Figure 71

Molecular structure of the ground substance of hyaline cartilage.

This schematic diagram shows the relationship of hyaluronate proteoglycan aggregates to type II collagen fibrils and chondrocytes in the ground substance of hyaline cartilage. A hyaluronic acid molecule forming a linear aggregate with many proteoglycan monomers

Hyaline cartilage matrix is highly hydrated to permit diffusion of small metabolites and resilience

Like other connective tissue matrices, cartilage matrix is highly hydrated. Sixty to eighty percent of the net weight of hyaline cartilage is water. Much of this water is bound tightly to the hyaluronate proteoglycan aggregates, which imparts resilience to the cartilage. Some of the water is bound loosely enough, however, to allow diffusion of small metabolites to and from the chondrocytes.

In articular cartilage, both transient and regional changes occur in water content during joint movement and when the joint is subjected to pressure. The high degree of hydration and the movement of water in the matrix allow the cartilage matrix to respond to varying pressure loads and contribute to cartilage's weight-bearing capacity. Throughout life, cartilage undergoes continuous internal remodeling as the cells replace matrix molecules lost through degradation. New evidence indicates that normal matrix turnover depends on the ability of the chondrocytes to detect changes in matrix composition. The chondrocyte then responds by synthesizing appropriate types of new molecules. In addition, the matrix acts as a signal transducer for the embedded chondrocytes. Thus, pressure loads applied to the cartilage, as in synovial joints, create mechanical, electrical, and chemical signals that help direct the synthetic activity of the chondrocyte. As the body ages, the composition of the matrix changes, and the chondrocytes lose their ability to respond to these stimuli.

Ground substance components of hyaline cartilage matrix are not distributed uniformly

Because the proteoglycans of hyaline cartilage contain a high concentration of bound sulfate groups, ground substance stains with basic dyes and with hematoxylin. Thus, the basophilia and metachromasia seen in stained sections of cartilage provide information on the distribution and relative concentration of sulfated proteoglycans. The highest concentration of these substances occurs immediately around the lacunae. This ring of intensely staining matrix is called the capsule or territorial matrix (Fig. 7.2). The matrix more removed from the immediate vicinity of chondrocytes has a lower concentration of sulfated proteoglycans and stains less intensely. These areas are designated the interterritorial matrix. In addition to these regional differences in the concentration of sulfated proteoglycans, the decrease in proteoglycan content that occurs as cartilage ages is also reflected by staining differences.

Chondrocytes are specialized cells that produce and maintain the extracellular matrix

In hyaline cartilage, chondrocytes are distributed either singly or in clusters called isogenous groups. When the chondrocytes are present in isogenous groups, they represent cells that have recently divided. As the newly divided chondrocytes produce matrix material, which surrounds them, they become dispersed.

The cytoplasm of chondrocytes varies in appearance relative to their activity. Chondrocytes that are active in matrix production display areas of cytoplasmic basophilia, indicating protein synthesis, and clear areas, reflecting the large Golgi apparatus (Fig. 7.3). Chondrocytes secrete not only the collagen present in the matrix but also all of the gly-

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