Читать книгу Musculoskeletal Disorders - Sean Gallagher - Страница 90

Hyaline cartilage is the most abundant type of cartilage (Figure 3.14). It is located at the ends of long bones (where it is called articular cartilage; Figure 3.14a,b), epiphyseal growth plates (Figure. 3.14c,d), ribs (where it is called costal cartilage), and in parts of the larynx, trachea, bronchi, and bronchial tubes. During growth, chondrocytes and hyaline cartilage are also present within the epiphyseal plate, becoming hypertrophic and releasing factors necessary for osteoblast, osteoclast, and endothelial cell invasion needed for bone lengthening (Figure 3.14c,d). Hyaline cartilage contains numerous chondrocytes responsible for manufacturing, secretion, organization, and maintenance of the organic components of the extracellular matrix (Nordin & Frankel, 2012). The ground substance is homogeneous and amorphous and is composed of fine collagen type II fibrils embedded in a concentrated solution of proteoglycans. Specifically, the collagen content of hyaline cartilage ranges from 15 to 20% of the wet weight. The matrix of hyaline cartilage contains three types of glycosaminoglycans (hyaluronic acid, chondroitin sulfate, and keratin sulfate). The chondroitin and keratin sulfates are joined together by a core protein to form a proteoglycan monomer. The proteoglycans account for 4–7% by wet weight (Nordin & Frankel, 2012; Ross, Romrell, & Kaye, 1995). About 80 proteoglycans are associated with each hyaluronic acid molecule in large aggregates reinforced by linking‐type proteins. These aggregates are bound to the thin collagen fibrils by electrostatic interactions and cross‐linking glycoproteins. The remainder of the matrix is composed of water (60–78%), inorganic salts, and small amounts of link proteins, glycoproteins, and lipids. Some of the water is loosely bound, allowing diffusion of small metabolites to the chondrocytes, which is key in this typically avascular tissue.

Figure 3.14 Hyaline cartilage. (a and b) Hyaline cartilage in the articular ends of the distal radius and a carpal bone of the radiocarpal joint. This is a plane‐type joint. The higher power image of panel B shows chondrocytes clustering at several sites. (c and d) Hyaline cartilage in an epiphyseal plate (growth plate) of a radial bone. Low to higher power images are shown.

Articular cartilage is a type of hyaline cartilage located in freely moving joints (Figure 3.14a,b) in which the joint is encased by an articular capsule and the bones connect with each other in a fluid‐filled cavity known as the synovial cavity. Articular cartilage is organized into zones termed superficial, middle, deep, and a calcified zone (deepest). The most superficial zone is in contact with the synovial fluid that contains nutrients that diffuse into the cartilage. This zone protects the deeper layers from shear stresses and constitutes 10–20% of the thickness of articular cartilage. The chondrocytes in this upper zone are relatively high in number and flattened in shape, and the collagen fibers are tightly packed and are aligned in parallel to the articular surface. This combined structure resists the sheer, tensile, and compressive forces imposed by articulation and aids the protection and maintenance of the deeper structures. The middle zone represents 40–60% of the total cartilage volume. In this zone, the chondrocytes are spherical in shape and low in density and the collagen is obliquely organized (to help resist compressive forces). The deep zone of articular cartilage zone represents around 30% of the total cartilage volume. The collagen fibrils in this zone are arranged perpendicular to the articular surface and are large in diameter. The chondrocytes are arranged in columns (Figure 3.14a,b) that are in parallel to the collagen fibers and perpendicular to the joint line. As a consequence, this zone provides the greatest resistance to compressive forces. The deepest zone is the calcified cartilage layer (and is separated from the deep zone by a histological stain detectable “tide mark”). Like the deep zone, the collagen fibrils are arranged perpendicular to the articular surface. Unlike the other zone, calcification is present and the cell population is scare and hypertrophic. Its main purpose is to anchor collagen fibrils to the underlying subchondral bone.