Cartilage: Composition and Repair

Joint cartilage consists of a matrix of several types of compounds suspended in matrix of water, as well as several other substances, including hyaluronate, dermatan sulfate, keratin sulfate, chondroitin, proteoglycans, and collagen. This is a hard but nearly transparent mixture of substances that are very strong, yet provide the bones what an ability to move in every direction, without wearing out due to friction. Joint cartilage allows the bones the ability to move in any direction required, but also to maintain space between the bones, and also to absorb shown due to gravity, and lifting and moving objects in space. The concentrations of these substances in our individual joints will vary, depending on the requirements for that particular joint. For example, the joints (intervertebral discs) that separate the spinal bones in our back are designed to be somewhat flexible and malleable. These discs are designed to collapse slightly when force is applied to out spine, such as when we jump down from a high point to a lower point. This slight bulge that occurs in the spine when a force is applied to it protects our body and spine by acting as a shock absorber. Other joints are far less flexible and their main job is separating articulating bones from one another.



Though the overall composition of cartilage includes between the substances above suspended in a solution composed of between 65-80% water, it is solid and hard. Two of the main cellular materials of cartilage - proteoglycans and collagen, are produced by chondrocytes, which are the cells involved in creating new joint cells. The chondrocytes that produce new joint tissue are located in the synovial tissue of the joint.

There are three different types of cartilage: fibrocartilage, hyaline cartilage, and elastic cartilage. Hyaline cartilage, also known as articular cartilage, is the type of cartilage that the joints of our body are composed of. Articulations are the space between two bones. These joint articulations provide support so that the bones do not pull away from each other, and also protection to that our bones don't become damaged as a result of rubbing or sliding together. Take for example, the long bones of our arm, the radius and the ulna. These two bones articulate with one another near the wrist, and closer to their attachment to our humerus of our upper arm. Without the joint spaces where the radius and ulna articulate, these bones would be prone to osteoarthritis and bone spurs as a result of the friction due to bone on bone friction.

The components of a healthy joint are composed of the following structures from the outside of the body, inwards: Skin, subcutaneous tissue, ligament, synovial membrane, synovial fluid, articular cartilage, meniscus, and bone. The structures from the synovial membrane to the deeper structures are highly vascular, which is one of the reasons that joint injuries are to treat. In order for unhealthy or damaged joints to be able to repair themselves, worn out cells must be broken down, and their used components must be absorbed back into the body. These worn out cells and tissues must be replaced by new and healthy cells and tissues.