The osteon or Haversian system is the fundamental functional anatomical unit of the compact or cortical bone tissue, which is that found in the body of the long bones and around the spongy bones.
It consists of a set of millimetric bone lamellae, rich in calcium, grouped in a cylindrical way. They are arranged in such a way that they form a central canal called Haversian duct, which opens the way for blood vessels and nerves that reach the bone.
Representation of osteone. By Laboratoires Servier - Smart Servier website: Images related to Osteon (Bone unit), Bone structure and Bones - Download in Powerpoint format.Flickr: Images related to Osteon (Bone unit), Bone structure and Bones (in French)., CC BY -SA 3.0, Osteons are separated by lacunar spaces in which osteocytes are located, which are mature bone cells. The system has a complex network of channels that communicate the osteons with the lagoons populated with osteocytes, thus ensuring the blood supply to all cells, even the most distant ones.
The first to describe this bone structure was the English anatomist Clopton Havers (1657-1702), who dedicated his professional life to the study of the formation and metabolism of bones.
The Havers system plays a fundamental role in the bone remodeling process that occurs both physiologically and when there is a fracture or fissure.
Anatomy and Histology
Anatomy
Compact tissue is found on the outside and in the body of long bones as well as in flat bone structures.
It is a type of very dense and resistant bone tissue, which contributes 80% of the bone mass of an adult skeleton. It gives the bones their characteristic color and consistency.
With the naked eye, it is not possible to distinguish its structure in a bone, so microscopic histological study is essential to understand it.
The English physician Clopton Havers was the first to describe the microscopic architecture of compact bones in his research work Osteologia nova, or some novel observations of the bones and their parts, with an emphasis on their structure and nutrition.
Dr. Havers' publications are still used for reference and the compact bone organization system is named after him.
Histology
The compact or cortical bone is formed by the union of millimeter bone lamellae that are divided into 3 groups, according to their location: external, internal and osteons or Haversian system.
The external lamellae are found on the most superficial face of the bone. They contain collagen-rich extensions called Sharpey fibers, which keep them firmly attached to the periosteum, which is the superficial layer that covers the bones.
Cross section of a bone. By Pbroks13 - Own work, CC BY 3.0, The internal lamellae are found on the inside of the bone, covering the medullary cavity that runs deep within it.
Havers system
The osteon or Haversian system is the main functional anatomical unit of compact bone; cancellous bone tissue does not contain osteons. Like the previous structures, it is composed of a set of bone lamellae that are grouped in a cylindrical way.
Its arrangement gives rise to a central canal called Haversian canal, within which are the blood vessels and neurological endings that supply and supply the bone.
Representation of the cross section of the fibula. By Source digital bitmap graphics: BDBRecreated in vector format: Nyq - Original analog graphics: Gray's Anatomy of the Human Body from the classic 1918 publication available online at Bartleby.com. Digital bitmap graphics: Transverse Section Of Bone.pngRecreated in vector format: Own work, CC BY-SA 4.0, Osteons communicate with each other through pathways that form as branches of the Haversian ducts. These branches are called Volkmann's ducts.
On the other hand, superficially they are separated in some points by spaces called osteocyte lagoons, which contain the bone cells called osteocytes. These spaces communicate with the Haversian ducts through narrow channels, or canaliculi.
Osteocytes form cell extensions that are located in the canaliculi, which allow these cells to reach the blood vessels to maintain their activity.
This form of communication and cellular nutrition is known as the lacuno-canalicular system.
Histology of bone tissue where osteocytes are visible From Posible2006 - Own work, CC BY-SA 4.0,
Function
The compact structure that forms the Havers system gives cortical bone its density and resistance, being much stronger than cancellous bone.
Through the communication pathways that form the Haversian ducts, Volkmann's ducts and the canaliculi, osteone ensures the irrigation and nutrition of the osteocytes. The blood supply to these cells would be impossible otherwise, due to the low porosity of compact bone.
Bone metabolism
The Havers system plays a fundamental role in bone remodeling. It works on bones that have little stress damage as well as those with a fracture.
Bone remodeling involves three types of bone cells that are responsible for the process of resorption, formation and stability of bone tissue; These are: osteocytes, osteoblasts and osteoclasts.
Osteocytes are the mature cells found in osteocyte lacunae, between osteons. These cells come from more primitive cells called osteoblasts, which are responsible for forming new bone tissue.
In compact bone, the oldest osteons, mature osteons, can be distinguished from the younger ones, since the former have a narrower Haversian duct.
Mature osteons are degraded by osteoclasts, which are also responsible for reabsorbing the destroyed bone matrix.
Bone cells By OpenStax College - Anatomy & Physiology, Connexions Web site. http://cnx.org/content/col11496/1.6/, Jun 19, 2013., CC BY 3.0, This process is mediated by the action of different hormones. Among the most important are parathyroid hormone (PTH) and calcitonin. Hormonal activation triggers the action of osteoclasts which, by releasing acidic enzymes, demineralize and destroy the bone surface.
It is these same hormones that participate in bone resorption. When this process occurs, calcium passes into the bloodstream which results in the regulation of that mineral in the body.
For their part, osteoblasts are responsible for forming new bone lamellae that will organize themselves, creating wide Haversian canals. Once they finish their work, these cells differentiate into osteocytes that rest in the lacunar spaces found between the osteons.
Bone remodeling. By Cancer Research UK - Original email from CRUK, CC BY-SA 4.0, Osteoblasts and osteoclasts work perfectly in sync to prevent further bone formation or degradation. Any alteration in this balance results in bone pathologies such as osteoporosis.
In addition to bone damage, hormones that activate bone metabolism are affected by decreased or increased levels of calcium and phosphorus in the blood and can trigger this mechanism for the body to achieve balance of these minerals.
Bone metabolism is a physiological process, that is, bone resorption and formation occurs in healthy individuals. Although it is very important for repair in the case of fractures, cells carry out this mechanism at all times.
References
- Clarke, B. (2008). Normal bone anatomy and physiology. Clinical journal of the American Society of Nephrology: CJASN. Taken from: ncbi.nlm.nih.gov
- Baig, M. A, Bacha, D. (2019). Histology, Bone. StatPearls, Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
- El Sayed SA, Nezwek TA, Varacallo M. (2019). Physiology, Bone. StatPearls, Treasure Island (FL). Taken from: ncbi.nlm.nih.gov
- Fernández-Tresguerres, I; Alobera, M; Canto, M; Blanco, L. (2006). Physiological bases of bone regeneration I: Histology and physiology of bone tissue. Oral Medicine, Oral Pathology and Oral Surgery. Taken from: scielo.isciii.es
- Pazzaglia, U. E; Congiu, T; Pienazza, A; Zakaria, M; Gnecchi, M; Dell'orbo, C. (2013). Morphometric analysis of osteonal architecture in bones from healthy young human male subjects using scanning electron microscopy. Journal of anatomy. Taken from: ncbi.nlm.nih.gov