Chapter 7: Cartilage

Cartilage SUMMARY OF KEY POINTS

• Cartilage is a tough, resilient type of connective tissue that structurally supports certain soft tissues, notably in the respiratory tract, and provides cushioned, low-friction surfaces in joints.

• Cells of cartilage, chondrocytes, make up a small percentage of the tissue’s mass, which is mainly a flexible mass of extracellular matrix (ECM).

• Chondrocytes are embedded within lacunae surrounded by the ECM.

• Cartilage ECM typically includes collagen as well as abundant proteoglycans, notably aggrecan, which bind a large amount of water.

• Cartilage always lacks blood vessels, lymphatics, and nerves, but it is usually surrounded by a dense connective tissue perichondrium that is vascularized.

• There are three major forms of cartilage: (1) hyaline cartilage, (2) elastic cartilage, and (3) fibrocartilage.

• The ECM of hyaline cartilage is homogenous and glassy, rich in fibrils of type II collagen and aggrecan complexes with bound water.

• The ECM has less collagen and more proteoglycan immediately around the lacunae, producing slight staining differences in this territorial matrix.

• Chondrocytes occur singly or in small, mitotically derived isogenous groups.

• Perichondrium is usually present, but not at the hyaline cartilage of articular surfaces or the epiphyses of growing long bones.

• Elastic cartilage generally resembles hyaline cartilage in its chondrocytes and major ECM components, but its matrix includes abundant elastic fibers, visible with special stains, which increase the tissue’s flexibility.

• Elastic cartilage provides flexible support for the external ear as well as certain structures of the middle ear and larynx; it is always surrounded by perichondrium.

• Fibrocartilage contains varying combinations of hyaline cartilage in small amounts of dense connective tissue.

• Histologically, it consists of small chondrocytes in a hyaline matrix, usually layered with larger areas of bundled type I collagen with scattered fibroblasts.

• Fibrocartilage provides very tough, strong support at tendon insertions and in intervertebral discs and certain other joints.

• All forms of cartilage form from embryonic mesenchyme.

• Cartilaginous structures grow by mitosis of existing chondroblasts in lacunae (interstitial growth) or formation of new chondroblasts peripherally from progenitor cells in the perichondrium (appositional growth).

• Repair or replacement of injured cartilage is very slow and ineffective, due in part to the tissue’s avascularity and low metabolic rate.