Chondrocytes form a temporary cartilaginous model by dividing and secreting a thick gel-like extracellular matrix. Once the chondrocytes undergo programmed cell death, osteoblasts enter the site of the cartilaginous model. The process of replacing the temporary cartilaginous model with bone in an ordered manner is called endochondral ossification. In endochondral ossification, not all of the cartilage is replaced by bone tissue. Some cartilage that performs a protective and supportive function remains intact.
Cartilage protects the ends of long bones. It also maintains the shape of flexible body parts such as the external ears and the larynx. These body parts contain a type of cartilage called elastic cartilage. The second type of cartilage, hyaline cartilage, is present in organs such as the nose, ends of ribs, and the ends of bones that make up the joints of the skeleton. Hyaline cartilage reduces friction between bones and provides flexibility to the joints. The third type of cartilage, fibrous cartilage, is found in tissues such as intervertebral discs and ligaments.
Cartilage can be damaged through wear and tear, such as sports injuries or osteoarthritis. People with damaged cartilage experience pain in joints, swelling, or stiffness. Cartilage is avascular, so nutrients must diffuse into the tissue; therefore, cartilage injuries often take time to heal. Additionally, cartilage is incapable of regeneration in contrast with bones that continuously undergo remodeling.
A mature bone consists of four types of tissues. The hard outer part of the bone is the compact tissue. Below that exists a sponge-like tissue called the cancellous tissue. The tissue at the ends of the bones, protected by cartilage, is called the subchondral tissue. Mechanical stress to a bone, such as a fracture, heals in several stages, which include hematoma formation, fibrocartilaginous callus formation, bony callus formation, and bone remodeling. The last stage, bone remodeling, is the longest and is where endochondral ossification occurs.
