osteoclasts and beginning to resorb.

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