Bone Remodeling - The Basics Of Bone: Osteoporosis

Content provided by the Faculty of the Harvard Medical School

Bone remodeling

Calcium is the principal currency of bone. It carries a lot of weight in the body, both literally and figuratively. Not only is it the major component of the cement-like hydroxyapatite, but it also plays an important role in other body systems.

Buoyed along in the blood, lymph, and other fluids, calcium bustles in and out of cells, transmitting signals to nerves and muscles. It is vital to many physical processes, including heart rate, blood pressure, and the regulation of the internal organs. However, the amount of calcium required to do this is slight — only about 1% of your body's total calcium stores. The rest — about 2.25 to 4.5 pounds' worth — is sequestered in your bones. When the supply of calcium in the blood drops below the critical level, it can be replenished from bone.

Tapping and replenishing calcium stores

The process by which calcium is released from bone is known as resorption. It is executed by osteoclasts — scavenger cells somewhat similar to those in the immune system. However, instead of gobbling up bacteria and cellular debris, osteoclasts go after intact bone. Their saw-toothed membranes enable them to attach to the surface of bone, and they are equipped with acids to dissolve hydroxyapatite and with enzymes to break down collagen and other proteins (see Figure 2). As they eat away at bone, they regurgitate the freed proteins and minerals into the bloodstream for reuse in other parts of the body. This recycling effort leaves tiny tunnels in the bone.

Figure 2: The cycle of bone construction and demolition Bone is constantly being constructed and demolished. During resorption (A), cells known as osteoclasts break down bone, releasing calcium into the bloodstream. The trenches that are left behind (B) are then filled in by construction cells known as osteoblasts. The osteoblasts release collagen into these troughs and eventually evolve into structural bone cells, or osteocytes (C). Once these osteocytes mix together with calcium, phosphate, and other minerals to form a cement-like substance known as hydroxyapatite, the process of replacing the lost bone is complete (D).

Resorption is coupled with another process known as formation, which is carried out by construction cells called osteoblasts. These cells move into the tunnels left by the osteoclasts and release strands of collagen into the void. Eventually, they become trapped in the web they have woven. Held by these moorings, they evolve into structural bone cells, or osteocytes.

Calcium, phosphate, and other minerals carried in the bloodstream also accumulate in the web. The minerals coalesce into the crystalline hydroxyapatite, and the formation process is complete. The bone that was removed has been fully replaced.

Other key players

This remodeling process not only liberates calcium, but also maintains the skeleton by replacing old bone with new. This important task in the body's housekeeping scheme requires more than osteoclasts and osteoblasts; a host of hormones work quietly behind the scenes to influence the behavior of cells.

Parathyroid hormone (PTH), which is secreted by small islands of tissue near the thyroid gland, is a primary force in resorption. Parathyroid hormone is secreted when the level of calcium in the blood falls below the amount needed by the body's cells. This hormone helps restore the appropriate levels of calcium in the blood in several ways. It promotes the absorption of calcium by the digestive system and slows the excretion of calcium into the urine. It also stimulates osteoclasts to break down bone to release calcium into the blood. When the calcium level in the blood is adequate, the production of parathyroid hormone falls.

It takes a sizeable squad of other hormones and substances to carry out bone formation. For example, vitamin D, which is actually a hormone, plays a pivotal role, limiting withdrawals of calcium from bone by promoting calcium absorption from the intestines.

A delicate balance

Although formation and resorption go hand in hand, the processes occur at different rates. As with any remodeling project, demolition is always speedier than reconstruction. So to preserve the skeleton, it's necessary to have more building sites than demolition projects. For most adults in their 30s, when bone mass is at its peak but no longer increasing, about 1% of bone is undergoing resorption and about 4% is under formation at any given moment.

Last updated:	January 23, 2007