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( Osteoclast)
An osteoclast (from the Greek words for "bone" and "broken") is a type of bone cell that removes bone tissue by removing its mineralized matrix. This process is known as bone resorption. Osteoclasts and osteoblasts are instrumental in controlling the amount of bone tissue osteoblasts form bone, osteoclasts resorb bone. Osteoclasts are formed by the fusion of cells of the monocyte-macrophage cell line.[1] Osteoclasts are characterized by high expression of tartrate resistant acid phosphatase (TRAP) and cathepsin K. An osteoclast is a large cell that is characterized by multiple nuclei and a cytoplasm with a homogeneous, "foamy" appearance. This appearance is due to a high concentration of vesicles and vacuoles.[2][3] At a site of active bone resorption, the osteoclast forms a specialized cell membrane, the "ruffled border", which touches the surface of the bone tissue.[1] The ruffled border, which facilitates removal of the bony matrix, is a morphologic characteristic of an osteoclast that is actively resorbing bone. The ruffled border increases surface area interface for bone resorption. The mineral portion of the matrix (called hydroxyapatite) includes calcium and phosphate ions. These ions are absorbed into small vesicles (see endocytosis) which move across the cell and eventually are released into the extracellular fluid, thus increasing levels of the ions in the blood. Osteoclasts formation requires the presence of RANK ligand (receptor activator of nuclear factor ?ß) and M-CSF (Macrophage colony-stimulating factor). These membrane bound proteins are produced by neighbouring stromal cells and osteoblasts; thus requiring direct contact between these cells and osteoclast precursors. M-CSF acts through its receptor on the osteoclast, c-fms (colony stimulating factor 1 receptor), a transmembrane tyrosine kinase-receptor, leading to secondary messenger activation of tyrosine kinase Src. Both of these molecules are necessary for osteoclastogenesis and are widely involved in the differentiation of monocyte/macrophage derived cells.
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Osteoclast Subcategories
Osteoclast Articles
Osteoporosis by Richard P. Huemer, MD
LE Magazine January 2005 Osteoporosis How Calcium Combines With Other Nutrients to Combat Bone Loss By Richard P. Huemer, MD A serious and disabling disorder, osteoporosis is the most common metabolic bone disease in the West...
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