The osteon, or Haversian

Clopton Havers

Clopton Havers was an English physician who did pioneering research on the microstructure of bone. He is believed to have been the first person to observe and almost certainly the first to describe what are now called Haversian canals and Sharpey's fibres.-Biography:He was born Stambourne, Essex,...

 system, is the fundamental functional unit of much compact bone

Cortical bone

Cortical bone, synonymous with compact bone, is one of the two types of osseous tissue that form bones. Cortical bone facilitates bone's main functions: to support the whole body, protect organs, provide levers for movement, and store and release chemical elements, mainly calcium. As its name...

. Osteons, roughly cylindrical structures that are typically several millimeters long and around 0.2mm in diameter

Diameter

In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

, are present in many of the bones of most mammals, birds, reptiles, and amphibians.

Each osteon consists of concentric layers, or lamellae, of compact bone tissue

Osseous tissue

Osseous tissue, or bone tissue, is the major structural and supportive connective tissue of the body. Osseous tissue forms the rigid part of the bone organs that make up the skeletal system.-Formation:Bone tissue is a mineralized connective tissue...

 that surround a central canal, the Haversian canal. The Haversian canal contains the bone's nerve and blood supplies. The boundary of an osteon is the cement line.

Between adjoining osteons there are angular intervals that are occupied by interstitial lamellae. These lamellae are remnants of osteons the greater parts of which have been destroyed. 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 osteocyte

Osteocyte

An osteocyte, a star-shaped cell, is the most abundant cell found in compact bone. Cells contain a nucleus and a thin ring of cytoplasm. When osteoblasts become trapped in the matrix they secrete, they become osteocytes...

s, each living within its own small space, or lacuna

Lacuna (histology)

In histology, a lacuna is a small space containing an osteocyte in bone or chondrocyte in cartilage.-Bone:The Lacuna are situated between the lamella, and consist of a number of oblong spaces. In an ordinary microscopic section, viewed by transmitted light, they appear as fusiform opaque spots...

. Osteocytes make contact with the cytoplasm

Cytoplasm

The cytoplasm is a small gel-like substance residing between the cell membrane holding all the cell's internal sub-structures , except for the nucleus. All the contents of the cells of prokaryote organisms are contained within the cytoplasm...

ic processes of their counterparts via a network of small canals, or canaliculi

Canaliculus (bone)

Canaliculi are microscopic canals between the various lacunae of ossified bone. The radiating processes of the osteocytes project into these canals. These cytoplasmic processes are joined together by gap junctions. Osteocytes do not entirely fill up the canaliculi. The remaining space is known...

. This network facilitates the exchange of nutrient

Nutrient

A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. They are used to build and repair tissues, regulate body processes and are converted to and used as energy...

s and metabolic

Metabolism

Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...

 waste.

Collagen

Collagen

Collagen is a group of naturally occurring proteins found in animals, especially in the flesh and connective tissues of mammals. It is the main component of connective tissue, and is the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content...

 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.

Osteons are separated from each other by interstitial lamellae between systems.

The space between osteons is occupied by interstitial lamellae, which are the remnants of osteons that were partially resorbed

Bone resorption

Bone resorption is the process by which osteoclasts break down bone and release the minerals, resulting in a transfer of calcium from bone fluid to the blood....

 during the process of bone remodelling

Bone

Bones are rigid organs that constitute part of the endoskeleton of vertebrates. They support, and protect the various organs of the body, produce red and white blood cells and store minerals. Bone tissue is a type of dense connective tissue...

.

Osteons are connected to each other and the periosteum

Periosteum

Periosteum is a membrane that lines the outer surface of all bones, except at the joints of long bones. Endosteum lines the inner surface of all bones....

 by oblique channels called Volkmann's canals

Volkmann's canals

Volkmann canals, also known as perforating holes, are microscopic structures found in compact bone. They run within the osteons perpendicular to the Haversian canals, interconnecting the latter with each other and the periosteum. They usually run at obtuse angles to the Haversian canals and contain...

.

Use in archaeology and forensics

Osteons carry their history so that given the right conditions they can be used to "read" the sex and age, as well as aspects of health history, motor history and diet of an individual whose bone is being studied. Osteons and their arrangement also vary according to taxon, so that genus and sometimes species can be differentiated using a bone fragment not otherwise identifiable. This is of use especially in archaeology, paleontology and forensics.

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 lamellae behind the advancing haversian canal.

See also

• Haversian canals
  Haversian canals
  Haversian canals are a series of tubes around narrow channels formed by lamellae. This is the region of bone called compact bone. Osteons are arranged in parallel to the long axis of the bone...
  
  
• Cortical bone
  Cortical bone
  Cortical bone, synonymous with compact bone, is one of the two types of osseous tissue that form bones. Cortical bone facilitates bone's main functions: to support the whole body, protect organs, provide levers for movement, and store and release chemical elements, mainly calcium. As its name...
  
   (compact bone)
• Cancellous bone (spongy bone containing no osteons)

External links

• SLIBS Bone Website: http://www.trinity.edu/rblyston/bone/intro2.htm
• Histology of osteons