Haversian System

The osteon, or Haversian system, is the fundamental functional unit of compact bone. Osteons 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 that surround a central canal, the Haversian canal. The Haversian canal contains the bone's nerve and blood supplies. Osteoblasts form the lamellae sequentially, from the most external inward toward the Haversian canal. Some of the osteoblasts develop into osteocytes, each living within its own small space, or lacuna. Osteocytes make contact with the cytoplasmic processes of their counterparts via a network of small canals, or canaliculi. This network facilitates the exchange of nutrients and metabolic waste.

Collagen 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 during the process of bone remodelling.

Osteons are connected to each other and the periosteum by oblique channels called Volkmann's canals.

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