 Calcification always occurs in cartilage that is about to be replaced by bone (endochondral ossification) during an individual’s growth period.
 Hyaline cartilage in the adult calcifies with time as part of the aging process.
In most of these situations, given sufficient time, cartilage that calcifies is replaced by bone. For example, in older individuals, portions of the cartilage rings in the trachea are often replaced by bone tissue (Fig. 7.13). Chondrocytes normally derive all of their nutrients and dispose of wastes by diffusion of materials through the matrix. When the matrix becomes heavily calcified, diffusion is impeded and the chondrocytes swell and die. The ultimate consequence of this event is removal of the calcified matrix and its replacement by bone.
A number of investigators believe the process of cartilage removal involves a specific cell type designated as a chondroclast. This cell is described as resembling an osteoclast in both
FIGURE 7.13 • Photomicrograph of a tracheal ring from an elderly individual, stained with H&E. The darker, somewhat basophilic areas on the left side of the micrograph represent normal cartilage matrix (C). The lighter and more eosinophilic areas represent bone tissue (B) that has replaced the original cartilage matrix. A large marrow cavity has formed within the cartilage structure and is visible in the center of the micrograph. 75.
morphology and lytic function. It is thought that these cells enter cartilage along newly sprouting blood vessels and may, in fact, be derived from perivascular or bone marrow stem cells. However, the precise origin of these cells is unknown. Early studies of chondroclast structure and function were carried out on the developing mandible, in which the resorption of Meckel’s cartilage is not followed by bone replacement (endochondral ossification). It is likely that chondroclasts are cells that occur wherever cartilage is being removed. Their role is discussed in regard to endochondral bone formation.
Cartilage is an avascular form of connective tissue composed of cells called chondrocytes and a highly specialized extracellular matrix. Three kinds of cartilage are described on the basis of characteristics of the matrix: hyaline cartilage (described here), elastic cartilage (described in Plate 9), and fibrocartilage (described in Plate 10). Hyaline cartilage has a homogeneous-appearing amorphous matrix. It contains type II collagen. Type II collagen appears with the transmission electron microscope (TEM) as thin fibrils, ~20 nm in diameter, in which the character-istic 68-nm banding may not be obvious. The fibrils are arranged in a three-dimensional felt-like pattern. The matrix also contains large amounts of glycosaminoglycans, most of which form proteoglycans and proteoglycan aggregates. Hyaline cartilage is found in the adult as the structural framework for the larynx, trachea, and bronchi; it is found on the articular ends of the ribs and on the surfaces of synovial joints. In addition, hyaline cartilage constitutes much of the fetal skeleton and plays an important role in the growth of most bones. Hyaline cartilage displays both appositional growth, the addition of new cartilage at its surface, and interstitial growth, the division and differentiation of chondrocytes within its substance.
Hyaline cartilage, trachea, human, H&E ×450.