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, which is then 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. Osteons can be arranged into woven bone or lamellar bone.
Woven bone Figure 1 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 haphazard 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, or, mature bone cells, 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 Figure 2. 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.
After a fracture, woven bone forms initially and is gradually replaced by lamellar bone during a process known as bony substitution.