Bones can grow in length, at least until an animal reaches maturity. Growth in length occurs at a growth plate (see Fig. 37.18), a region of cartilage between the middle region, called the diaphysis, and the end, called the epiphysis. The growth plate remains in place when the rest of the cartilage is transformed into bone. The growth plate adds new cartilage toward the bone’s diaphysis, enabling bone length to continue to increase after birth (see Fig. 37.18). At maturity, each growth plate fuses as the remaining cartilage is replaced with bone, preventing further growth in length. The fusion of growth plates is typical of most mammals and birds. In amphibians and reptiles, the growth plates often do not fuse and growth may continue at a slower pace over much of their lifetime.

Limb bones grow in diameter when osteoblasts deposit new bone on the bone’s external surface. At the same time, bone is removed from the inner surface, expanding the marrow cavity. Bone removal is slower than bone growth, thickening the walls during growth. Bone is removed from the marrow cavity by a group of cells called osteoclasts that secrete digestive enzymes and acid to dissolve the calcium mineral and collagen. These dissolved compounds are reabsorbed and recycled for bone formation in other regions of the skeleton. Through this process, the vertebrate skeleton serves as an important store of calcium and phosphate ions. For example, in female birds, calcium removed from the skeleton is regularly used to form the eggshell.

A great advantage of endoskeletons compared with exoskeletons is that a damaged endoskeleton can be repaired by osteoblasts and osteoclasts forming and removing mineralized tissue in particular regions of the bone. Generally, physically active younger adults have thicker bones than less active younger adults because physical loading stimulates osteoblasts to produce more bone during growth. However, in older adults, bone tissue is gradually lost as osteoclasts remove more bone than is produced by osteoblasts. Bone loss is particularly severe in women after menopause, in part because of hormone shifts as well as reduced physical activity, and can lead to osteoporosis, a condition that significantly increases the risk of bone fracture.