Cartilage is made primarily of water, collagen fibers, and large sugar-protein molecules called proteoglycans, all produced and maintained by a small population of specialized cells called chondrocytes. Water alone accounts for 65% to 80% of cartilage’s total weight. The remaining solid material is a dense mesh of proteins and sugars that gives cartilage its signature combination of firmness and flexibility.
The Extracellular Matrix
About 90% of cartilage tissue is extracellular matrix, the structural scaffolding that surrounds and supports the cells living inside it. Only 5% to 10% of the tissue is actual cells. That means cartilage is overwhelmingly a material, not a living mass of cells like muscle or liver tissue. The matrix is what does the mechanical work of cushioning joints, supporting airways, and shaping your nose and ears.
Three main ingredients make up this matrix: water, collagen, and proteoglycans. Each plays a distinct role.
Water
Water is the single largest component by weight. Near the surface of joint cartilage, water makes up roughly 80% of the tissue. Deeper down, closer to the underlying bone, it drops to about 65%. This water isn’t just sitting there. It moves in and out of the cartilage when you load and unload your joints (walking, standing, sitting), which helps distribute pressure evenly and ferry nutrients to the cells inside.
Collagen
Collagen provides the structural framework. In most cartilage, the dominant form is type II collagen, which makes up roughly 10% to 20% of the tissue’s wet weight but forms the backbone of the entire solid structure. These collagen fibers are arranged in a dense, crisscrossing network that resists stretching and gives cartilage its tensile strength. Smaller amounts of other collagen types (IX and XI) weave through the matrix, helping to stabilize and cross-link the primary fibers.
Proteoglycans
Proteoglycans are the third key ingredient, and they’re responsible for cartilage’s ability to resist compression. Each proteoglycan molecule has a core protein with long chains of sugar molecules (called glycosaminoglycans) attached to it, giving it a bottle-brush shape. These sugar chains carry a strong negative electrical charge, which attracts and holds water. When you press down on cartilage, that trapped water resists being squeezed out, creating a cushioning effect. The main proteoglycan in cartilage is aggrecan, which binds to a backbone of hyaluronic acid to form enormous molecular complexes.
Chondrocytes: The Cells Inside
The cells responsible for building and maintaining all of this matrix are chondrocytes. They occupy only about 10% of the total tissue volume in adult joint cartilage, sitting alone or in small clusters within tiny pockets called lacunae. Despite their small numbers, chondrocytes do all the heavy lifting metabolically. They continuously produce collagen, proteoglycans, and other matrix proteins, and they break down old or damaged components so the tissue can renew itself.
Chondrocytes originate during embryonic development from two sources. Most come from the mesoderm, the middle layer of the embryo that also gives rise to muscle and bone. But in the head and face, chondrocytes arise from a different cell population called the neural crest. Once mature, chondrocytes are relatively isolated. They sit far apart from one another, surrounded by the matrix they created, and have limited ability to migrate or multiply quickly. This is one reason cartilage repairs itself so poorly after injury.
Three Types of Cartilage
Not all cartilage is identical. Your body makes three types, each with a slightly different recipe suited to its job.
Hyaline cartilage is the most common type. It covers the ends of bones inside joints (where it’s called articular cartilage), forms the rings that hold your windpipe open, and shapes much of your nose. Its matrix is rich in type II collagen and aggrecan, giving it a smooth, glassy appearance. This is the cartilage most people picture when they hear the word.
Elastic cartilage contains the same basic ingredients as hyaline cartilage but adds a dense, threadlike network of elastic fibers woven through the matrix. These elastic fibers allow the tissue to bend and snap back to its original shape. You’ll find elastic cartilage in your outer ear, the epiglottis (the flap that covers your windpipe when you swallow), and parts of the ear canal.
Fibrocartilage is the toughest variety. It has a higher proportion of thick collagen fibers (including type I collagen, the same kind found in tendons and ligaments) and fewer proteoglycans than hyaline cartilage. This makes it better at resisting heavy loads and shearing forces. Fibrocartilage forms the discs between your spinal vertebrae, the menisci in your knees, and the ring around the hip socket.
How Cartilage Gets Its Nutrients
One of cartilage’s most distinctive features is that it has no blood vessels. Nutrients reach the chondrocytes mainly through diffusion from the surrounding fluid. In joints, that fluid is synovial fluid, the slippery liquid that fills the joint capsule. Small molecules like oxygen and glucose slowly seep from the fluid into the matrix and eventually reach the cells.
Movement helps this process. When you walk, bend, or bear weight, the cartilage compresses slightly and then rebounds, pushing fluid out and drawing it back in like a sponge. This pumping action speeds up nutrient delivery and waste removal. It’s one reason regular physical activity is important for cartilage health, and why prolonged immobility can be harmful to joints.
In cartilage outside of joints (like in the ribs or trachea), a thin outer membrane called the perichondrium supplies blood vessels that feed nutrients into the tissue’s outer layers.
How Cartilage Grows
Cartilage expands through two mechanisms. Interstitial growth happens when chondrocytes already inside the tissue divide and produce new matrix between themselves, pushing the tissue outward from within. This is the primary growth method during childhood and adolescence. Appositional growth occurs when new cells on the outer surface of the cartilage lay down fresh layers of matrix on top of existing tissue, adding thickness from the outside in. Both processes slow dramatically after skeletal maturity.
Why Cartilage Is Hard to Repair
The same features that make cartilage an effective cushion also make it difficult to heal. Without blood vessels, the tissue can’t mount a normal inflammatory response or deliver repair cells to a damaged area the way skin or bone can. Chondrocytes are sparse and relatively inactive in adults, so they can’t quickly ramp up production to fill a defect. And the dense matrix itself acts as a physical barrier, preventing cells from migrating to the injury site.
Small surface damage to joint cartilage often goes unrepaired entirely. Deeper injuries that reach the underlying bone can trigger a partial healing response because blood from the bone marrow reaches the defect, but the tissue that forms is typically fibrocartilage rather than the original hyaline cartilage. Fibrocartilage is functional but less smooth and less durable, which is why cartilage injuries can lead to long-term joint problems.