Smooth, nerveless tissue that keeps joints quiet
Cartilage is resilient, semi-transparent connective tissue, usually wrapped in fibrous perichondrium. It caps long-bone ends as articular padding, props open airways with tracheal rings, and comes in three flavours—hyaline, elastic, and fibrocartilage—built by chondrocytes that never enjoy a direct blood supply.
Chondrocytes pour out collagen-rich matrix packed with proteoglycans and elastin. Because vessels and nerves generally skip this tissue, it feels nothing; nutrition arrives by diffusion, helped when joint compression or elastic bending pumps fluid through the matrix. Knee menisci are a partial exception among fibrocartilage, holding some blood supply at the margins.
Rigidity lets cartilage hold tubes open—the cricoid and carina among tracheal examples. In the embryo, mesoderm-derived mesenchyme condenses and chondrifies: chondroblasts secrete aggrecan and type II collagen until a cartilaginous scaffold exists for much of the early skeleton.
After the embryo stage, cartilage grows mostly by maturing rather than by adding mass, because its cells divide very slowly. Its behaviour under load comes from the matrix: aggrecan, the main proteoglycan, clumps with hyaluronan into negatively charged aggregates that trap water. Testing that wet, viscoelastic material is awkward, so researchers use confined compression in creep or relaxation mode, pressing a disc inside a sealed fluid-filled chamber; the aggregate modulus of articular cartilage typically lands between 0.5 and 0.9 megapascals. Indentation tests with tips under 0.8 millimetres can take hours to settle. Cartilage also works as a graded bridge between soft tissue and bone: layers near bone are stiffer and mineralised with hydroxyapatite, with collagen fibres anchored straight into it, which spreads stress that would otherwise concentrate at a sharp interface. Osteoarthritis and ageing usually strike the soft, slippery superficial zone first, and the glycoprotein lubricin, abundant in cartilage and joint fluid, helps guard against that wear.
Source: Cartilage
