What is the articular cartilage in adults?

Comprising a very small percentage of total joint tissue volume, the articular cartilage is a highly specialized connective tissue that lines the end of bones where they connect to form movable joints. This unique tissue is essential for smooth, pain-free movement throughout adulthood. It allows bones to glide against one another with extremely low friction and provides shock absorption, protecting the underlying bone from wear and tear.

Quick Summary

Articular cartilage is a smooth, elastic, and dense tissue covering the ends of bones in synovial joints, primarily functioning to facilitate low-friction movement and absorb impact. Unlike many other tissues, it lacks blood vessels and nerves, giving it a limited capacity for self-repair after injury or age-related degeneration.

Key Points

• Essential for Joint Function: Articular cartilage is the specialized tissue lining bone ends in movable joints, providing smooth, low-friction movement and shock absorption.

• Avascular and Aneural: Unlike many tissues, it has no blood vessels or nerves, which explains its extremely limited capacity for self-repair in adulthood.

• Layered Microstructure: The tissue consists of four distinct zones, each with a unique arrangement of collagen fibers and cells designed to handle different types of stress.

• Prone to Age-Related Wear: As a result of constant mechanical stress and aging, articular cartilage can degenerate, often leading to conditions like osteoarthritis.

• Healing Challenges: Damage from trauma or degeneration is difficult to reverse due to the limited replication potential of its primary cells, chondrocytes, and inefficient nutrient supply.

• Importance of the Extracellular Matrix: Its composition of water, Type II collagen, and proteoglycans is what gives cartilage its vital compressive and tensile strength.

• Protection for Underlying Bone: By cushioning the joint, articular cartilage protects the more fragile subchondral bone from damaging forces.

A Closer Look at the Structure of Articular Cartilage

Articular cartilage, a type of hyaline cartilage, is a complex, layered tissue adapted to withstand significant biomechanical forces. It is avascular (lacking blood vessels), aneural (lacking nerves), and alymphatic (lacking lymphatic vessels), which impacts its metabolism and poor healing capacity.

The Composition and Microstructure

This resilient tissue is mainly composed of water (65–80%) and an extracellular matrix (ECM) rich in Type II collagen fibers and proteoglycans.

• Type II Collagen: Provides tensile strength and a structural framework.
• Proteoglycans: Trap water for compressive resilience.
• Chondrocytes: Sparse cells that maintain the ECM but have limited replication potential in adults.

The Four Distinct Zones of Cartilage

Articular cartilage has four zones, each with a different cellular and fiber arrangement.

1. Superficial Zone: Outermost layer (10–20%), resists shear and tensile forces with parallel collagen fibers.
2. Middle Zone: Beneath the superficial layer (40–60%), resists compressive forces with oblique collagen fibers.
3. Deep Zone: Approximately 30%, resists compressive forces with perpendicular collagen fibrils and highest proteoglycan content.
4. Calcified Zone: Deepest layer, anchors cartilage to bone.

The Critical Functions of Articular Cartilage

Articular cartilage is vital for proper joint function.

• Load Bearing and Shock Absorption: Compresses under load to absorb shock and distribute forces.
• Low-Friction Articulation: Allows bones to glide easily with a low coefficient of friction.
• Protection of Subchondral Bone: Cushions and protects the underlying bone from stress.

Understanding Damage and Limited Repair

Due to its lack of blood supply, adult articular cartilage has limited healing capacity, making it vulnerable to degeneration and osteoarthritis.

Causes of Articular Cartilage Damage

Damage can result from various factors:

• Traumatic Injury: Acute events like sports injuries.
• Degenerative Changes: Chronic wear and tear.
• Osteoarthritis: Progressive breakdown and loss of cartilage.

The Impact of Aging on Cartilage

Aging alters cartilage structure and mechanics, decreasing hydration and increasing stiffness, which reduces cushioning and accelerates degeneration.

Comparison of Cartilage Types

Articular (Hyaline) Cartilage differs from Fibrocartilage and Elastic Cartilage.

Feature	Articular (Hyaline) Cartilage	Fibrocartilage	Elastic Cartilage
Location	Ends of bones in movable joints	Meniscus, intervertebral discs	External ear, epiglottis
Composition	Primarily Type II collagen, high water content	Dense, coarse bundles of Type I collagen	Network of elastic fibers
Function	Smooth movement, shock absorption	Provides tough, strong support and shock absorption	Provides flexible, elastic support
Characteristics	Smooth, glossy, resilient	Tough, very strong, less flexible	Yellowish color, highly flexible
Vascularity	Avascular (no blood supply)	Avascular	Avascular

The Role of Chondrocytes and Metabolism

Chondrocytes maintain the ECM but have limited repair potential in adults. They rely on nutrient diffusion from synovial fluid.

Management and Future Outlook

Management for damaged cartilage often focuses on symptom relief and preserving function, including therapy, medication, or surgery. Regenerative medicine offers hope for future repair strategies.

For more in-depth information, authoritative sources like the National Institutes of Health provide resources (e.g., [https://pmc.ncbi.nlm.nih.gov/articles/PMC3445147/]).

Conclusion

Articular cartilage is a vital tissue for joint movement and shock absorption. Its unique structure and composition are crucial for function, but its avascular nature limits its repair capacity, making it susceptible to age-related degeneration and injury. Understanding this tissue is key to maintaining joint health and mobility.

Frequently Asked Questions

What is the primary function of articular cartilage in adults?

The main function is to provide a smooth, lubricated surface for bones to move over each other with minimal friction. It also acts as a shock absorber to protect the bone from high-impact forces during activity.

Why does articular cartilage have a limited ability to heal?

Articular cartilage is avascular, meaning it lacks a direct blood supply. Without blood vessels to deliver nutrients and repair cells, the chondrocytes responsible for maintaining the tissue cannot effectively repair significant damage on their own.

Where is articular cartilage found in the body?

It is located at the ends of bones that form movable, or synovial, joints. Common examples include the knees, hips, shoulders, and ankles.

What happens when articular cartilage is damaged?

When damaged, the smooth surface can become rough and irregular, increasing friction and potentially leading to pain, stiffness, and inflammation. This can progress into osteoarthritis if left unchecked.

How does articular cartilage differ from other types of cartilage?

Articular cartilage (hyaline cartilage) is characterized by its smooth, glossy appearance and its function in joints. Fibrocartilage, like that in the meniscus, is tougher and denser, while elastic cartilage, found in the ear, is more flexible.

Can diet or supplements help with articular cartilage health?

Some supplements, like glucosamine and chondroitin, are popular for joint health, but their effectiveness is debated. Maintaining a healthy diet rich in vitamins and minerals is generally beneficial for overall joint health. It is always best to consult a healthcare professional.

How do chondrocytes get nutrients if there is no blood supply?

Chondrocytes receive their nutrition through a process of diffusion from the synovial fluid that bathes the joint. This is a slower process and is why exercise and movement are important for maintaining joint health by promoting fluid circulation.

What role does aging play in the health of articular cartilage?

Aging can lead to a decrease in hydration and a reduction in the size of proteoglycan aggregates in the cartilage matrix. These changes reduce the cartilage's elasticity and resilience, making it more susceptible to wear and tear over time.