Bone is a type of skeletal tissue. Bones are also organs made largely of this tissue and surrounded by a membrane called the periosteum. This study of bones is called osteology.

Bone tissue contains living cells embedded in a hard matrix. This matrix consists mostly of calcium phosphate (as hydroxylapatite) and other calcium minerals held together by collagen (a protein) and other organic substances.

The major types of bone tissue exist. Compact bone tissue has many rodlike Haversian systems, each containing many concentric cylindrical layers (lamellae) surrounding a central blood vessel. Between these layers the bone cells (osteocytes) lie within spaces called acunae. Tiny pores (canalculi) connect the osteocytes with one another and with the Blood vessel, allowing oxygen and nutrients to reach the cells.

Cancellous, or spongy bone tissue has a bone network of rigid beams (trabeculae). The spaces between these  trabeculae contain marrow, a blood forming tissue. Young marrow is red because many new blood cells are forming. Older marrow becomes yellow with the deposition of fat cells.

Bone tissue renews (itself throughout life, constantly tearing down and rebuilding its mineralized framework. Chemical factors secreted by immune system cells apparently regulate the balance between bone resorption and formation. Bone absorbing cells (osteoclasts) dissolve old tissue, changing into mature osteoblasts. New trabeculae and Haversian systems are mainly oriented by parallel to the direction of greatest stress.

The single most important function of bones is the voluntary  muscles on bones and joint produces locomotion in vertebrates. A second important function is the protection of softer structures, especially those of the nervous system. A third is the production of blood cells in the marrow. Cells there called stem cells are the ultimate source of all types of blood cells; long sought, they were finally isolated in the middle ear of higher animals conduct sound waves and make hearing possible.

Bone can be long, short (wrist bones), flat (many skull bones), irregular (vertebratae), or sesamoid (kneecap). Most limb bones are long and roughly cylindrical. The shaft (diaphysis) consist of compact bone surrounding an interior (the medullary cavity) or mostly spongy bone and tissue, capped the cartilage at the joints. During youth, rapid growth occurs in the epiphyseal cartilage, which lies between the flared section of the bone (metaphysis) and the epiphysis.


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