Bones

Bones are rigid structures that form the body's skeleton, or framework. Contrary to some people's understanding, bone is a dynamic structure composed of both living tissues, such as bone cells, fat cells, and blood vessels, and nonliving materials, including water and minerals. Throughout life, bone tissue undergoes continual breakdown and restoration in response to the body's demands. During childhood and adolescence, much more bone tissue is deposited than broken down, so the skeleton grows in size and strength. During early adulthood, breakdown slowly begins to exceed deposits. As a person ages, bone tissue is gradually depleted and bones become less dense.

Bone Fact: An adult human has 206 bones, which account for 14 percent of the body's total weight. The longest and strongest bone is the thighbone, which at maturity is about 50 cm (20 in) long and 2.5 cm (1 in) wide. The smallest bone, the stirrup bone, is one of three tiny bones buried within the middle ear; it is only 0.18 cm (0.07 in) long.

More than 300 bones are present in an infant, several of which fuse as the infant matures.

Cartilage

Made up of water (70%); collagen (15%); chondrocytes (5%); and proteoglycans (10%) cartilage is a translucent, tough, flexible and elastic connective tissue that acts as a cushion or shock absorber when you put weight on the joints. Its smooth, slippery surface also allows the bone ends to move freely.

• Water helps absorb shocks and jolts and contributes to cartilage lubrication and nourishment
• Collagen provides strength and provides the framework for other cartilage components
• Living cells called chondrocytes build cartilage by producing collagen and proteoglycans (amino sugars). They also release enzymes that break down ageing collagen and proteoglycans
• Proteoglycans comprise both glucosamine sulphate and chondroitin sulphate which stimulate the chondrocytes to make collagen and more proteoglycans. These have a high propensity to attract and hold water which can be squeezed out into the joint space to assist in lubrication. The water is reabsorbed when the load is reduced.