The Skeleton - Bone as living tissue

Our bones, like all our tissues, change as we grow up, mature, and finally grow old. There are changes in the chemical activity and composition of bone representative of each stage of life.

In young children, the ends and edges of bones are mainly cartilage, forming a growing surface on the bone that is gradually replaced by hard bone as full size is attained. The bones of a child are more pliable and less likely to break than those of a full-grown adult.

Similarly, as an adult ages, the bones turn from a resilient hardness to a more brittle hardness. This accounts for the much greater danger of broken bones in older people.

These changes with age are an indication of the great amount of chemical activity going on within bone. We sometimes forget that our bones are amply supplied and penetrated by blood vessels. There is a constant building up and breaking down, an interchange of materials between blood and bone.

The Composition of Bone

A living bone does not have a single uniform composition, but instead is composed of several different kinds of tissue. To begin with, there are actually two types of bone tissue in the same bone: compact and spongy. In addition, bone is sheathed in a tough membranous tissue called the periosteum , interlaced with blood vessels. Finally, within most of the larger and longer bones of the body, as well as in the interior of the skull bones and vertebrae, are two more kinds of tissue: red marrow and yellow marrow.

Marrow

Within the spongy bone areas, red marrow produces enormous numbers of red blood cells, at a rate of millions per minute. These are needed for growth as well as for replacement of red cells, which also die in enormous numbers. Children's bones contain greater proportions of red marrow than adults’. With age, yellow marrow , composed mainly of fat cells, begins to fill the interior bone cavities formerly occupied by red marrow.

Calcium

Bone also serves as a storage and distribution center for one of the most important elements in our body. Calcium, in the form of calcium phosphate, is the basic chemical of bone tissue, but this element also must always be present in the bloodstream at a certain level to ensure normal heartbeat, blood clotting, and muscle contraction. When the calcium level in the blood is deficient, the bones release some calcium into the bloodstream; when the blood has a surplus of calcium, the bones reabsorb it.

Fractures

Like most other tissues, broken bone can repair itself, and it is a remarkable process to observe. It is a process, however, that will proceed even if the ends are not aligned or set—an important reason why any suspected fracture should be checked by a physician.

A break in a bone causes a sticky material to be deposited by the blood around the broken ends. This material begins the formation of a kind of protective, lumpy sleeve, called a callus , around the broken ends. Mainly cartilage, the callus hardens into spongy bone, normally within a month or two. Then, the spongy bone begins to be reduced in size by bone-dissolving cells produced in the marrow, while at the same time the spongy bone in the area of the break is beginning to be replaced by hard bone.

Depending on the particular bone involved and the severity of the fracture, the broken bone can be completely healed within four to ten months.