Haversian canals (sometimes canals of Havers, osteonic canals or central canals) are a series of microscopic tubes in the outermost region of bone called

cortical bone A bone is a rigid organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red and white blood cells, store minerals, provide structure and support for the body, an ...

. They allow blood vessels and nerves to travel through them to supply the

osteocyte An osteocyte, an oblate-shaped type of bone cell with dendritic processes, is the most commonly found cell in mature bone. It can live as long as the organism itself. The adult human body has about 42 billion of them. Osteocytes do not divide an ...

Structure

Each Haversian canal generally contains one or two

capillaries A capillary is a small blood vessel, from 5 to 10 micrometres in diameter, and is part of the microcirculation system. Capillaries are microvessels and the smallest blood vessels in the body. They are composed of only the tunica intima (the in ...

and many nerve fibres. The channels are formed by concentric layers called lamellae, which are approximately 50 μm in

diameter In geometry, a diameter of a circle is any straight line segment that passes through the centre of the circle and whose endpoints lie on the circle. It can also be defined as the longest Chord (geometry), chord of the circle. Both definitions a ...

. The Haversian canals surround

blood vessels Blood vessels are the tubular structures of a circulatory system that transport blood throughout many animals’ bodies. Blood vessels transport blood cells, nutrients, and oxygen to most of the tissues of a body. They also take waste an ...

and nerve cells throughout bones and communicate with

s (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 A bone is a Stiffness, rigid Organ (biology), organ that constitutes part of the skeleton in most vertebrate animals. Bones protect the various other organs of the body, produce red blood cell, red and white blood cells, store minerals, provi ...

its strength.

Active transport In cellular biology, active transport is the movement of molecules or ions across a cell membrane from a region of lower concentration to a region of higher concentration—against the concentration gradient. Active transport requires cellula ...

is used to move most substances between the blood vessels and the osteocytes. Haversian canals are contained within

osteon In osteology, the osteon or haversian system (; named for Clopton Havers) is the fundamental functional unit of much compact bone. Osteons are roughly cylindrical structures that are typically between 0.25 mm and 0.35 mm in diameter. Thei ...

s, 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

Clinical significance

Fracture

Blood vessels in the Haversian canals are likely to be damaged by

bone fracture A bone fracture (abbreviated FRX or Fx, Fx, or #) is a medical condition in which there is a partial or complete break in the continuity of any bone in the body. In more severe cases, the bone may be broken into several fragments, known as a ''c ...

. This can cause haematoma.

Rheumatoid arthritis

Haversian canals may be wider in patients with

rheumatoid arthritis Rheumatoid arthritis (RA) is a long-term autoimmune disorder that primarily affects synovial joint, joints. It typically results in warm, swollen, and painful joints. Pain and stiffness often worsen following rest. Most commonly, the wrist and h ...

. They are also more likely to contain

osteoclast An osteoclast () is a type of bone cell that breaks down bone tissue. This function is critical in the maintenance, repair, and bone remodeling, remodeling of bones of the vertebrate, vertebral skeleton. The osteoclast disassembles and digests th ...

s that break down bone structure. These differences are studied with light microscopy.

History

Haversian canals were first described (and probably discovered) by

British British may refer to: Peoples, culture, and language * British people, nationals or natives of the United Kingdom, British Overseas Territories and Crown Dependencies. * British national identity, the characteristics of British people and culture ...

physician A physician, medical practitioner (British English), medical doctor, or simply doctor is a health professional who practices medicine, which is concerned with promoting, maintaining or restoring health through the Medical education, study, Med ...

Clopton Havers, after whom they are named. He described them in his 1691 work ''Osteologica Nova.''

In different animals

Human bones are densely vascularized as in many other mammals. Even though some authors tried to identify a correlation between

endotherm An endotherm (from Greek ἔνδον ''endon'' "within" and θέρμη ''thermē'' "heat") is an organism that maintains its body at a metabolically favorable temperature, largely by the use of heat released by its internal bodily functions inst ...

y and secondary Haversian reconstruction, this feature is absent in many living mammals (e.g.

monotreme Monotremes () are mammals of the order Monotremata. They are the only group of living mammals that lay eggs, rather than bearing live young. The extant monotreme species are the platypus and the four species of echidnas. Monotremes are typified ...

s, '' 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 Turkey, officially the Republic of Türkiye, is a country mainly located in Anatolia in West Asia, with a relatively small part called East Thrace in Southeast Europe. It borders the Black Sea to the north; Georgia (country), Georgia, Armen ...

, helmeted guineafowl). Scattered Haversian canals are also found in ectotherms like cryptodire turtles. Among extinct groups, dense Haversian vascularization is only present in stem-birds (dinosaurs) and stem-mammals (therapsids) 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.

Notes

References

External links

* http://www.lab.anhb.uwa.edu.au/mb140/ * Video of haversian canal system within cortical bone.

Additional images

File:Osso por Descalcificação – HE – 40x.JPG, Bone by decalcification (40x): File:Osso por Descalcificação2 – HE – 100x.JPG, Bone by decalcification (100x): {{Bone and cartilage Skeletal system