III.

Development and Maintenance

During the early development of a baby within its mother’s body, the skeletal structure consists of cartilage. At about the eighth week of fetal development, calcium and phosphorus salts begin to deposit around the cartilage. At 40 weeks of development, however, the fetal bones still consist primarily of soft cartilage. The skull consists of several cartilage plates that are not completely joined. The spaces between the cartilage plates are called soft spots, or fontanels. The soft cartilage and the fontanels enable the skull to be compressed as it passes through the birth canal. During childhood, cartilage gradually is replaced by bone through the activity of osteoblasts. More than 300 bones are present in an infant, several of which fuse as the infant matures.

Throughout life, bone tissue undergoes continual breakdown and restoration in response to the body’s demands. For example, calcium must always be present in blood at a certain concentration. If calcium blood levels drop, cells called osteoclasts break down bone to release calcium into the bloodstream. If exercise increases muscle mass, bones must thicken so that the pull of the stronger muscle does not break the bone. In this case, osteoblasts create new bone.

During childhood and adolescence, much more bone tissue is deposited than broken down, so the skeleton grows in size and strength. During early adulthood, breakdown slowly begins to exceed deposits. As a person ages, bone tissue is depleted, and bones are weakened and increasingly susceptible to breaks. Exercise and proper diet are critical for maintaining healthy bone growth at all stages of life. Nutrients—particularly sufficient calcium, phosphorus, and vitamin D, and hormones—including growth hormone, parathyroid hormone, calcitonin, and sex hormones—all influence bone growth.

Fractures, or breaks, are very common injuries to bones. The repair process requires the interplay of several processes. About a week after a fracture occurs, cells from the periosteum invade the damaged area and produce a fibrous network. Then, other cells produce cartilage in the network. Finally, osteoblasts enter the network and convert the cartilage to bone. Complete healing may take weeks or even months, depending on the individual’s general health, age, and other factors. Some fractures are treated with a splint, a firm object that supports the area surrounding the broken bone and restricts motion. Other fractures must be completely immobilized to heal because movement can cause a new fracture in the area. These fractures may be immobilized with a cast, plastic or plaster wrapped around the area that surrounds the broken bone.