types of cartilage, namely hyaline (articular) cartilage, fibrocartilage, and elastic cartilage, can be distinguished from each other by the type of fibers within the matrix.
Organization
While the cell types are similar in each type of cartilage, the organization of cells and collagen/elastin fibrils differ extensively between types. The characteristics of each are described in detail next.
Hyaline Cartilage
Structure
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.
Function
The large amounts of hyaluronic acid and other components in hyaline cartilage help retain water in the extracellular matrix. As a consequence, this type of cartilage provides resilience and pliability and is well adapted to serve in a weight‐bearing joint, especially at points of movement. Hyaline cartilage also reduces friction and protects bony surfaces.
Fibrocartilage
Structure
Fibrocartilage is located at the intervertebral discs between the vertebrae (Figure 3.15), pubic symphysis, and in the menisci of knees. Fibrocartilage is distinct from hyaline cartilage in composition and structure. Fibrocartilage has a higher dry weight of collagen and less water, making it tougher and less resilient than articular cartilage. Its collagen fibers are thick (and therefore visible) and densely deposited in the tissue. In fact, the collagen fibers are so tightly packed that there is little evidence of an extracellular matrix. The chondrocytes are scattered among the many bundles of collagenous fibers.
Function
The structure of fibrocartilage combines both strength and rigidity; therefore, it is key to the support and fusion of joints in which it is present. Fibrocartilage is an anisotropic material that exhibits different strength capacities depending on the direction of loading (Murphy, Black, & Hastings, 2016). For example, it has been shown that fibrocartilage is strong during tension occurring in parallel to the orientation of the collagen fibers, but weaker during shear loading (Mansour, 2009).
Elastic Cartilage
Structure
The ligamentum flava joining the laminae of vertebrae is a type of elastic cartilage. Elastic cartilage is also present in the epiglottis, external ear, and auditory tubes. It is distinguished from other types of cartilage by the presence of elastic fibers in the matrix, in addition to the normal components of hyaline cartilage. As shown in Figure 3.16, a thread‐like network of elastic fibers surrounds chondrocytes in this type of cartilage.
Figure 3.15 Fibrocartilage in the intervertebral discs between the vertebrae. (a) Low power image showing the disc between two rat lumbar level vertebrae. (b) Mid‐power image showing fibrocartilage encircling the nucleus pulposus (the gelatinous interior of intervertebral
