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 discs) located in the middle of the disc. (c) High power image showing the presence of chondrocytes within thick collagen fibers in the fibrocartilage.
Function
The elastic materials in this type of cartilage give it elastic properties in addition to the resilience and pliability of its other hyaline cartilage components. Related to musculoskeletal tissue function, the elastic fibers in the ligamentum flava of the vertebra aid in the rebound of vertebrae from a flexed position to an upright position. Thus, elastic cartilage gives support and maintains the shape of its inclusionary structure. Unlike hyaline cartilage, the matrix of elastic cartilage does not calcify.
Bone
Bone is a complex and dynamic tissue that continuously undergoes renewal and repair throughout life to fulfill its functions. It bears the weight of the body and provides the framework (i.e., helps the body maintain its shape). The anchoring of muscles to bone also permits mechanical movement and locomotion by providing levers for the muscles (Clarke, 2008). Bone is composed of several cell types and a predominantly collagenous extracellular matrix (the osteoid) that becomes mineralized by the deposition of calcium hydroxyapatite. Bone also hosts hematopoietic cells and provides the environment for hematopoiesis within marrow spaces (which is also the storage site of iron) (Yang, 2010). Its general characteristics are summarized in Table 3.5.
Figure 3.16 Elastic cartilage.
Table 3.5 Summary of Cells, Extracellular Matrix (ECM), Subtypes, and Function of Bone Under Normal Conditions
| Characteristic | Description |
|---|---|
| Tissue type | Dense mineralized connective tissue |
| Cells | Main cell types: Osteoblasts, bone lining cells, osteocytes, osteoclasts, bone marrow–derived mesenchymal stem cellsAdditional cell types: Hematopoietic cells in marrow spaces |
| ECM | Collagen type I (~70%), 25% water, inorganic minerals (e.g., calcium, phosphorus) |
| Subtypes | Cortical bone (compact bone), trabecular (cancellous/spongy bone) |
| Function | Strength, stability, lever at points of attachment, storage of minerals/lipids/nutrients, blood cell formation |
Bone Structure
Cells
The