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An osteoclast is a multinucleated cell that degrades and reabsorbs old bone. Osteoclasts arise from haemopoietic cells of the monocyte/macrophage 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. 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, such as calcium and phosphate ions are absorbed into small vesicles (see endocytosis) which move across the cell and eventually get released into the extracellular fluid.
Osteoclasts are regulated by several hormones, including the parathyroid hormone (PTH) 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 resorption and bone formation.
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