17: Cartilage - Structure & Function

Elastic cartilage (LM)

- Normally indiscernible from hyaline cartilage - *Certain histological stains will permit identification of elastin fibers* - e.g. Verhoeff Stain - elastin appears dark brown/black

Elastic cartilage

- Similar structure as hyaline cartilage - Found in external ears, Auditory tubes, epiglottis cuneiform cartilage of larynx - Contains large proportion of elastic fibers: Greater pliability, elasticity - Macroscopically, gives cartilage a yellowish tinge

appositional growth

• Along internal edge of perichondrium • Portion of cells differentiate into chondrocytes • Growth along periphery • *post-natal cartilage growth* • With aging, chondrocytes hypertrophy, hyaline cartilage matrix calcifies, and chondrocytes dies, resulting in "asbestiform degeneration"

Avascular nature of cartilage

• Blood would promote calcification, hardening of cartilage matrix (eg. Myositis ossificans) • Chondrocytes release *anti-angiogenic factors* (e.g. chondromodulin-I (ChM-I)) to inhibit local angiogenesis • Also lacks nerve supply, lymphatic drainage • Dependent on diffusion of nutrients from adjacent capillary beds (facilitated through periodic compression/decompression) • Limits thickness of cartilage • *Limits metabolic activity of chondrocytes* (glycolysis, lactic acid production)

Cartilage formation

• Cartilage forms from mesenchyme through chondrogenesis • *Mesenchymal cells* condense, differentiate into *chondroblasts* (synthesize/deposit cartilage matrix) • Chondroblasts further differentiate into *chondrocytes* • Differentiation occurs from inside out; in a given cartilage model • *In mature cartilage, chondroblasts lay dormant in perichondrium*

Fibrocartilage (LM)

• Characterized by *organized rows of chondrocytes* aligned parallel to stress forces • Exceptionally *dense collagen network*

Fibrocartilage (EM)

• Chondrocytes have highly developed RER network • Due to high collagen synthesis/secretion • Less pronounced Golgi apparatus, compared to hyaline, elastic cartilage • Less proteoglycan production, less need for post-translational modification

Extracellular matrix (ECM)

• Collagen fibrils • Glycosaminoglycans (GAGs) -polysaccharides consisting of a repeating disaccharide unit (*hydrophilic*) • *Proteoglycans* - heavily glycosylated proteins • Chondronectin • Chemical structure dictates physical (gel-like) properties

Hyaline cartilage (LM)

• Relatively *dense, irregular aggregation of cells separated by thin ECM* • Can typically identify distinct transitional zone from matrix to perichondrium

Cartilage

• *Firm yet flexible* type of connective tissue • Composed of chondrocytes embedded in *extracellular matrix* (ECM) rich in protein filaments, ground substance • 3 distinct types (hyaline, elastic, fibrocartilage)

Fibrocartilage

• *Intermediate* structure between *dense connective tissue* and *hyaline cartilage* • Found in intervertebral discs, symphysis pubis • Strong resistance to pulling forces • *Lacks distinct perichondrium* • Type I provides greater tensile strength than type II collagen • Can form either irregular or parallel bundles, in response to routine stresses • Lower concentrations of proteoglycans

Chondronectin

• *Linker* glycoprotein • Binds chondrocytes to ECM through adhesions to collagen, GAGs, integrin molecules

Perichondrium

• Dense connective tissue surrounding cartilage • *Fibroblasts secrete type 1 collagen* • Supports neurovascular supply • Gradual transition between cartilage ECM, perichondrium • H&E staining - *more eosinophilic than inner matrix*

Chondrocytes

• Matrix producing cells • Appearance affected by location - *Medullary chondrocytes have rounded appearance* - may appear in isogenous aggregates - *Chondrocytes along periphery of matrix take on elliptical appearance* • Occupy distinct cavities within matrix called *lacuna* • *Territorial matrix* surrounds periphery of each lacunae

Interstitial growth

• Mitotic division of preexisting chondrocytes deep within cartilage matrix - Matrix secreted between daughter chondrocytes, forming 2 separate lacuna • *Expansion of cartilage matrix from within* Seen in... • Early phase of cartilage formation • Articular cartilage (lacking perichondrium) • Growth of long bones at epiphyseal plates • Less pronounced in developed cartilage • Dense matrix makes division, expansion from within difficult

Hyaline cartilage

• Most abundant form • Found in costal cartilage, nose cartilage, respiratory tract, lining articular surfaces of bones • Consists primarily of *type II collagen* • Encased in perichondrium • NB: perichondrium not found in cartilage of articular lining

What are the functions of cartilage?

• Shock absorption from mechanical stress • *Viscoelastic properties* - flexible, yet resists permanent distortion • Covers articulating surfaces to reduce friction; facilitate joint movement • Provides framework for bone formation during embryonic/fetal development • Important role in bone growth throughout adolescence