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The osteon is the primary structure of the hard outer layer of bones known as cortical bone. Cortical bone, or compact bone, is dense, mature bone found in vertebrates such as mammals, amphibians, reptiles, and birds. It is this dense hard layer that provides strength in the long bones of vertebrates; approximately 80% of vertebrate skeletons is composed of cortical bone. The osteon structure is comprised of the Haversian canal, Volkmann's canals, osteocytes, and canaliculi.
Osteons are cylindrical in shape and run parallel to the long axis of the cortical bone. Each one contains a central canal, called the Haversian canal, along with concentric layers of bone called interstitial lamellae. Lamellae are composed of bone matrix, collagen fibers, and mineral crystals. Interstitial lamellae are contained in the spaces between osteons and perform the important function of providing stability to the long bones. The Haversian canal is surrounded with rings of lamellae.
Within the Haversian canal is a layer of endosteum, connective tissue that contains nerve fibers and blood vessels. The blood cells contained within the canal supply essential nutrients to the osteon itself. These systems of canals and lamellae are also called osteons, and the term is often used synonymously with Haversian system. They were named after 17th century English physician Clopton Havers, who is considered the first person to discover this central canal. He discussed the structure of bone in his book Osteologia Nova.
Volkmann's canals are also components of osteons. They are situated perpendicular to the long axis of the bone and connect the blood supply and nerves to the periosteum, which covers the surface of bones, to the Haversian canals. Found within spaces called lacunae, osteocytes sense strain or stress in bones, and they maintain the bone matrix found within the lamellae.
Another component of osteons are the canaliculi, very narrow canals that join the osteocytes together in order to facilitate the distribution of nutrients and the elimination of waste. Canaliculi radiate outward from the center, like the spokes of a wheel, and they connect the Haversian canal with other canaliculi.
Osteons are found only in cortical bone and are not present in trabecular bone. Trabecular bone is also called spongy bone or cancellous bone and is usually found at the ends of long bones. Cortical bone can be found in long bones, such as the femur or tibia. The trabecular bone is normally surrounded by cortical bone at the end of joints and the vertebrae.
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