Bone

Bone, special form of connective tissue which is rigid and supports the soft tissues of the body. It constitutes the principal component of almost all adult vertebrate skeletal structures, which protects vital organs, permits locomotion, and plays a vital role in calcium homeostasis (balance) in the body. It exists in either dense or spongy form, known respectively as compact and cancellous bone (see Skeleton). It is distinguished from another type of firm connective tissue known as cartilage or gristle.

Bone consists of a chemical mixture of inorganic salts (65 to 70 per cent) and various organic substances (30 to 35 per cent) and is both hard and elastic. Its hardness is derived from inorganic constituents, principally calcium phosphate and calcium carbonate, with small amounts of fluorides, sulphates, and chlorides; its elasticity is derived from such organic substances as gelatin, collagen, and traces of elastin, cellular material, and fats. Internal tubular structures called Haversian canals contain nerve tissues and blood vessels that provide bones with organic nourishment. Surrounding these canals is a somewhat porous tissue composed of thin plates, known as lamellae, and usually containing cavities filled with a network of connective tissue called marrow or myeloid tissue. Bone marrow accounts for from 2 to 5 per cent of the body weight of a person and consists of tissue of two types. Yellow bone marrow is made up principally of fat, and red bone marrow is tissue in which red and white blood cells and blood platelets originate. The external portions of bones, enclosing all the components mentioned above, include the compact and hardest of all bone tissue, which is in turn generally sheathed by a vascular, fibrous membrane known as the periosteum.

The body continually replaces bone through a complex interaction among minerals in the blood, particularly calcium and phosphorus; certain hormones; specialized bone cells called osteoclasts and osteoblasts; and the stresses and strains of bodily activity. In the disease known as rickets, the mineral matter in bones is deficient, and the two flexible leg bones bend under the weight of the body. In older people, the rate of bone resorption exceeds that of bone formation, resulting in the condition known as osteoporosis. Symptoms include a reduction of height and a susceptibility to fracture.

Many radioactive elements have an affinity for bone. Excessive amounts of radiation may be stored in bone tissue and initiate a cancer. Moreover, bone marrow is particularly susceptible to radiation. A single exposure of 25 roentgens produces a detectable drop of circulating lymphocytes (white blood cells). In acute radiation syndrome, first the number of white blood cells and later the number of red blood cells are reduced. Both symptoms are directly related to damage done to the blood-forming tissue of which bone marrow is an important component. See Radioactive Fallout; Transplantation, Medical.