Haversian canal

Each Haversian canal generally contains one or two capillaries and many nerve fibres. The channels are formed by concentric layers called lamellae, which are approximately 50 μm in diameter. The Haversian canals surround blood vessels and nerve cells throughout bones and communicate with osteocytes (contained in spaces within the dense bone matrix called lacunae) through connections called canaliculi. This unique arrangement is conducive to mineral salt deposits and storage which gives bone tissue its strength. Active transport is used to move most substances between the blood vessels and the osteocytes.{{Citation|last1=Dahl|first1=A. C. E.|title=5.18 - Mechanobiology of Bone|date=2011-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780080885049004190|work=Comprehensive Biotechnology (Second Edition)|pages=217–236|editor-last=Moo-Young|editor-first=Murray|place=Burlington|publisher=Academic Press|language=en|isbn=978-0-08-088504-9|access-date=2021-01-15|last2=Thompson|first2=M. S.}}

Haversian canals are contained within osteons, which are typically arranged along the long axis of the bone in parallel to the surface. The canals and the surrounding lamellae (8-15) form the functional unit, called a Haversian system, or osteon.

Blood vessels in the Haversian canals are likely to be damaged by bone fracture.{{Citation|last1=White|first1=Tim D.|title=Chapter 4 - BONE BIOLOGY & VARIATION|date=2005-01-01|url=http://www.sciencedirect.com/science/article/pii/B9780120884674500077|work=The Human Bone Manual|pages=31–48|editor-last=White|editor-first=Tim D.|place=San Diego|publisher=Academic Press|language=en|isbn=978-0-12-088467-4|access-date=2021-01-15|last2=Folkens|first2=Pieter A.|editor2-last=Folkens|editor2-first=Pieter A.}} This can cause haematoma.

Haversian canals may be wider in patients with rheumatoid arthritis.{{Citation|last=Aeberli|first=D.|title=Skeleton, Inflammatory Diseases of|date=2014-01-01|url=http://www.sciencedirect.com/science/article/pii/B978012801238300026X|work=Reference Module in Biomedical Sciences|publisher=Elsevier|language=en|isbn=978-0-12-801238-3|access-date=2021-01-15}} They are also more likely to contain osteoclasts that break down bone structure. These differences are studied with light microscopy.

Haversian canals were first described (and probably discovered) by British physician Clopton Havers, after whom they are named.{{Cite journal|last1=Sparks|first1=David S.|last2=Saleh|first2=Daniel B.|last3=Rozen|first3=Warren M.|last4=Hutmacher|first4=Dietmar W.|last5=Schuetz|first5=Michael A.|last6=Wagels|first6=Michael|date=2017-01-01|title=Vascularised bone transfer: History, blood supply and contemporary problems|url=http://www.sciencedirect.com/science/article/pii/S1748681516301723|journal=Journal of Plastic, Reconstructive & Aesthetic Surgery|language=en|volume=70|issue=1|pages=1–11|doi=10.1016/j.bjps.2016.07.012|pmid=27843061|issn=1748-6815|url-access=subscription}} He described them in his 1691 work Osteologica Nova.{{Cite book|last=Havers|first=Clopton|url=https://books.google.com/books?id=tx52uqkSVzIC&pg=PA7|title=Osteologia Nova: Or, Some New Observations of the Bones, and the Parts Belonging to Them; with the Manner of Their Accretion and Nutrition: Communicated to the Royal Society in Several Discourses ... To which is Added, a Fifth Discourse, of the Cartilages. The Second Edition. By Clopton Havers ..|date=1729|publisher=W. Innys|language=en}}

Human bones are densely vascularized as in many other mammals. Even though some authors tried to identify a correlation between endothermy and secondary Haversian reconstruction, this feature is absent in many living mammals (e.g. monotremes, Talpa, flying foxes, Herpestes, Dasypus) and birds (Aratinga, Morococcyx, Nyctidromus, Momotus, Chloroceryle) while others possess only scattered Haversian systems (e.g. artiodactyls, Didelphis, Anas, Gallus, turkey, helmeted guineafowl). Scattered Haversian canals are also found in ectotherms like cryptodire turtles.{{Cite journal |last=Bouvier |first=Marianne |date=1977 |title=DINOSAUR HAVERSIAN BONE AND ENDOTHERMY |url=https://academic.oup.com/evolut/article/31/2/449/6871095 |journal=Evolution |language=en |volume=31 |issue=2 |pages=449–450 |doi=10.1111/j.1558-5646.1977.tb01028.x|doi-access=free |url-access=subscription }} Among extinct groups, dense Haversian vascularization is only present in stem-birds (dinosaurs) and stem-mammals (therapsids){{Cite book |last=Enlow |first=Donald H. |title=Biology of the Reptilia |publisher=Academic Press |year=1969 |volume=1 |pages=45–81 |language=en |chapter=The bones of reptiles}} while scattered Haversian systems can be found in ichthyosaurs, phytosaurs, basal stem-mammals (e.g. Ophiacodon), Limnoscelis, and temnospondyls. When endosteal Haversian systems are considered, the phylogenetic distribution becomes even broader.

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• http://www.lab.anhb.uwa.edu.au/mb140/ {{Webarchive|url=https://web.archive.org/web/20101201063032/http://www.lab.anhb.uwa.edu.au/mb140/ |date=2010-12-01 }}
• {{cite web |title=Bone Anatomy - Haversian canals within bone |date=November 4, 2006 |url=https://www.youtube.com/watch?v=XtV7GdogTPA |archive-url=https://ghostarchive.org/varchive/youtube/20211213/XtV7GdogTPA |archive-date=2021-12-13 |url-status=live|via=YouTube }}{{cbignore}} Video of haversian canal system within cortical bone.

File:Osso por Descalcificação – HE – 40x.JPG|Bone by decalcification (40x): {{olist |Volkmann's canal |Haversian canal |Blood vessel}}

File:Osso por Descalcificação2 – HE – 100x.JPG|Bone by decalcification (100x): {{olist |Volkmann's canal |Haversian canal |Lacuna |Lamella}}

{{Bone and cartilage}}

Category:Skeletal system