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Bones are composed of bone matrix, which has both organic and inorganic components. Bone matrix is laid down by osteoblasts as collagen, also known as osteoid. Osteoid is hardened with inorganic salts, such as calcium and phosphate, by the chemicals released from the osteoblasts through a process known as mineralization.
The basic microscopic unit of bone is an osteon (or haversian
Osteons are roughly cylindrical structures that can measure
several millimeters long and around 0.2 mm in diameter.
Each osteon consists of lamellae of compact bone
tissue that surround a central canal (haversian
canal). The haversian canal contains the bone's blood supplies. The cement
line is the boundary of an osteon.
Osteons can be arranged into woven bone or lamellar bone.
Woven bone is found on the growing ends of an immature skeleton or, in adults, at the site of a healing fracture. Woven bone is characterized by the irregular organization of collagen fibers and is mechanically weak, but forms quickly. The criss-cross appearance of the fibrous matrix is why it is referred to as woven. It has a high proportion of osteocytes to hard inorganic salts, which leads to its mechanical weakness.
Woven bone is replaced by lamellar bone during development. In contrast to woven bone, lamellar bone is highly organized in concentric sheets with a much lower proportion of osteocytes to surrounding tissue. The regular parallel alignment of collagen into sheets, or, lamellae, causes lamellar bone to be mechanically strong.
Lamellar bone makes up the compact or cortical bone in the skeleton, such as the long bones of the legs and arms. In cross-section, the fibers of lamellar bone run in opposite directions in alternating layers, much like in plywood, assisting in the bone's ability to resist torsion forces. When the same lamellar bone is loosely arranged, it is referred to as trabecular bone. Trabecular bone gets its name because of the spongey pattern that it displays on an x-ray. The spaces within trabecular bone are filled with active bone marrow.