Cartilage is a type of dense connective tissue. It is a tough, semitransparent, flexible tissue that is composed of cartilage cells (chondrocytes) and tough fibers that are surrounded by a dense material made of fats and protein (sort of like a fruit salad suspended inside a bowl of Jell-O). Among the many tissues affected by OA, cartilage is the most seriously damaged.
Chondrocyte
A cartilage cell.
In the joints, cartilage covers the surface of the bones and is referred to as articular cartilage. A dense fibrous membrane called the perichondrium covers this cartilage. The perichondrium helps to protect the cartilage from wear by allowing one bone to slide over another easily, which reduces friction and prevents damage. Additionally, the thick layer of cartilage found in weight-bearing joints, such as the hips and knees, acts as a shock absorber. In fact, cartilage can resist compressive forces up to 65 times body weight, which also helps to prevent injuries to the bones. Articular cartilage lacks an arterial blood supply and venous and lymphatic drainage. These additional tissues would compromise the cartilage's elasticity and toughness. Given that it does not have its own blood supply, the cartilage derives its nutrition primarily from the surrounding synovial fluid and, to a lesser extent, from the blood supply of the adjacent bone. As a consequence of this lack of vasculature, cartilage, once damaged, does not heal readily.
Articular cartilage
Tough, rubbery tissue that forms the surface of bones within joints.
Synovial fluid
A lubricating fluid secreted by the synovial membrane.
Cartilage's toughness and flexibility make it an ideal tissue for lining joints and providing mechanical support to many tissues in the body. For example, it forms part of the structure of the skeleton in the ribs, where it joins them to the breastbone (sternum). Cartilage is found in the tip of the nose, in the external ear, and in the walls of the windpipe (trachea) and the voice box (larynx), where it provides both support and shape. In a human embryo, the entire skeleton is made of cartilage. Even after we are born, many of our bones are little more than cartilage. As we grow and develop, however, these structures absorb calcium and phosphate, causing the bones to become hard and inflexible.