Cartilage, a tough and durable form of supporting connective tissue, characteristically displayss an extracellular matrix (ECM) with very high concentrations of GAGs and proteoglycans, all interacting with collagen and elastic fibers. Structural features of its matrix make cartilage ideal for a variety of mechanical and protective roles within the adult skeleton and elsewhere (Figure 7–1).
Distribution of cartilage in adults.
(a) There are three types of adult cartilage associated with areas of the skeleton, particularly in joints and where pliable support is useful, as in the ribs, ears and nose. Cartilage support of other tissues throughout the respiratory tract is also prominent. The photomicrographs show the key features of (b) hyaline cartilage, (c) elastic cartilage and (d) fibrocartilage. Dense connective tissue of perichondrium appears here with hyaline and elastic cartilage.
Cartilage ECM has a firm yet flexible consistency that allows the tissue to bear mechanical stresses without permanent distortion. In the respiratory tract, ears and nose, cartilage forms the framework supporting softer tissues. Because of its resiliency and smooth, lubricated surface, cartilage provides cushioning and sliding regions within skeletal joints and facilitates bone movements. As described in Chapter 8, cartilage also guides development and growth of long bones, both before and after birth.
Cartilage consists of cells called chondrocytes (Gr. chondros, cartilage + kytos, cell) and chondroblasts embedded in the ECM which, unlike connective tissue proper, contains no other cell types. Chondroblasts undergo mitosis and secrete ECM components, maturing as nondividing chondrocytes which also produce the ECM and maintain it longterm. As cartilage develops the cells occupy matrix cavities called lacunae.
The physical properties of cartilage depend on electrostatic bonds between type II collagen fibrils, hyaluronan, and the sulfated GAGs on densely packed proteoglycans. Its semirigid consistency relates to the water binding the negatively charged hyaluronan and GAG chains that extend from proteoglycan core proteins, all components enclosed within a complex meshwork of thin type II collagen fibrils. The high content of bound water allows cartilage to serve as a shock absorber, an important functional role.
All types of cartilage lack vascular supplies with chondrocytes receiving nutrients and oxygen by diffusion from capillaries in surrounding connective tissue (the perichondrium). In some skeletal elements, large blood vessels do traverse cartilage to supply other tissues, but these vessels release few nutrients to the chondrocytes. As expected of cells in an avascular tissue, chondrocytes exhibit low metabolic activity. Cartilage also lacks nerves.
The perichondrium (Figure 7–2), a sheath of dense connective tissue surrounding most cartilaginous structures, forms an interface between the cartilage and tissues supported by the cartilage. The perichondrium harbors the blood supply serving the cartilage and a small neural component. Articular cartilage, which covers the ends of bones in movable ...