 The intermediate (transitional) zone lies below the superficial zone and contains round chondrocytes randomly distributed within the matrix. Collagen fibrils are less organized and are arranged in a somewhat oblique orientation to the surface.
 The deep (radial) zone is characterized by small, round chondrocytes that are arranged in short columns perpendicular to the free surface of the cartilage. The collagen fibrils are positioned between columns parallel to the long axis of the bone.
 The calcified zone is characterized by a calcified matrix with the presence of small chondrocytes. This zone is separated from the deep (radial) zone by a smooth, undulating, heavily calcified line called the tidemark. Above this line, proliferation of chondrocytes within the cartilage lacunae provides the new cells for interstitial growth. In articular cartilage renewal, chondrocytes migrate from this region toward the joint surface.
The renewal process of mature articular cartilage is very slow. This slow growth is a reflection of the highly stable type II collagen network and the long half-life of its proteoglycan molecules. Also, in healthy articular cartilage, metalloproteinase (MMP-1 and MMP-13) activity is low.
Elastic cartilage is distinguished by the presence of elastin in the cartilage matrix.
In addition to containing the normal components of hyaline cartilage matrix, elastic cartilage matrix also contains a dense network of branching and anastomosing elastic fibers and interconnecting sheets of elastic material (Fig. 7.11 and Plate 9, page 214). These fibers and lamellae are best demonstrated in paraffin sections with special stains such as resorcinfuchsin and orcein. The elastic material gives the cartilage elastic properties in addition to the resilience and pliability that are characteristic of hyaline cartilage.
Elastic cartilage is found in the external ear, the walls of the external acoustic meatus, the auditory (Eustachian) tube, and the epiglottis of the larynx. The cartilage in all of these locations is surrounded by a perichondrium similar to that found around most hyaline cartilage. Unlike hyaline cartilage, which calcifies with aging, the matrix of elastic cartilage does not calcify during the aging process.
Fibrocartilage consists of chondrocytes and their matrix material in combination with dense connective tissue.
Fibrocartilage is a combination of dense regular connective tissue and hyaline cartilage. The chondrocytes are dispersed among the collagen fibers singularly, in rows, and in isogenous groups (Fig. 7.12 and Plate 10, page 216). These chondrocytes appear similar to the chondrocytes of hyaline cartilage, but they have considerably less cartilage matrix material. There is also no surrounding perichondrium as in hyaline and elastic cartilage. In a section containing fibrocartilage, a population of cells with rounded nuclei and a small amount of surrounding amorphous matrix material can typically be seen. These nuclei belong to the chondrocytes. Within the fibrous areas are nuclei that are flattened or elongated. These are fibroblast nuclei.
Fibrocartilage is typically present in intervertebral discs, the symphysis pubis, articular discs of the sternoclavicular and temporomandibular joints, menisci of the knee joint, the triangular fibrocartilage complex of the wrist, and certain places where tendons attach to bones. The presence of fibrocartilage in these sites indicates that resistance to both compression and shearing forces is required of the tissue. The cartilage serves much like a shock absorber. The degree to which such forces occur is reflected in the amount of cartilage matrix material present.
Extracellular matrix of fibrocartilage is characterized by the presence of both type I and type II collagen fibrils.