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continuing process of synthesis and destruction that gives bone its mature structure and maintains normal calcium levels in the body. Destruction, or resorption, of bone by large cells called osteoclasts releases calcium into the bloodstream to meet the body's metabolic needs and simultaneously allows the bone-which is inhibited by its inorganic component from growing by cell division like other tissues-to alter size and shape as it grows to adult proportions. Osteoclasts act on the inner surfaces of bones, in the marrow cavity and the spaces of cancellous bone, to widen these cavities; they also act on the outer surfaces to reduce bony processes, such as the epiphyseal swellings at the ends of the long bones of the arm and leg. Osteoclast activity takes place behind the epiphyseal growth zone to reduce former swellings to the width of the lengthening shaft. Within the bone, osteoclastic destruction helps to convert immature bone (called woven bone) into mature compact bone (lamellar bone) by clearing long tubular spaces that will serve as centres for the development of osteons, the bony structures through which blood vessels pass.