Chapter 5: Bone
Three distinct types of cartilage exist, and their location depends on the required function.
Hyaline cartilage is the most common type of cartilage in the body. It covers the surfaces of articular joints and is the cartilage present in physes. Hyaline cartilage is primarily composed of water, present as a viscoelastic hydrated gel, reinforced by type II collagen. Water is bound by proteoglycan, the largest of which is a molecule called aggregin. Collagen fibers within the cartilage provide tensile strength and contribute to shock absorption. The deepest aspect of cartilage is mineralized (the junction between mineralized and unmineralized cartilage is the “tidemark”), which anchors cartilage to the bone. Turnover of proteoglycan in cartilage is high and controlled by chondrocytes. This is in contrast to collagen, which has low turnover. Adult chondrocytes have limited replicative potential.
Fibrocartilage is stronger and more robust than hyaline cartilage due to abundant collagen that is arranged in bundles. These bundles are interspersed by islands of cartilage. This cartilage can be found in menisci and the annulus fibrosus of intervertebral discs. Fibrocartilage is sparsely vascularized and has limited response to injury. Elastic cartilage is similar to hyaline cartilage but contains many elastin fibers. These fibers are best demonstrated with special stains. Elastic cartilage is strong but very flexible; elastic cartilage is best demonstrated in the pinna.
Hyaline cartilage is separated from the surrounding tissue by perichondrium. Like periosteum, it has an outer fibrous layer and an inner cellular layer (but has chondrogenic rather than osteogenic potential). This cellular layer is only prominent during periods of growth or injury. The perichondrium is lost from the surface of weight-bearing articular hyaline cartilage (although a thin rim of perichondrium is maintained around the articular edge and is continuous with the periosteum). Perichondrium also covers the periphery of physes (‘perichondrial ring’), supporting lateral expansion of the physis by appositional growth.
Chondroblasts are only found within the perichondrium. Morphologically, chondroblasts are small and oval with hyperchromatic nuclei and clear space surrounding the cell (this is an artifact). Chrondroblasts lie in small lacunae, and are often found in groups. Chondroblasts undergo hypertrophy into chondrocytes, taking an oval shape. Chondrocytes are the most frequent cell within hyaline cartilage. Ultrastructurally, chondrocytes have extensive cellular processes, but these are difficult to appreciate by light microscopy. The function of chondrocytes is to produce and maintain the surrounding chondroid matrix. Cartilage can undergo both appositional growth (cells/matrix are deposited on top of each other from the perichondrium) and interstitial growth (growth from within the cartilage model). Hyaline cartilage is also avascular and must be nourished by synovial fluid. This process is facilitated during periods of heavy loading.