cartilage engineering

Instructor: Dr. Nick Bryan

Recap the Functions and Anatomical Location of Cartilage: Understand the roles that cartilage plays within the human body, including load bearing, providing structure and support, and facilitating movement. Anatomically, cartilage is primarily located in joints, the rib cage, the ear, nose, bronchial tubes, and intervertebral discs.
Importance in Regenerative Medicine: Explore why cartilage is a prime focus for regenerative medicine and tissue engineering initiatives, particularly due to its limited ability to heal after injury. This section will examine cartilage's mechanical properties, its role in joint function, and the implications of cartilage degeneration, as seen in conditions such as osteoarthritis.
Review of Primary Research: Delve into key publications concerning cartilage tissue engineering, summarizing major findings and experimental approaches that have contributed to advancements in the understanding of chondrogenesis and the development of effective therapies for cartilage repair.

Types of Cartilage

Cartilage is categorized into three primary types based on rigidity and structure, each having distinct roles and characteristics:

Hyaline Cartilage: More rigid than muscle but less than bone. It provides support while allowing for flexibility. It is most commonly found in the embryonic skeleton, articulating surfaces of bones, and respiratory structures.
Elastic Cartilage: Contains abundant elastic fibers, allowing it to maintain shape while providing flexibility. It is crucial for structures that require resilience, such as the auricle of the ear and the epiglottis, where flexibility is essential during swallowing.
Fibrocartilage: The toughest type, which is designed to resist compressive forces due to its dense collagen fiber arrangement. It is found in areas subjected to high levels of stress, such as intervertebral discs, the menisci in knees, and the pubic symphysis.

Stiffness Comparison

Cartilage: Displays intermediate rigidity between muscle and bone, allowing it to effectively distribute loads while supporting movement and flexibility.
Types:
- Hyaline Cartilage
- Elastic Cartilage
- Fibrocartilage

Avascular Nature of Cartilage

Cartilage is considered atypical due to its avascular nature:

Lacks an internal blood supply, which complicates nutrient delivery and waste removal, significantly affecting its healing capacity.
Contains resident cells known as Chondrocytes, which are crucial for maintaining the cartilage matrix and are situated within small cavities called Lacunae.
Often enveloped by Perichondrium, a dense irregular connective tissue that assists in nutrient supply for the surrounding cartilage.

Hyaline Cartilage

Appearance: It is typically colorless, transparent, or 'glassy'; the term 'hyalos' means glass in Greek, reflecting its clear properties.
Locations: Found in joints, trachea, nose, larynx, and between ribs and sternum. It provides smooth surfaces for joint movement and flexibility of respiratory structures.
Articular Cartilage: When it lines the joints, it is termed 'articular cartilage'; its chondrocytes produce primarily Type II Collagen, which is critical for resistance to tensile and compressive forces.
- Collagen Fibers: These are thin and optically similar to the ground substance, making them difficult to resolve with light microscopy, yet vital for the structural integrity of cartilage.

Chondrocytes and Lacunae

Chondrocytes reside within the lacunae in the cartilage matrix, synthesizing and maintaining the extracellular matrix.
The matrix is rich in Glycosaminoglycans, which attract water and lubricate the tissue, contributing to its slippery properties essential for joint function.

Elastic Cartilage

Contains a combination of collagen and a high density of elastic fibers, which provide an exceptional ability to return to its original shape after deformation. This allows structures to maintain flexibility and shape.
Found in elastic structures such as the lobe of the ear and the epiglottis, it plays a crucial role in areas where both support and flexibility are required.

Structural Components

Perichondrium: Surrounding connective tissue that supplies nutrients and aids in growth and repair.
Chondrocytes in Lacunae: Similar to hyaline cartilage, but additionally contribute to the elasticity through their unique extracellular matrix composition.

Fibrocartilage

Recognized as the toughest cartilage type, it is essential for resisting compressive forces and providing tensile strength, making it integral to joints that experience high pressure.
Commonly found in intervertebral discs, tendons, ligaments, and the symphysis pubis, it contains densely packed collagen fibers, potentially including Type I collagen, which enhances durability and support.
Characterized by chondrocytes often arranged in distinct rows through lacunae, indicating an ordered structural arrangement that contributes to its mechanistic properties.

Importance of Articular Cartilage

A major focus for tissue engineers due to the high incidence of defects and disorders causing debilitating conditions such as osteoarthritis and trauma.
Consequences of Damage: Damage leads to severe pain and dysfunction from bone-on-bone friction due to the loss of cartilage, resulting in limited movement and decreased quality of life. Understanding the pathology of cartilage injury will inform better treatment strategies.
Poor Healing Capacity: The avascularity of cartilage restricts the supply of nutrients and the removal of waste, severely limiting its intrinsic repair capabilities and complicating the healing process following injury.

Unique Tissue Properties

Despite its appearance of simplicity, articular cartilage exhibits a complex hierarchical structure and specific mechanical properties essential for its function, particularly in shock absorption and load distribution near bone surfaces.
Gradients in collagen types and proteoglycan content reflect the dense and adaptable nature of articular cartilage.
Zones of the tissue include:
- Hypertrophic Zone: Predominantly consists of Type X collagen, which plays a crucial role in endochondral ossification.
- Articular Surface: Primarily composed of Type II collagen, forming 90-95% of the collagen in the extracellular matrix (ECM), vital for load-bearing and joint lubrication.

Chondrocyte Function and Matrix Interaction

Chondrocytes maintain cartilage homeostasis through complex interactions with both pericellular and territorial matrices, responding dynamically to physical, chemical, and mechanical stimuli.
Specific cellular environments are pivotal in influencing chondrocyte functionality; consequently, careful consideration must be given in tissue engineering efforts to replicate these conditions to avoid exacerbating patient outcomes or diminishing the regenerative potential.