Regardless of its specialized function, all cartilage consists of cells—chondrocytes and chondroblasts. These cells synthesize and deposit around them an elaborate matrix of macromolecules that are some of the largest in nature. The mechanical properties of cartilage tissue are derived primarily from the properties of the complex extracellular matrix.
On gross examination and on light microscopy, all cartilage appears smooth and homogeneous. However, electron microscopy reveals that its basic fibrillar structure consists of a meshwork of collagen fibers and large proteoglycans in approximately equal amounts. In addition, water is a major component of cartilage, contributing 65% to 80% of its weight. Type II collagen, the major fibrillar component of cartilage matrix, contributes tensile strength and form to the tissue. Proteoglycans, by their ability to entrap large amounts of water (tissue fluid) in their macromolecular domains, give cartilage a resiliency and stiffness to compression (see Plate 2-25). The exact mechanisms by which collagen and proteoglycans interact in the various types of cartilage remain unclear. However, another function of collagen is to trap proteoglycans and restrain their swelling pressure.
In addition to properties shared with other types of hyaline cartilage, articular cartilage has a complex internal structure. Electron microscopy and biochemical studies reveal four poorly demarcated zones: a small superficial, or tangential, zone (I); a larger intermediate, or transitional, zone (II); a deep vertical zone (III), which occupies the greatest volume; and a zone of calcified cartilage (IV), which lies adjacent to the subchondral bone. On light microscopy, the boundary between zones III and IV is demarcated by an undulating plate, referred to as a tidemark.
The cells in the four zones differ dramatically in size, shape, orientation, and number, as well as in the relative composition, proportion, and orientation of macromolecules in their matrix. Even small differences in the composition and organization of the matrix give each zone slightly different mechanical properties.
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