Cartilage's strength comes from its unique composition and structure.
Composition of Cartilage
Cartilage's impressive strength, while remaining flexible, is primarily due to the following components:
- Chondrocytes: These are specialized cartilage cells embedded within the matrix. They produce and maintain the matrix.
- Extracellular Matrix: This is the main component of cartilage, providing structural support and resilience. It consists of:
- Collagen Fibers: Primarily type II collagen, these fibers provide tensile strength, resisting pulling forces. The arrangement of these fibers varies depending on the type of cartilage (hyaline, elastic, or fibrocartilage), which influences its specific properties.
- Proteoglycans: These are large molecules consisting of a protein core attached to glycosaminoglycans (GAGs), such as chondroitin sulfate and keratin sulfate. Proteoglycans are hydrophilic, meaning they attract and hold water. This water content contributes to cartilage's ability to resist compression. The high concentration of negatively charged GAGs within the proteoglycans creates osmotic pressure, drawing water into the matrix.
- Water: Water makes up a significant portion of cartilage (60-80% by weight). It's crucial for nutrient transport to chondrocytes (since cartilage lacks blood vessels) and for the tissue's ability to withstand compressive forces. The water held within the proteoglycan network acts like a shock absorber.
Structure of Cartilage
The arrangement of these components further contributes to cartilage's strength:
- Lack of Blood Vessels and Nerves: Cartilage is avascular and aneural, relying on diffusion for nutrient supply. This unique characteristic affects its healing ability but also contributes to its specialized mechanical properties.
- Specialized Zones: In articular cartilage (the type that covers joint surfaces), there are distinct zones with varying collagen fiber orientations. This zonal arrangement is optimized to distribute stress across the cartilage, protecting the underlying bone.
In summary, cartilage's strength stems from the interaction of its collagen fibers providing tensile strength, its proteoglycans attracting and holding water for compressive resistance, and its unique structural organization. The absence of blood vessels also plays an indirect role by creating a unique mechanical environment.
