Osteoclast

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An osteoclast is a multinucleated cell that degrades and reabsorbs bone. They are involved in the natural turnover of bone tissue along with osteoblasts. Osteoclasts arise from haemopoietic cells of the monocyte/neutrophil lineage, specifically CFU-GM.

Osteoclasts lie in a small cavity called Howship's lacuna, formed from the digestion of the underlying bone. The sealing zone is the attachment of the osteoclast's plasmalemma to the underlying bone. The osteoclast releases hydrogen ions (H₂O + CO₂ → HCO₃^- + H⁺) into the cavity, acidifying and dissolving the mineralized bone matrix into Ca²⁺, H₃PO₄, H₂CO₃ and water. Hydrogen ions are pumped against a high concentration gradient by proton pumps, specifically a unique vacuolar-ATPase. This enzyme has been targeted in the prevention of osteoporosis. In addition, several hydrolytic enzymes are released to digest the organic components of the matrix, such as Collagenase and Cathepsin K. These enzymes are released into the compartment by lysosomes.

The osteoclast has a ruffled border which enhances its surface area. Products of digestion of the bone matrix (a mineral called hydroxyapatite), include calcium and phosphate ions. These ions are absorbed into small vesicles (see endocytosis) which move across the cell and eventually get released into the extracellular fluid, thus increasing levels of the ions in the blood.

Osteoclasts are regulated by several hormones, including parathyroid hormone (PTH) from the Parathyroid gland, calcitonin from the Thyroid gland, and growth factor interleukin 6 (IL-6). This last hormone, IL-6, is one of the factors in the disease osteoporosis, which is an imbalance between bone reabsorption and bone formation.

Osteoclastic activity is also the basis for straightening of the teeth with dental braces. The pressure exerted on the periodontal ligament of the tooth (by the braces) causes osteoclasts to absorb the alveolar bone. Osteoblasts then relay the bone to coincide with less pressure on the tooth, thus manipulating of the orientation of the tooth.

An osteoclast can also be an instrument used to fracture and reset bones (the origin is Greek osteon:bone and klastos:broken). To avoid confusion, the cell was originally termed osotoclast. When the surgical instrument went out of use, the cell became known by its present name.

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References

Vaananen, H.K., et al., The cell biology of osteoclast function. J Cell Sci, 2000. 113 (Pt 3): p. 377-381. PMID 10639325
Hankermeyer, C.R., et al., Dissolution rates of carbonated hydroxyapatite in hydrochloric acid. Biomaterials, 2002. 23(3): p. 743-750. PMID 11771694

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