The osteon is the basic building block and fundamental functional unit of compact bone.
It is also called a Haversian system, named after Clopton Havers, an English physician who worked extensively on the microstructure of the bone.
Structurally, osteons are roughly cylindrical structures which are several millimeters long and around 0.2 mm in diameter.
Structure of Osteon or Haversian System
[Reada about bone anatomy and physiology]
The Haversian system has a central canal, called the Haversian canal.
This central canal or Haversian canal is surrounded by concentric layers of matrix. The layers of the matrix are called lamellae.
Each lamellar layer is laid over the previous layer with the newer layer being inside the older one. Thus each successor layer is contained inside the preceding one.
These lamellae are laid down one after the other over time.
The collagen fibers in a single lamella run parallel to each other but the orientation of collagen fibers across separate lamellae is oblique.
The collagen fiber density is lower at the border between adjacent lamellae.
This is responsible for the distinctive appearance of an osteon.
In between, the concentric lamellae are present osteocytes.
Osteocytes are the most abundant cell type of bone. They account for 95% all bone cells in skeletally mature tissue.
Osteocytes are terminally differentiated osteoblasts, the bone-producing cells.
Osteoblasts are responsible for producing the matrix of the bone. After being entrapped in the matrix they produce, the osteoblasts are now termed osteocytes.
Each osteocyte is within its own small space also known as the lacuna.
Osteocytes play an important role in bone and mineral homeostasis.
Osteon (purple) are round, concentric layers of lamellae surrounding a central canal or Haversian Canal (dark purple)
Haversian Lamellae (purple) are concentric layers of calcified matrix
Lacunae (black) – oblong spaces found between lamellae containing single osteocytes
Cementing Line (yellow) is the boundary that surrounds an osteon
Interstitial Lamellae (blue) are remnants of osteons from bone remodeling
Image Credit: Histology Guides
Osteocytes are located between concentric lamellae and connected to each other and the central canal by cytoplasmic processes through the canals called canaliculi.
Bone canaliculi are microscopic canals between the lacunae of ossified bone which contain the radiating processes of the osteocytes called filopodia joined together by gap junctions.
The space around the osteocytes in the canaliculi is called the periosteocytic space. It is filled with periosteocytic fluid which contains substances which are too large for transportation through the gap junctions connecting osteocytes.
This network permits the exchange of nutrients and metabolic waste.
Osteons are separated from each other by cement lines. Collagen fibers and canaliculi do not cross cement lines.
The space between separate osteons is occupied by interstitial lamellae. Interstitial lamellae are formed by preexisting osteons which have been reabsorbed.
Haversian systems connect to each other and the periosteum by oblique channels called Volkmann’s canals. Volkman canals are usually run at obtuse angles to the Haversian canals.
Volkmann’s canals contain anastomosing vessels between Haversian capillaries. Volkmann’s canals were named after German physiologist Alfred Volkman
The Haversian canal contains the bone’s nerve and blood supplies which anastomose with other Haversian canals via Volkmann’s canal.
Functions of Osteonal System
- Provides channels for the vascular supply to the cortex
- Act like cables to provide strength to cortical bone
- Limit propagation of microcracks by diverting cracks along cement lines
Drifting osteons
It is an osteon which runs both longitudinally as well as transversely through the cortex.
It can drift in one direction or even change the directions.
The phenomenon is not completely understood.
Use
Osteons in a bone fragment can be used to determine the sex and age of an individual. Not only this, it can tell about diet and health.
