Human bone is a living, growing tissue. Bone tissue (or osseous tissue) is a type of dense connective tissue composed mainly of collagen (or ossein fibers) and bone cells called osteocytes.
There are two types of bone tissue: cortical bone and cancellous bone.
- Cortical bone (compact bone) forms the extremely hard, strong, stiff and dense exterior of bones and facilitates the main function of bones: to support the whole body, protect organs, provide levers for movement, and store and release chemical elements, mainly calcium. Cortical bone makes up 80% of the weight of a human skeleton.
- Cancellous bone (trabecular or spongy bone) is softer and weaker than cortical bone, but is considerably more flexible. Typically found at the ends of long bones (near joints and within the interior of vertebrae), cancellous bone is highly vascular and frequently contains red bone marrow where haematopoiesis (the production and maturation of blood cells) occurs.
Bone tissue is covered by the periosteum – a dense membrane layer of vascular connective tissue consisting of an outer fibrous layer and an inner cellular layer (cambium).
- The outer layer is composed mostly of collagen and contains nerve fibers that cause pain when the tissue is damaged. It also contains many blood vessels – branches of which penetrate the bone to supply the osteocytes (bone cells). These perpendicular branches pass into the bone along channels known as Volkmann canals to the vessels in the haversian canals, which run the length of the bone. Fibers from the inner layer also penetrate the underlying bone, assisting the blood vessels to bind the periosteum to the bone.
- The inner layer of the periosteum contains osteoblasts (bone-producing cells) and is most prominent in fetal life and early childhood, when bone formation is at its peak. In adulthood these cells are less evident, but they retain their functional capacities and are vital to the constant remodeling of bone that goes on throughout life.