The cells in fibrocartilage synthesize a wide variety of extracellular matrix molecules not only during its development stage but also during its mature, fully differentiated state. This allows the fibrocartilage to respond to changes in the external environment (such as mechanical forces, nutritional changes, and changing levels of hormones and growth factors). The extracellular matrix of fibrocartilage contains significant quantities of both type I collagen (characteristic of connective tissue matrix) and type II collagen (characteristic of hyaline cartilage). The relative proportions of these collagens can vary. For example, menisci of the knee joint contain only a small quantity of the type II collagen, whereas the intervertebral disc contains equal amounts of type I and type II collagen fibers. The ratio between type I and type II collagen in fibrocartilage changes with age. In older individuals, there is more type II collagen because of the metabolic activity of chondrocytes, which constantly produce and discharge type II collagen fibrils into the surrounding matrix. In addition, the extracellular matrix of fibrocartilage contains larger amounts of versican (a proteoglycan monomer secreted by fibroblasts) than aggrecan (produced by chondrocytes). Versican can also bind hyaluronan to form highly hydrated proteoglycan aggregates (see Table 6.4, page 179).
(tangential) zone intermediate (transitional) zone deep (radial) zone tidemark calcified zone subchondral bone cancellous bone ababarticularcartilageSZIZDZCZbonearticularcartilageSZIZDZCZboneFIGURE 7.10 • Diagram and photomicrograph of articular cartilage. a. This diagram shows the organization of the collagen network and chondrocytes in the various zones of articular cartilage. b. Photomicrograph of normal articular cartilage from an adult. The superficial zone (SZ ) exhibits elongated and flattened chondrocytes. The intermediate zone (IZ ) contains round chondrocytes. The deep zone (DZ ) contains chondrocytes arranged in short columns. The calcified zone (CZ ), which borders the bone, exhibits a calcified matrix and lacks chondrocytes. Also, this zone is lighter staining than the matrix of the more superficial zones. The tidemark is indicated by the dashed line. 160.
FIGURE 7.11 • Photomicrograph of elastic cartilage from the epiglottis. This specimen was stained with orcein and reveals the elastic fibers, stained brown, within the cartilage matrix. The elastic fibers are of various sizes and constitute a significant part of the cartilage. Chondrocyte nuclei are evident in many of the lacunae. The perichondrium is visible at the top of the photomicrograph. 180.