Like other connective tissue matrices, cartilage matrix is highly hydrated. From 60% to 80% of the net weight of hyaline cartilage is intercellular water (see Fig. 7.2). Much of this water is bound tightly to the aggrecan– hyaluronan aggregates, imparting 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. Normal matrix turnover depends on the ability of the chondrocytes to detect changes in matrix composition. The chondrocytes then respond 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 chondrocytes. As the body ages, however, the composition of the matrix changes, and the chondrocytes lose their ability to respond to these stimuli.
Chondrocytes are specialized cells that produce and maintain the extracellular matrix.
In hyaline cartilage, chondrocytes are distributed either singularly or in clusters called isogenous groups (Fig. 7.4). When the chondrocytes are present in isogenous groups, they represent cells that have recently divided. As the newly divided chondrocytes produce the matrix material that surrounds them, they are dispersed. They also secrete metalloproteinases, enzymes that degrade cartilage matrix, allowing the cells to expand and reposition themselves within the growing isogenous group.
The appearance of chondrocyte cytoplasm varies according to chondrocyte activity. Chondrocytes that are active in matrix production display areas of cytoplasmic basophilia, which are indicative of protein synthesis, and clear areas, which indicate their large Golgi apparatus (Fig. 7.5). Chondrocytes not only secrete the collagen present in the matrix but also all of the glycosaminoglycans and proteoglycans. In older, less active cells, the Golgi apparatus is smaller; clear areas of cytoplasm, when evident, usually indicate sites of extracted lipid droplets and glycogen stores. In such specimens, chondrocytes also display considerable distortion resulting from shrinkage after the glycogen and lipid are lost during preparation of the tissue. In the transmission electron microscope (TEM), the active chondrocyte displays numerous profiles of rough-surfaced endoplasmic reticulum (rER), a large Golgi apparatus, secretory granules, vesicles, intermediate filaments, microtubules, and actin microfilaments (Fig. 7.6).
Components of the hyaline cartilage matrix are not uniformly distributed.
Because the proteoglycans of hyaline cartilage contain a high concentration of bound sulfate groups, ground substance stains with basic dyes and hematoxylin (Plate 7, page 210). Thus, the basophilia and metachromasia seen in stained sections of cartilage provide information about the distribution and relative concentration of sulfated proteoglycans.