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Osteoclast
An osteoclast (from the Greek words for "bone" (ὀστέον), and "broken" (κλαστός)) is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and remodelling of bones of the vertebral skeleton. The osteoclast disassembles and digests the composite of hydrated protein and mineral at a molecular level by secreting acid and a collagenase, a process known as ''bone resorption''. This process also helps regulate the level of blood calcium.
==Morphology==
Osteoclasts were discovered by Kolliker in 1873. An osteoclast is a large multinucleated cell and human osteoclasts on bone typically have five nuclei and are about 150-200 µm in diameter. Giant osteoclasts can occur in some diseases, including Paget's disease of bone and bisphosphonate toxicity. When osteoclast-inducing cytokines are used to convert macrophages to osteoclasts, very large cells that may reach 100 µm in diameter occur. These may have dozens of nuclei, and typically express major osteoclast proteins but have significant differences from cells in living bone due to the not-natural substrate. The size of the multinucleated assembled osteoclast allows it to focus the ion transport, protein secretory and vesicular transport capabilities of many macrophages on a localized area of bone.
In bone, osteoclasts are found in pits in the bone surface which are called resorption bays, or Howship's lacunae. Osteoclasts are characterized by a cytoplasm with a homogeneous, "foamy" appearance. This appearance is due to a high concentration of vesicles and vacuoles. These vacuoles include lysosomes filled with acid phosphatase. This permits characterization of osteoclasts by their staining for high expression of tartrate resistant acid phosphatase (TRAP) and cathepsin K. Osteoclast rough endoplasmic reticulum is sparse, and the Golgi complex is extensive.〔Standring S., Ed. Gray's Anatomy. 39th ed. 2005, Elsevier〕〔 p. 378〕
At a site of active bone resorption, the osteoclast forms a specialized cell membrane, the "ruffled border," that opposes the surface of the bone tissue. This extensively folded or ruffled border facilitates bone removal by dramatically increasing the cell surface for secretion and uptake of the resorption compartment contents and is a morphologic characteristic of an osteoclast that is actively resorbing bone.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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