OSTEOCLAST

large, multinucleate cell responsible for the dissolution and absorption of bone during the process of bone remodeling, or renewal. Bone is a dynamic tissue, continuously being broken down and restructured in response to such influences as structural stress and the body's requirement for calcium. The osteoclasts are the mediators of the continuous destruction of bone. Osteoclasts occupy small depressions on the bone's surface called Howship's lacunae; the lacunae are thought to be caused by erosion of the bone by the osteoclast's enzymes. Osteoclasts appear to be formed by the fusion of many smaller cells and may have as many as 200 nuclei, although most have only 5 to 20. The side of the cell closest to the bone contains many small projections (microvilli) that extend into the bone's surface, forming a brush border that is the cell's active region. Osteoclasts produce a number of enzymes, chief among them acid phosphatase and collagenase, that dissolve both the organic collagen and the inorganic calcium and phosphorus of the bone. The solid bone is first broken into fragments by dissolving the mucopolysaccharide glue that holds it together; the osteoclast then engulfs the fragments and digests them within cytoplasmic vacuoles. Calcium and phosphorus liberated by the breakdown of the bone are released into the bloodstream. Osteoclast function is regulated by parathyroid hormone released in response to changes in blood calcium levels. It was formerly thought that parathyroid hormone caused bone-forming cells, or osteoblasts (q.v.), to differentiate into osteoclasts, but it is now believed that the two cells, which have opposite functions in maintaining bone structure, originate independently of one another. See also bone.