OSTEON

also called haversian system the chief structural unit of compact bone, consisting of lamellae, or layers of bone, surrounding a long hollow passageway, the haversian canal. Osteons are quite evident in a cross section of a long bone (e.g., the femur) as a pattern of many groups of concentric circles, with a haversian canal at the centre of each group of circles. The haversian canal contains small blood vessels responsible for the blood supply to osteocytes (individual bone cells). Osteons themselves are concentric cylindrical elements several millimetres long and about 0.2 millimetre in diameter. Both the canal and the system are named for Clopton Havers, a 17th-century English physician. Osteons are formations characteristic of mature bone and take shape during the process of bone remodeling, or renewal. New bone may also take this structure as it forms, in which case the structure is called a primary osteon. The process of the formation of osteons and their accompanying haversian canals begins when immature, woven bone and primary osteons are destroyed by large cells called osteoclasts, which hollow out a channel through the bone, usually following existing blood vessels. Layers of bone-forming cells, or osteoblasts, follow the osteoclasts, laying down new bone on the sides of the channel; the layers of bone built up in this way slowly narrow the channel until a tunnel not much larger than the central blood vessel remains. The blood supply for the bone cells, or osteocytes, then passes through these channels, which are the haversian canals. The spaces between adjacent osteons are filled with interstitial lamellae, layers of bone that are often remnants of previous haversian systems. Transverse pathways called Volkmann's canals connect adjacent osteons and also connect the blood vessels of the haversian canals with the periosteum, the tissue covering the bone's outer surface. See also bone.