Bone remodeling is a continuous process of bone resorption and formation for the purpose of maintaining normal bone mass. Normal bone mass indicates healthy bones that are strong and free from problems like osteoporosis. This process goes on inside the human body as long as the person is living. Cells that play important roles in it are osteoclasts, which are responsible for bone resorption; osteoblasts, which are vital in the formation of bones; and osteocytes, which send the signals that bones are being exposed to stress or injury.
Constant remodeling allows bones to perform their many functions, including structural support to the whole body and important storage sites of calcium. With bone remodeling, the body is also able to repair small bone fractures that occur from daily physical activities. Old bone is being replaced by new bone during the remodeling cycle. In adults, this occurs at a rate of about 10% each year. This is a natural process to ensure maintenance of normal bone mass as a person ages.
The remodeling cycle usually starts when injury or mechanical stresses occur in bones. Growth hormones stimulate the production of osteoclasts, which then release enzymes capable of dissolving the bone matrix, creating pits in most bone surfaces. Their lifespan is approximately two weeks, and then they die naturally through a programmed process of cell death, or apoptosis.
Osteoblasts are stimulated by growth hormones as well. They are responsible for filling the pits created by osteoclasts in bone surfaces. As the bone matrix thickens, osteoblasts incorporate minerals such as calcium and phosphorus into the bone in a process known as mineralization. After their lifespan of about three months, most mature into osteocytes, which reside mostly in the bone matrix and are the ones that give signals of mechanical stress and injury to growth hormones. Other osteoblasts become lining cells in the bone surfaces and are responsible for the release of calcium into the blood stream, while still others die naturally.
The actions of osteoclasts, osteoblasts, and osteocytes are regulated by prohormones and hormones inside the body. These include vitamin D, parathyroid hormone (PTH), calcitonin, testosterone, and estrogen. Any derangement in the actions of these chemicals can lead to certain medical conditions. For example, some studies indicate estrogen deficiency in menopausal women can cause a delay in cell death of most osteoclasts, thus exposing bones to their enzymatic actions longer and promoting osteoporosis.