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Bone Remodeling

Bone remodeling involves resorption (the removal of old bone tissue) and ossification (the formation of new bone). Cells called osteoclasts resorb calcified bone matrix, leaving a collagen framework exposed. Cells called osteoblasts perform ossification by secreting new bone matrix onto the collagen framework. This matrix becomes calcified. An osteoblast may become completely surrounded by bone matrix it has secreted. As this happens, it matures into an osteocyte, which maintains the bone tissue. Remodeling is important in the homeostasis of blood calcium levels. When blood calcium levels are low, calcium is harvested from bone tissue. When blood levels are high, excess calcium is deposited into bone tissue. The remodeling activity of osteoclasts and osteoblasts allows growth of bone in length and width, as well as changes overall shape, especially in resistance to stress. During childhood and adolescence, ossification occurs faster than resorption so bones grow larger. Long bones are typically growing in length as well. During the early to middle years of adulthood, ossification and resorption occur at about the same rate. Between the ages of 35 and 40, resorption starts to occur faster than ossification. Inactivity and lack of exercise cause increased bone resorption, which may lead to osteoporosis. When bones are stressed through exercise such as walking, jogging, and other weight-bearing exercises, ossification increases, which can help prevent osteoporosis.