Bone is a dynamic and intricate tissue that constantly renews itself throughout life. Its complex internal organization allows it to perform various functions. Its strength and adaptability come from its microscopic architecture, enabling it to withstand daily stresses and support the body.
Defining the Osteon
The osteon, also known as the Haversian system, is the fundamental structural unit of compact bone. This dense tissue forms the hard outer layer of most bones, including the long shafts of bones like the femur. Osteons are cylindrical structures, typically 0.2 to 0.35 millimeters in diameter and several millimeters to one centimeter in length. Aligned parallel to the bone’s long axis, they are responsible for compact bone’s mechanical properties, including its strength and rigidity.
The Intricate Structure of an Osteon
Each osteon is a highly organized microscopic cylinder. At its core lies the central, or Haversian, canal, which runs longitudinally through the osteon. This canal houses small blood vessels, nerve fibers, and lymphatic vessels that provide nourishment and communication to the bone tissue.
Surrounding the central canal are concentric rings of bone matrix called lamellae. These 5 to 20 layers, each 3 to 7 micrometers thick, are composed of collagen fibers and mineral salts. The collagen fibers within each lamella are arranged parallel, but their orientation shifts in adjacent layers, resisting twisting forces.
Within these lamellae are small, oval-shaped spaces called lacunae, where mature bone cells, osteocytes, reside. Each lacuna contains a single osteocyte, measuring approximately 5 to 7 micrometers in diameter. Radiating outward from the lacunae are tiny channels called canaliculi. These canals connect the lacunae to each other and to the central Haversian canal, forming an extensive network.
The Essential Roles of Osteons
The organized structure of osteons directly supports their functions in bone health and mechanical performance. The network of canaliculi serves as microscopic pathways for the diffusion of nutrients, oxygen, and signaling molecules from the central canal’s blood supply to the osteocytes within the lacunae. This system also facilitates the removal of waste products from these embedded bone cells, maintaining their viability.
The concentric arrangement of the lamellae provides significant mechanical strength to compact bone. This layered structure allows the bone to withstand stress and bear weight without fracturing. Osteons also play a part in the ongoing maintenance of bone tissue. Osteocytes, nestled within the lacunae, sense mechanical forces applied to the bone and communicate these signals through the canaliculi, influencing bone renewal and adaptation to changing demands.