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{{Short description|Fundamental anatomical unit of compact bone}}
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{{Infobox anatomy
| Name        = Osteon
| Latin       = Osteon
| Image       =
| Caption     =
| Image2      = Transverse section of bone en.svg
| Caption2    = Diagram of compact bone from a transverse section of a long bone's cortex.
| System      =
}}

[[File:Illu compact spongy bone.jpg|right|371x371px|thumb|Diagram of a typical long bone showing both compact (cortical) and cancellous (spongy) bone.]]
[[File:Bone connective tissue.jpg|thumb|Osteons on cross-section of a bone]]

In [[osteology]], the '''osteon''' or '''haversian system''' ({{IPAc-en|h|ə|ˈ|v|ɝ|.|ʒ|ən}}; named for [[Clopton Havers]]) is the fundamental functional unit of much [[cortical bone|compact bone]]. Osteons are roughly cylindrical structures that are typically between 0.25 mm and 0.35 mm in [[diameter]].<ref name=":0" /> Their length is often hard to define,<ref>{{cite book |last1=Patterson |first1=Janet C. |last2=Firth |first2=Elwyn C. |title=The athletic horse : principles and practice of equine sports medicine |isbn=978-0-7216-0075-8 |edition=Second |url=https://www.sciencedirect.com/book/9780721600758/the-athletic-horse|date=2013-06-06 |publisher=Elsevier Health Sciences }}</ref> but estimates vary from several millimeters<ref name=brittanica>"Osteon," ''Encyclopædia Britannica Online'' (2009); retrieved 23 June 2009.</ref> to around 1 centimeter.<ref name=":0">{{Cite book |title=General Anatomy and Musculoskeletal System (Thieme Atlas of Anatomy) |url=https://archive.org/details/generalanatomymu00mich |url-access=limited |publisher=Thieme |year=2006 |isbn=978-1-60406-292-2 |editor1-last=Ross |editor1-first=Lawrence M. |page=[https://archive.org/details/generalanatomymu00mich/page/n48 35] |editor2-last=Lamperti |editor2-first=Edward D.}}</ref> They are present in many bones of most mammals and some bird, reptile, and amphibian species.

==Histogenesis==
The Haversian system forms during the process of endochondral ossification, which starts with a cartilage template that is gradually replaced by bone tissue. [[Osteoblast]]s, the bone-forming cells, secrete the organic components of bone matrix [osteoid] and then initiates its mineralization. As osteoblasts become surrounded by the bone matrix, they differentiate into [[osteocyte]]s, which reside in the lacunae and maintain bone tissue. The osteocytes connect to each other and the [[Haversian canal]] via tiny canals called ''[[Canaliculus (bone)|canaliculi]]''.

==Structure==
[[File:Compact bone histology 2014.jpg|left|thumb|226x226px|Histology of compact bone showing osteon]]
Each osteon consists of concentric layers, or ''lamellae'', of [[Osseous tissue|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'''.

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 (histology)|lacuna]]. Osteocytes make contact with the [[cytoplasm]]ic processes of their counterparts via a network of small transverse canals, or ''canaliculi''. This network facilitates the exchange of [[nutrient]]s and [[metabolism|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 [[bone resorption|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.

=== Drifting osteons ===
Drifting osteons are a phenomenon that is not fully understood. A "drifting osteon" is classified as one that runs both longitudinally as well as transversely through the cortex. An osteon can "drift" in one direction or change directions several times, leaving a tail of lamella behind the advancing haversian canal.<ref>{{cite journal |last1=Robling |first1=Alexander D. |last2=Stout |first2=Sam D. |year=1999 |title=Morphology or the Drifting Osteon |journal=Cells Tissues Organs |volume=164 |issue=4 |pages=192–204 |doi=10.1159/000016659 |pmid=10436327 |s2cid=11335682 }}</ref>

