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In osteology, the osteon or haversian system (; named for Clopton Havers) is the fundamental functional unit of much cortical bone. Osteons are roughly cylindrical structures that are typically between 0.25 mm and 0.35 mm in diameter. Their length is often hard to define, but estimates vary from several millimeters"Osteon," Encyclopædia Britannica Online (2009); retrieved 23 June 2009. to around 1 centimeter. They are present in many bones of most mammals and some bird, reptile, and amphibian species.
Each Haversian canal is surrounded by varying number (5-20) of concentrically arranged lamellae of bone matrix. 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 processes of their counterparts via a network of small transverse canals, or canaliculi. This network facilitates the exchange of and metabolism 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 bone resorption 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.
Osteons and their arrangement vary according to taxon, so that genus and sometimes species can be differentiated using a bone fragment not otherwise identifiable. However, there is considerable variability among the different bones of a skeleton, and features of some faunal osteons overlap with those of human osteons; therefore, examination of osteons is not of primary use in the analyses of osteological remains.Martiniaková, Monika. 2006. Differences in Bone Microstructure of Mammalian Skeletons. Faculty of Natural Sciences, Constantine the Philosopher University in Nitra: Nitra, Slovakia More research is needed, but osteohistology has the potential to positively affect the studies in bioarchaeology, paleontology and forensic science.
In recent decades, osteohistological studies of dinosaur fossils have been used to address a number of issues, such as the periodicity of growth of dinosaurs and whether it was uniform across species and the question of whether dinosaurs were warm-blooded or not.
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