Cartilage — Types, Histology, Locations, Growth and Link to Ossification

Cartilage resists compression, adds resilience to bone and supports regions where flexibility is needed. Its only cell is the chondrocyte, sitting in a space called a lacuna inside a matrix of fibres, proteoglycans and glycosaminoglycans. It is derived from mesoderm by chondrogenesis; mesenchymal cells become chondroblasts that secrete the matrix, chiefly aggrecan and type II collagen.

• Avascular: chondrocytes are fed by diffusion from adjacent tissue, helped by compressive loading. This is why cartilage heals slowly.
• Aneural: cartilage has no nerve supply. Pain in cartilage disease, for example osteoarthritis, comes from surrounding joint and bone.
• No lymphatics.
• Matrix is not calcified in the normal state: unlike bone, its matrix has no calcium; it is rich in chondroitin, which gives elasticity. When cartilage calcifies, the chondrocytes die and bone-like tissue replaces it.
• Perichondrium: a fibrous membrane around most cartilage; it is absent in articular cartilage and fibrocartilage.

Cartilage has limited ability to regenerate. The perichondrium is not efficient at regenerating cartilage, and chondrocytes divide little after the initial chondrification, so the size and mass of cartilage change little in adults.

Hyaline cartilage is the most abundant type. It gives a resilient, minimal-friction surface and resists compressive forces where bones articulate. With age it becomes drier, thinner and more yellow. It forms the template of most fetal bones, and the epiphyseal growth plate is hyaline cartilage.

Articular cartilage is a subset of hyaline cartilage lining joint surfaces, with no perichondrium. Tissue fluid forms 65–80% of its weight, and type II collagen makes up most of the collagen; proteoglycans retain water and give compressive stiffness. It is organised into four zones: superficial, middle, deep and calcified. The deepest, calcified zone lies next to the subchondral bone.

• Articular cartilage depends on synovial fluid for nutrition; fibrocartilage is fed by diffusion from nearby connective tissue and vessels; elastic cartilage by its perichondrium.
• Osteoarthritis thins and wears the articular cartilage, giving 'bone-on-bone' contact, reduced range of motion and pain.

Elastic cartilage is dull yellow and is found in the larynx, ear, epiglottis and Eustachian tube. It has a perichondrium and gives flexibility together with resistance to pressure. Its matrix contains a dense, branching network of elastic fibres supported by type II collagen; the fibres are highlighted by special stains such as orcein, Verhoeff's stain or resorcin-fuchsin, which make elastic cartilage easy to separate from hyaline cartilage on a slide.

Fibrocartilage is rich in type I collagen and has much less proteoglycan than hyaline cartilage, so it resists high tension and compression. It has no perichondrium. It occurs in tendons, ligaments, intervertebral discs, menisci and some articular surfaces, and forms part of the enthesis, the junction of tendon or ligament with bone.

Disc herniation: the outer ring of the intervertebral disc, the annulus fibrosus, is fibrocartilage. When its structure fails (degeneration, lifting injury), the nucleus pulposus can herniate into the spinal canal and press on nerves. Fibrocartilage also forms the menisci and the triangular fibrocartilage complex of the wrist; see knee ligament injuries.

Cartilage is a simple tissue of one cell type in a deformable matrix. It grows by interstitial growth: chondrocytes divide inside the matrix and secrete more matrix around themselves, so the tissue expands from within. It can also grow by appositional growth, recruiting new cells from the surrounding tissue (the perichondrium), which differ from bone, where growth is only by apposition on free surfaces.

Cartilage heals poorly because it is avascular, nutrients arrive only by diffusion, the chondrocytes divide little after chondrification, and the perichondrium is not efficient at regenerating cartilage. The recovery after injury is slow.

Bone forms in two ways. Intramembranous ossification converts mesenchyme directly into bone and forms the flat bones of the skull and the clavicle. Endochondral ossification first builds a hyaline cartilage model, later replaced by bone, and forms the remainder of the axial skeleton and the long bones.

1. Mesenchymal cells become chondrocytes and form a hyaline cartilage model with a perichondrium.
2. Chondrocytes at the centre hypertrophy and add collagen X and fibronectin, which allows the matrix to calcify.
3. Calcification cuts off nutrients, chondrocytes undergo apoptosis, and blood vessels invade the empty spaces, bringing osteogenic cells; the perichondrium becomes periosteum.
4. Osteoblasts lay down a periosteal collar of compact bone: the primary ossification centre in the diaphysis.
5. After birth, secondary ossification centres form in the epiphyses; cartilage persists as the epiphyseal growth plate and as articular cartilage.

Other embryology links are covered in pharyngeal arches and pouches (laryngeal and ear cartilages arise from arch derivatives) and germ layers (cartilage is mesodermal).