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osteon
The osteon or haversian system (named for Clopton Havers) is the fundamental functional unit of much compact bone. Osteons are roughly cylindrical structures that are typically several millimeters long and around 0.2mm in diameter.〔"Osteon," ''Encyclopædia Britannica Online'' (2009); retrieved 23 June 2009.〕 They are present in many bones of most mammals and some bird, reptile, and amphibian species.
==Description==
Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a central canal, the haversian canal. The haversian canal contains the bone's blood supplies. The boundary of an osteon is the cement line.
Between adjoining osteons, there are angular intervals that are occupied by interstitial lamellae. These lamellae are remnants of osteons that have been mostly destroyed. Near the surface of the compact bone, the lamellae are arranged parallel to the surface; these are called circumferential lamellae. Some of the osteoblasts develop into osteocytes, each living within its own small space, or lacuna. Osteocytes make contact with the cytoplasmic processes of their counterparts via a network of small transverse canals, or canaliculi. This network facilitates the exchange of nutrients and metabolic waste. Collagen fibers in a particular lamella run parallel to each other, but the orientation of collagen fibers within other lamellae is oblique. The collagen fiber density is lowest at the seams between lamellae, accounting for the distinctive microscopic appearance of a transverse section of osteons. The space between osteons is occupied by ''interstitial lamellae'', which are the remnants of osteons that were partially resorbed during the process of bone remodeling.
Osteons are connected to each other and the periosteum by oblique channels called Volkmann's canals or perforating canals.
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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