==Investigative applications==
In [[bioarchaeology|bioarchaeological]] research and in forensic investigations, osteons in a bone fragment can be used to determine the sex of an individual<ref>{{cite journal |first1=Monika |last1=Martiniaková |first2=Radoslav |last2=Omelka |first3=Birgit |last3=Grosskopf |first4=Alexander V. |last4=Sirotkin |first5=Peter |last5=Chrenek |year=2008 |title=Sex-related variation in compact bone microstructure of the femoral diaphysis in juvenile rabbits |journal=Acta Vet Scand |volume=50 |issue=1 |pages=15ff |doi=10.1186/1751-0147-50-15 |pmc=2435232 |pmid=18522730 |doi-access=free }}</ref> and age, as well as aspects of taxonomy, diet, health and motor history.<ref>{{cite journal |last1=Pfeiffer |first1=Susan |last2=Crowder |first2=Christian |last3=Harrington |first3=Lesley |last4=Brown |first4=Michael |year=2006 |title=Secondary osteon and haversian canal dimensions as behavioral indicators |journal=American Journal of Physical Anthropology |volume=131 |issue=4 |pages=460–468 |doi=10.1002/ajpa.20454 |pmid=16685724 }}</ref>

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.<ref>Martiniaková, Monika. 2006. ''Differences in Bone Microstructure of Mammalian Skeletons''. Faculty of Natural Sciences, Constantine the Philosopher University in Nitra: Nitra, Slovakia</ref>  More research is needed, but osteohistology has the potential to positively affect the studies in [[bioarchaeology]], [[paleontology]] and [[forensic science|forensic investigations]].

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<ref>{{cite journal|last=Chinsamy|first=Anusuya|year=1995|title=Ontogenetic Changes in the Bone Histology of the Late Jurassic Ornithopod 'Dryosaurus Lettowvorbecki'|journal=Journal of Vertebrate Paleontology|volume=15|issue=1|pages=96–104|doi= 10.1080/02724634.1995.10011209}}</ref>  and the question of whether dinosaurs were warm-blooded or not.<ref>{{cite journal|last=Reid|first=R. E. H.|year=1984|title=Primary Bone and Dinosaurian Physiology|journal=Geological Magazine|volume=121|issue=6|pages=589–598|doi=10.1017/S0016756800030739|bibcode=1984GeoM..121..589R|s2cid=128981835 }}</ref>

== See also ==
* [[Cancellous bone]] (spongy bone)
* [[Cortical bone]] (compact bone)

== References ==

<references/>

==Bibliography==
*{{cite journal |last1=Cooper |first1=Reginald R. |last2=Milgram |first2=James W. |last3=Robinson |first3=Robert A. |year=1966 |title=Morphology of the Osteon: An Electron Microscopic Study |journal=Journal of Bone and Joint Surgery |volume=48 |issue= 7|pages=1239–1271 |doi= 10.2106/00004623-196648070-00001|pmid=5921783 }}
* [[Frank H. Netter|Netter, Frank H.]] (1987), ''Musculature system: anatomy, physiology, and metabolic disorders''. Summit, New Jersey: Chiba-Geiger Corporation {{ISBN|0-914168-88-6}}

== External links ==
* SLIBS Bone Website:  http://www.trinity.edu/stonily/bone/intro2.htm {{Webarchive|url=https://web.archive.org/web/20150304175645/http://www.trinity.edu/stonily/bone/intro2.htm |date=2015-03-04 }}
* {{BiowebUW|zoolab/Table_of_Contents/Lab-1b/Bone_Model/bone_model.htm|Bone }}
* [https://web.archive.org/web/20080125233621/http://science.nhmccd.edu/biol/tissue/bone.html Histology of osteons]
* {{cite web |title=Video explaining osteons |url=https://www.youtube.com/watch?v=ylmanEGjRuY  |archive-url=https://ghostarchive.org/varchive/youtube/20211212/ylmanEGjRuY| archive-date=2021-12-12 |url-status=live|via=[[YouTube]] }}{{cbignore}}

{{Bone and cartilage}}

[[Category:Skeletal system]]