Chapter 7: Cartilage: Structure & Growth

Cartilage: Structure & Growth details cartilage, a crucial form of connective tissue primarily composed of specialized cells known as chondrocytes and an extensive extracellular matrix (ECM), which accounts for over 95% of the tissue's total volume. As an avascular tissue, the survival of the sparsely populated chondrocytes relies entirely on the diffusion of substances through the highly hydrated and resilient ECM, which is largely maintained by the large ratio of glycosaminoglycans (GAGs) to Type II collagen fibers. The chapter distinguishes between three main categories of cartilage based on their matrix characteristics: Hyaline cartilage, which features a homogeneous, glassy matrix providing a low-friction surface and serving as the precursor for the fetal skeleton (endochondral ossification). Its matrix includes Type II collagen fibrils as the bulk, along with cartilage-specific collagens (Types VI, IX, X, and XI), and abundant aggrecan proteoglycan monomers that bind large amounts of water. Hyaline cartilage matrix is regionally distinct, divided into the heavily stained capsular, surrounding territorial, and interterritorial matrices. Most hyaline cartilage is encased by a dense irregular connective tissue layer called the perichondrium, except for articular cartilage and epiphyseal plates. The breakdown of articular cartilage, which is organized into superficial, intermediate, deep, and calcified zones separated by the tidemark, is characteristic of Osteoarthritis. Elastic cartilage possesses all the components of hyaline cartilage but is augmented with a dense network of elastic fibers and lamellae, granting it increased flexibility; it is found in structures like the external ear and epiglottis, and notably, it does not calcify with age. Fibrocartilage is a hybrid tissue combining dense regular connective tissue with hyaline cartilage, characterized by the presence of both Type I and Type II collagen fibrils, and functions as a shock absorber in areas requiring resistance to compression and shearing forces, such as intervertebral discs and menisci; fibrocartilage lacks a perichondrium. Cartilage development, or chondrogenesis, begins with mesenchymal cell aggregation and the expression of transcription factor SOX-9. Cartilage grows via two mechanisms: appositional growth, which adds new material at the surface from chondroblasts in the perichondrium, and interstitial growth, which involves chondrocyte division within the existing matrix. Due to its avascular nature and limited chondrocyte mobility, cartilage has a poor capacity for repair, often resulting in dense connective tissue scar formation, and hyaline cartilage is prone to calcification and replacement by bone as a function of aging. Malignancies of cartilage, such as Chondrosarcomas, are generally slow-growing tumors characterized by the secretion of a cartilaginous matrix.