By the time an individual reaches adulthood, the number of bones in the body totals about 206.
Structure of Bones
Bone is living tissue that is constantly being renewed throughout life. Three types of bone cells take part in this process: osteoblasts, osteocytes, and osteoclasts (osteon is the Greek word meaning "bone").
Osteoblasts are the principal bone-building cells. They produce hard calcium compounds and flexible collagen (a fibrous protein), which combined form the nonliving part of bone called the bone matrix. The matrix makes bone strong, hard, and slightly elastic. In the process of forming the bone matrix, osteoblasts become trapped in it. Once they are trapped, they develop into osteocytes or mature bone cells. Osteocytes help maintain the hard bone tissue by removing and replacing the calcium compounds in the matrix. In mature adults (whose bones are no longer growing), osteocytes are the most numerous bone cells. Finally, osteoclasts are the bone-destroying cells. They break down bone matrix, releasing calcium and phosphate ions into the blood (this is important when blood calcium levels drop below normal).
About 98 percent of the calcium and 90 percent of the phosphorus in the body are stored in bones and teeth.
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