Bone

Bone

Bone is the major component of the adult vertebrate skeleton. It is a hard connective tissue comprised of living material, including bone cells, fat cells, and blood vessels, and an inorganic matrix, which is made up largely of water and minerals.

All connective tissues support and connect various parts of the body, and the specific functions of bones are diverse. As the main element of the skeleton, they provide structure and support to vertebrate bodies. They also act as levers for body movement, their position controlled by the muscles attached to them. Bones also protect the delicate internal organs from external impact. For example, the skull encases and protects the brain, and the rib cage houses the lungs and heart.

As well as serving these structural and protective functions, bones play two important physiological roles. They serve as deposits for calcium, a mineral that makes the bones stronger and is essential for the operation of nerves and muscles. Red blood cells, white blood cells, and platelets are all manufactured in the core of the bone, or bone marrow.

Bones change and develop along with the rest of the body. During the early stages of embryonic development, the vertebrate skeleton consists entirely of cartilage. As the fetus grows, calcium and phosporous deposits form around the cartilage as the mineralization process begins. At birth, the skeleton still consists mostly of cartilage and experiences further changes as the child matures. For instance, the bones of an infant's skull do not fuse until several months after birth. A newborn human has over 300 bones, which over time fuse into the 206 bones of an adult. Cartilage gradually replaces bone through the process of ossification , which is achieved through the activity of osteoblasts , the bone precursor cells.

Bone is made up of osteocytes, living bone cells that are surrounded by the matrix. Osteoblasts secrete the matrix and collagen, a protein that gives bone a slightly elastic quality and prevents it from shattering when bearing weight. The osteoblasts also secrete mineral salts, which harden the bone. As the bone matures, the osteoblasts are transformed into osteocytes, and new osteoblasts are released into the system to build more bone.

Bone tissue can be categorized as compact or spongy. Compact bone, also called cancellous bone, has a honeycomb structure that is designed to withstand stress from multiple directions. Compact bone is denser and harder than spongy bone, and is present in the main bones of the arms and legs. It is made up of long, cylindrical units called osteons, which help the bone bear weight. Blood vessels and nerves run through the center of each osteon.

Many bones are composed of an outer layer of compact bone and an inner core of spongy bone. The skull, pelvis, ribs, breastbone, and vertebrae all contain spongy bone, as do the ends of the arm and leg bones. Trabeculae are the bony struts that create the criss-cross formation of spongy bone. Bone marrow fills the spaces between the trabeculae. A thin, two-layered membrane called the periosteum surrounds and protects both bone types. Nerves and blood vessels run throughout the outer layer of the periosteum into the bone. Osteoblasts are the main constituent of the inner layer.

Bones are connected to each other at junctions called joints. There are several types of joints, each with a different range and pattern of movement. The fused joints of the skull do not permit movement, the hinge joints of the elbow and knee allow movement in one direction, and the pivot joints found between certain neck vertebrae permit side-to-side twisting motions. The ball-and-socket joint in the shoulder allows a wide range of movement.

Bone is a dynamic tissue with a structure and composition that adapt to environmental stresses. It undergoes constant breakdown and rebuilding. As a calcium deposit, bone is responsible for maintaining required levels of this mineral in the blood. When calcium levels drop, cells called osteoclasts break down bone to release calcium into the blood. Through the activity of osteoblasts, bones also thicken in response to exercise and impact.

When a bone breaks, several processes contribute to its repair. First, cells from the periosteum transfer to the site of the break and create a fibrous network. Then other cells produce cartilage around this network. In the final step, osteoblasts arrive and convert the cartilage into bone. This healing process may take weeks or months, depending on the severity and location of the injury and the individual's age and general health.

As an individual ages, the rate at which bone breaks down slowly begins to exceed the rate at which it is formed. The bone is weakened, and its size reduced. Developing and maintaining proper exercise and nutrition habits at an early age ensures that bones remain healthy in old age.

see also Skeletons.

Judy P. Sheen

Bibliography

Harris, William H., and Judith S. Levy, eds. The New Columbia Encyclopedia. New York: Columbia University Press, 1975.

Parker, Sybil P., ed. Encyclopedia of Science and Technology, 8th ed. London: McGraw-Hill, 1997.