Cartilage

Cartilage is a firm but bendable connective tissue. Its cells, called chondrocytes, sit in small pockets known as lacunae inside a rubbery matrix of collagen fibers and proteoglycans, large sugar-rich molecules built around chondroitin sulfate that hold water. Most cartilage is wrapped in a layer of dense connective tissue called the perichondrium.

There are three main types. Hyaline cartilage, the most common, covers the ends of bones inside joints and forms the rib connections, the nose and the rings of the airways. Fibrocartilage, packed with thick bundles of collagen, resists squeezing in the intervertebral discs and the menisci of the knee. Elastic cartilage contains elastic fibers that let the outer ear bend and spring back into shape.

Cartilage has no blood vessels, so nutrients must diffuse through the matrix to reach its cells, which is why damaged cartilage heals so slowly. In the embryo, hyaline cartilage forms a model of most of the skeleton that is later replaced by bone, and plates of cartilage near the ends of long bones let them keep growing until early adulthood.