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Also called the Haversian system (named for Clopton Havers ) is the fundamental functional unit of many compact bones.
Osteons are roughly cylindrical structures typically several millimeters long and about 0.2 mm in diameter.
They are present in many bones of most mammals and some species of birds, reptiles, and amphibians.
Osteon, the central structural unit of compact (cortical) bone, consisting of concentric bony layers called lamellae, surrounding a long, hollow passage, the Haversian canal (named for Clopton Havers, a 17th century English physician).
The Haversian canal contains tiny blood vessels that supply blood to osteocytes (individual bone cells). Osteons are several millimeters long and approximately 0.2 millimeters (0.008 inches) in diameter; they tend to run parallel to the long axis of a bone.
Osteons are characteristic formations of mature bone and take shape during bone remodeling or renewal. New bone can also take on this structure as it forms, in which case the structure is called a primary osteon.
The formation of osteons and their accompanying Haversian channels began when large cells called osteoclasts to destroy immature tissue bone and primary osteons, which hollow a channel through the bone, usually following existing blood vessels.
The layers of bone-forming cells or osteoblasts follow the osteoclasts and deposit new bone on the sides of the canal; the layers accumulated in this way slowly narrow the canal until there is a tunnel not much larger than the central blood vessel.
The blood supply for the osteocytes then passes through these channels, the Haversian channels. The spaces between adjacent osteons are filled with interstitial lamellae, layers of bone that are often remnants of anterior Haversian systems.
The transverse vessels, which run perpendicular to the long axis of the cortex, are called Volkmann canals; Volkmann’s canals connect adjacent osteons and connect the Haversian canals’ blood vessels to the periosteum, the tissue that covers the outer surface of the bone.
Structure
Each osteon consists of concentric layers, or lamellae, of compact bone tissue surrounding a central canal, the Haversian canal. The Haversian canal contains the blood supplies of the bone. The limit of an osteon is the cement line.
Each Haversian canal is surrounded by a variable number (5-20) of concentrically arranged lamellae of bone matrix. Near the surface of compact bone, the lamellae are arranged parallel to the surface; These are called circumferential lamellae.
Some osteoblasts develop into osteocytes, each living within a small space or pond. Osteocytes come into contact with the cytoplasmic processes of their counterparts through a network of small transverse channels or canaliculi.
This network facilitates the exchange of nutrients and metabolic waste. The collagen fibers in a particular lamella run parallel to each other, but the orientation of the collagen fibers within other lamellae is oblique.
Collagen fiber density is lowest at the junctions between the lamellae, which accounts for the distinctive microscopic appearance of a cross-section of osteons.
The space between osteons is occupied by interstitial lamellae, which are the remains of osteons that were partially reabsorbed during the bone remodeling process.
Osteons are connected, and the periosteum by oblique canals called Volkmann canals or perforator canals.
Drifting osteons
Drifting osteons are a phenomenon that is not fully understood. A “drifting osteon” is classified as one that extends both longitudinally and transversely through the cortex.
An osteon can “drift” in one direction or change direction several times, leaving a trail of lamellae behind the advancing Haversian canal.
Research Applications
In bioarchaeological and forensic investigations, osteons in a bone fragment can be used to determine an individual’s sex and age and aspects of taxonomy, diet, health, and motor history.
Osteons and their arrangement vary by taxon, so genus and sometimes species can be differentiated using an otherwise unidentifiable bone fragment.
However, there is considerable variability between the different bones of a skeleton. The characteristics of some faunal osteons overlap with those of human osteons; therefore, the examination of osteons is not of primary use in the analysis of osteological remains.
More research is needed, but osteohistology has the potential to positively affect studies in bioarchaeology, paleontology, and forensic investigations.
