Cartilage is a specialized form of connective tissue found throughout the body. Although often mistaken for a simple protein, its unique characteristics depend entirely on the proteins and protein-sugar complexes that form its structure. The tissue is softer and more flexible than bone, yet stiffer than muscle, allowing it to perform various mechanical and structural roles. Cartilage consists of specialized cells embedded within a dense, protein-rich substance called the extracellular matrix, which provides resilience and flexibility.
The Core Components of Cartilage
The structure of cartilage consists of a sparse population of cells housed within a vast extracellular matrix (ECM). These specialized cells, called chondrocytes, are the only cell type present in mature cartilage. Chondrocytes are responsible for synthesizing and maintaining the entire matrix, and they reside in small, fluid-filled spaces within the matrix known as lacunae.
The ECM is primarily composed of water, making up 74 to 80% of the tissue’s wet weight. The dry weight is dominated by structural macromolecules, primarily collagen and proteoglycans. Collagen, specifically Type II collagen, provides the meshwork that gives cartilage its tensile strength, accounting for 90 to 95% of the total collagen content.
Interwoven within this collagen framework are large molecules called proteoglycans, which are complexes of protein and long chains of sugar molecules (glycosaminoglycans). The most abundant proteoglycan is aggrecan, which has a core protein with attached sugar chains like chondroitin sulfate. These highly negatively charged proteoglycans bind water, creating a swelling pressure that resists compressive forces. This molecular architecture allows cartilage to absorb and distribute mechanical stress like a resilient, water-filled sponge.
The Three Main Forms of Cartilage
The body utilizes three forms of cartilage, each tailored to specific mechanical requirements and location. Hyaline cartilage is the most common type, characterized by a smooth, glassy appearance. It is primarily found lining the ends of bones in joints (articular cartilage), but also forms the structural support for the nose, trachea, and ribs. This type is built almost exclusively on a framework of Type II collagen fibers.
Fibrocartilage is the toughest and least flexible form, designed for areas requiring high resistance to pulling forces and compression. Its matrix contains thick, visible bundles of both Type I and Type II collagen fibers. This durable tissue is found in structures like the intervertebral discs of the spine and the menisci within the knee joint.
Elastic cartilage is the most pliable variety, containing a dense network of elastin fibers in addition to collagen. This specialized composition allows it to withstand repeated bending and quickly return to its original shape. Elastic cartilage gives form to flexible structures, such as the external ear and the epiglottis.
How Cartilage Functions in the Body
The primary function of cartilage in joints is to provide a smooth, lubricated surface that facilitates bone movement. Articular cartilage operates with a coefficient of friction lower than that of ice sliding on ice, allowing bones to glide past each other without grinding. This low-friction environment minimizes wear and tear on the joint.
Cartilage also acts as a shock absorber that cushions the ends of bones during movement and weight-bearing activities. The water-retaining proteoglycan aggregates resist compressive loads placed on the joint, distributing the force evenly across the bone surface. This protects the underlying bone tissue from impact damage.
Cartilage serves a structural role by offering flexible yet firm support. For example, the rings of cartilage in the trachea prevent airways from collapsing, and the tissue maintains the shape of the outer ear and the tip of the nose.
Cartilage is fundamental to human development, providing the initial blueprint for most of the skeletal system. During fetal development, hyaline cartilage templates are gradually replaced by bone tissue through endochondral ossification. This process ensures the formation of long bones and provides a framework for the mature skeleton.
Maintaining Cartilage Health
A significant factor in cartilage health is its inability to heal following injury. Cartilage is avascular, meaning it lacks a direct blood supply, and it is also without nerve endings. Without blood vessels, chondrocytes cannot directly receive the nutrients and oxygen required for repair, resulting in a very slow or non-existent healing response.
Chondrocytes rely on the diffusion of nutrients from the surrounding environment, such as the synovial fluid that bathes the joints. This passive process is aided by the mechanical loading and unloading of the joint. Compression pushes waste out, and relaxation allows nutrients to diffuse in, making regular, controlled movement essential for tissue nourishment.
When synthesis and degradation are disrupted, often due to excessive mechanical stress or inflammation, the matrix begins to break down, a process associated with conditions like osteoarthritis. To support joint health, maintaining a healthy body weight reduces pressure on the articular cartilage. A diet including adequate Vitamin C and Omega-3 fatty acids can support collagen production and help manage inflammation, preserving matrix integrity.