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Bone is a living tissue that is constantly being broken down and replenished. It is made mostly of collagen (a protein that provides a soft framework) and calcium phosphate (a mineral that adds strength). Almost all of the calcium in the body is stored in the bones and teeth.

What Is Bone?

Made mostly of collagen, bone is living, growing tissue. Collagen is a protein that provides a soft framework, and calcium phosphate is a mineral that adds strength and hardens the framework. This combination of collagen and calcium phosphate makes bone strong and flexible enough to withstand stress.
More than 99 percent of the body's calcium is contained in the bones and teeth. The remaining 1 percent is found in the blood.


There are two types of bone found in the body: cortical and trabecular. The cortical type is dense and compact. It forms the outer layer of the bone. The trabecular type makes up the inner layer of the bone and has a spongy, honeycomb-like structure.


Throughout life, bone is constantly renewed through a two-part process called remodeling. This process consists of resorption and formation. During resorption, old bone tissue is broken down and removed by special cells called osteoclasts. During bone formation, new tissue is laid down to replace the old. This task is performed by special cells called osteoblasts. Osteoclast and osteoblast function is regulated by several hormones, including calcitonin, parathyroid hormone, vitamin D, estrogen (in women), and testosterone (in men), among others.
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Last reviewed by: Arthur Schoenstadt, MD
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