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How do osteoclasts break down bone tissue?

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Internal structure of a human long bone — bone A long bone of the human body, showing a magnified cross section of the interior.

Osteoclasts are large, multinucleated cells that break down bone tissue through a process called resorption. To facilitate this, osteoclasts occupy small depressions on the bone surface known as Howship lacunae.12

The cell membrane closest to the bone surface features numerous small projections called microvilli, which form a "ruffled" or "brush" border. Through this active region, the osteoclast secretes enzymes—most notably acid phosphatase—and hydrogen ions onto the bone surface.13

These secretions simultaneously dissolve the inorganic mineral components (calcium and phosphorus) and the organic collagen matrix. Once the mineralized bone is broken into fragments, the osteoclast engulfs and digests them within cytoplasmic vacuoles. The resulting calcium and phosphorus are then released into the bloodstream to help maintain the body's metabolic needs. This process is highly efficient, often occurring more rapidly than the formation of new bone.132