Cartilage

Introduction

Cartilage is a specialized connective tissue that provides support, flexibility, and smooth surfaces for joint movement. It plays a crucial role in skeletal development and maintenance. Despite being firm, cartilage is resilient and capable of withstanding mechanical stress.

Definition and General Characteristics

Definition of Cartilage

Cartilage is a firm, flexible connective tissue composed of chondrocytes embedded within an extracellular matrix. It is found in various regions of the body including joints, respiratory structures, and the ear.

Key Features

• Avascular nature: Cartilage lacks blood vessels, receiving nutrients via diffusion from surrounding tissues.
• Presence of extracellular matrix: The matrix contains collagen, proteoglycans, and water, giving cartilage its strength and resilience.
• Chondrocytes as cellular components: These specialized cells produce and maintain the cartilage matrix.
• Elasticity and resilience: Cartilage can deform under pressure and return to its original shape, enabling it to absorb shock and support mechanical functions.

Types of Cartilage

Hyaline Cartilage

• Location in the body: Found in the nose, trachea, larynx, and at the ends of long bones forming articular surfaces.
• Structural features: Contains a homogeneous, glassy matrix with fine collagen fibers and scattered chondrocytes within lacunae.
• Function: Provides smooth surfaces for joint movement, supports respiratory structures, and contributes to bone growth during development.

Elastic Cartilage

• Location in the body: Present in the external ear, epiglottis, and auditory tube.
• Structural features: Contains abundant elastic fibers in the matrix, giving it flexibility while maintaining shape.
• Function: Provides elasticity and structural support in regions requiring flexibility.

Fibrocartilage

• Location in the body: Found in intervertebral discs, pubic symphysis, menisci of the knee, and the temporomandibular joint.
• Structural features: Contains dense bundles of type I collagen fibers, with chondrocytes arranged in rows.
• Function: Resists compressive forces and provides tensile strength to areas subjected to mechanical stress.

Histology of Cartilage

Cellular Components

• Chondrocytes: Mature cartilage cells responsible for maintaining the extracellular matrix, located in lacunae.
• Chondroblasts: Immature cells found near the perichondrium that actively produce matrix components and differentiate into chondrocytes.

Extracellular Matrix

• Collagen fibers: Provide tensile strength; type II collagen is predominant in hyaline cartilage, while type I collagen dominates fibrocartilage.
• Proteoglycans: Include aggrecan which retains water and provides compressive resistance.
• Water content: High water content (65-80%) allows elasticity and shock absorption.

Perichondrium

• Structure: Dense connective tissue surrounding most cartilage, consisting of an outer fibrous layer and inner cellular layer.
• Functions: Supplies nutrients to avascular cartilage, facilitates appositional growth, and provides mechanical support.

Physiology and Function

• Mechanical support: Cartilage provides a stable framework for soft tissues and maintains the shape of structures such as the nose and ear.
• Shock absorption: Its viscoelastic properties allow it to absorb compressive forces in joints and intervertebral discs, protecting underlying bone.
• Facilitation of smooth joint movement: Articular cartilage reduces friction and enables smooth gliding of bones within synovial joints.
• Growth and development of long bones: Cartilage serves as a precursor in endochondral ossification, guiding bone formation during development.

Cartilage Growth and Development

Interstitial Growth

Interstitial growth occurs when chondrocytes divide within lacunae and secrete new matrix, leading to expansion of cartilage from within. This type of growth is essential during early development and in the lengthening of bones.

Appositional Growth

Appositional growth involves the addition of new cartilage layers by chondroblasts located in the inner layer of the perichondrium. This increases the thickness of cartilage and contributes to surface expansion.

Role in Endochondral Ossification

During endochondral ossification, cartilage acts as a template for bone formation. Hyaline cartilage in the growth plates is gradually replaced by bone, allowing longitudinal growth of long bones until skeletal maturity is reached.

Cartilage Repair and Regeneration

• Limitations due to avascularity: The absence of blood vessels in cartilage limits its intrinsic ability to heal, making repair slow and often incomplete.
• Spontaneous repair mechanisms: Minor injuries may be repaired by limited proliferation of chondrocytes and matrix deposition, but large defects rarely heal naturally.
• Current therapeutic approaches:
  • Autologous chondrocyte implantation: Harvesting and expanding the patient’s own chondrocytes for implantation into the defect site.
  • Stem cell therapy: Using mesenchymal stem cells to differentiate into chondrocytes and promote repair.
  • Tissue engineering: Employing biomaterial scaffolds combined with cells and growth factors to regenerate cartilage tissue.

Clinical Conditions Involving Cartilage

Osteoarthritis

Osteoarthritis is a degenerative joint disease characterized by progressive loss of articular cartilage, leading to pain, stiffness, and decreased joint mobility.

Chondromalacia

Chondromalacia refers to the softening or degeneration of cartilage, commonly affecting the patella, and resulting in anterior knee pain and impaired movement.

Cartilage Injuries

Traumatic injuries such as tears or fractures can damage cartilage. Due to limited repair capacity, these injuries may lead to chronic pain and joint dysfunction if not managed appropriately.

Congenital and Genetic Disorders

• Achondroplasia: A genetic disorder causing abnormal cartilage formation, resulting in dwarfism and skeletal abnormalities.
• Multiple epiphyseal dysplasia: A condition affecting cartilage development at the growth plates, leading to joint deformities and early-onset arthritis.

Imaging and Diagnostic Evaluation

• X-ray findings: Useful for assessing joint space narrowing, bone changes, and indirect signs of cartilage degeneration.
• MRI evaluation: Provides detailed visualization of cartilage thickness, surface integrity, and internal structure, allowing early detection of lesions.
• Arthroscopy: A minimally invasive technique that allows direct visualization of cartilage surfaces and assessment of injury severity, often combined with surgical repair.

Current Research and Advances

• Cartilage tissue engineering: Development of bioengineered cartilage using cell-seeded scaffolds for repairing defects and degenerative lesions.
• Biomaterials and scaffolds: Investigation of synthetic and natural materials that support chondrocyte growth and matrix production.
• Gene therapy approaches: Experimental strategies targeting genes involved in cartilage repair, inflammation modulation, and extracellular matrix synthesis to enhance regeneration.

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