Unlike bone, cartilage does not contain blood vessels. Its cells rely on the diffusion of nutrients from the tiny blood vessels in surrounding tissues. Because cartilage is dense and fibrous, it can support weight despite its extreme flexibility.

Cartilage is gradually replaced by bone during the process of bone formation called ossification (ahs uh fih KAY shun). Ossification begins up to seven months before birth. Bone tissue forms as cells called osteoblasts secrete mineral deposits that replace the cartilage in developing bones. As bone tissue completes its development, most osteoblasts mature into osteocytes. Osteocytes help to maintain the minerals in bone tissue and continue to strengthen the growing bone.

Many long bones, including those of the arms and legs, have growth plates at either end. The growth of cartilage at these plates causes the bones to lengthen. Gradually, this cartilage is replaced by bone tissue, and the bones become larger and stronger. During late adolescence or early adulthood, growth plates become completely ossified, and the person “stops growing.” Cartilage remains in those parts of the body that are flexible, such as the tip of the nose and the external part of ears. As you will read later, cartilage also cushions the areas where bones meet, such as in the knee.

Bone Remodeling and Repair In many ways, a bone is never finished growing. Bones are remodeled throughout life by small numbers of osteoblasts, which continue to build bone tissue, and osteoclasts—cells that break down bone minerals. Both functions are important because they enable bones to remodel and strengthen in response to exercise and stress. Without the continuous breakdown of old bone tissue and buildup of new bone tissue, bones would become brittle and weak. Both types of cells work together to repair broken and damaged bones.

Some older adults, especially women, develop a disorder called osteoporosis. In osteoporosis, osteoclasts break down bone much faster than osteoblasts rebuild it. Osteoporosis leads to weak bones due to excessive decrease in bone density. Research suggests that consuming plenty of calcium and performing weight-bearing exercise such as walking could help to prevent this serious problem.

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