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Building Up and Breaking Down: Your Bones Day by Day

Posted Apr 24 2009 7:22am

Bone is constantly reshaping itself in a complex process of building and remodeling. You don't notice this reshaping because it happens on a microscopic level. The major players involved in bone building and remodeling are

  • The cells: Osteoblasts, osteoclasts, and osteocytes
  • The hormones: The directors of cellular function

The essential minerals: Most commonly calcium and phosphorus

Blasts, clasts, and cytes (oh my)

An intricate balance between the activities of two major cell types referred to as the osteoblast and osteoclast determine a person's total bone mass.

An easy way to remember the work of osteoblasts, osteoclasts, and osteocytes is
Osteoblasts giveth.
Osteoclasts taketh away.
Osteocytes maintaineth.

Osteoblasts are the builders and make collagen and hydroxyapatite. Some of the osteoblasts become buried in their matrix and then they are referred to as osteocytes. The rest of the osteoblasts cover the new bone's surface. Waves of osteoblasts that move into the area form new layers of bone.

Osteoclasts are larger cells whose function is to dissolve bone by acting on the mineral matrix. They make enzymes such as collagenase, which breaks down collagen. Osteoclasts also secrete various acids that can dissolve the hydroxyapatite structure.

There are a variety of signals that control the function of osteoblasts and osteoclasts. Interestingly, osteoblasts make small proteins, one of which is called OPG (osteoprotegrin). OPG can prevent osteoclasts from being activated. Osteoblasts change their shape and become buried in their matrix, connected to each other only by thin processes called canaliculi. After the osteoblasts are buried in bone, they're referred to as osteocytes. Osteocytes account for 90 percent of all cells in the skeleton.

Bone remodeling starts with resorption, which the osteoclasts orchestrate. Osteoclasts break down bone by dissolving mineral and resorbing the matrix that osteoblasts have formed.

More research into the function of these cells will undoubtedly result in new drugs to treat osteoporosis. Scientists now understand that the process of building up bone and resorption of bone is critical because abnormalities in these processes lead to bone diseases.

Fine-tuning your bones with hormones

Hormones are the directors of the entire process of keeping bone in proper balance. Many hormones contribute to the balance, but the hormones noted here are the most important.

Vitamin D

Vitamin D is a critical hormone that you need for proper bone mineralization. The body mainly absorbs it through your skin from sunlight, although you do absorb some through the stomach by way of diet and supplements.

When vitamin D is absorbed in the skin, it's an inactive hormone and requires special changes that occur in both the kidney and liver. Certain drugs interfere with vitamin D metabolism and therefore cause soft fragile bone. Vitamin D deficiency can also occur from a poor diet and lack of exposure to sunlight.

Vitamin D has many important functions in addition to its role in mineralization. For example, vitamin D helps to maintain normal blood levels of calcium by promoting calcium absorption in the intestine. Hence, vitamin D helps keep bones from becoming thin, brittle, or misshapen. An adequate amount of vitamin D in your diet or through vitamin D supplements prevents rickets in children and osteomalacia, a condition where bones are soft and brittle, in adults.

Parathyroid hormone (PTH)

Parathyroid hormone (also known as PTH) is another key director. The parathyroid gland, which is actually a set of four small glands located near your thyroid gland, produces this hormone.

PTH provides for the exquisite regulation of calcium metabolism. For example, when the serum level of calcium drops, the parathyroid gland synthesizes more hormone. PTH instructs the kidney to hold onto more calcium. It also directs how much calcium is allowed to be stored in the bone.

Basically, PTH is the traffic director of calcium, regulating how much calcium you absorb with your diet, how much calcium your kidneys secrete, and how much calcium your bones store. Osteoblasts have receptors for PTH. When these receptors are activated, the osteoblasts make less OPG. This small molecule in turn regulates the activity of osteoclasts.

Calcitonin

Calcitonin, a hormone produced by the thyroid gland, inhibits bone removal by osteoclasts, and promotes bone formation by osteoblasts. Calcitonin is also one of the older drugs used to treat osteoporosis.

Estrogen

Estrogen is a hormone that is instrumental in regulating women's menstrual cycles. Estrogen also works with the parathyroid glands to keep calcium levels in balance. The drop in estrogen levels at menopause is one of the reasons why women begin to develop osteoporosis.

Estrogen deficiency is one of the most important factors in the development of bone fragility. For some reason, estrogen deficiency results in the production of more osteoclasts and more active osteoclasts.

Testosterone

Although you may associate the hormone testosterone with men, both men and women produce testosterone. Testosterone helps maintain strong bone and muscles, and stimulates bone formation. Testosterone deficiency clearly is associated with osteoporosis.

Orchestrating bone growth with minerals

The two most important minerals your body needs to orchestrate bone growth are calcium and phosphorus. Calcium is the most common mineral found in your body and contributes to bone strength. Remember that the crystal hydroxyapatite is composed of calcium.

Osteoblasts add calcium to your bones, and doctors don't completely understand just how the crystals are formed. Osteoclasts remove calcium from your bones. In fact, an interesting tidbit: The 206 bones in your body contain about three pounds of calcium! Also, remember that when the rest of your body needs calcium, the bone tissue will supply it and the integrity of your bones may suffer as a result!

Phosphorus is the second most important mineral found in your body, because it's the other major component of hydroxyapatite. Your bones and teeth store approximately 85 percent of the phosphorus.

Phosphorus and calcium work together to build healthy bones. Your body attempts to achieve a balanced ratio of calcium to phosphorus. When this balance is disrupted, various bone diseases can result. Too little or too much phosphorus is harmful. Many other cells use phosphorus, like calcium, to keep you healthy.

Although an adequate amount of phosphorus is not only good but also essential to getting through the day, an excess amount of phosphorus can be detrimental. A diet high in phosphorus, such as a high-protein diet or a high intake of soft drinks, can decrease calcium in your bones because the excess phosphorus looks for calcium to bind to, removing it from bone if necessary. Many American diets contain too much phosphorus in sodas and not enough calcium.

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