Cartilage

Cartilage

Specialized connective tissue with a unique hydrated ECM; firm, gelatinous, compressible, tough, and durable; low regeneration capacity and low metabolic activity; contains collagen and elastic fibers.

Cartilage ECM

Made of glycosaminoglycans (GAGs), proteoglycans, and glycoproteins; highly hydrated; GAGs are negatively charged and bind water, interacting with type II collagen to form a compressible, semi-rigid gel.

Vascularization and Innervation of Cartilage

Cartilage is avascular and lacks nerves.

Macromolecules in Cartilage Matrix

Includes chondroitin sulfate, keratin sulfate (GAGs), aggrecan (proteoglycan), chondronectin (glycoprotein), type II collagen, and hyaluronan.

Perichondrium

A dense connective tissue surrounding cartilage, except fibrocartilage and articular cartilage; composed of an outer fibrous layer and an inner chondrogenic layer.

Function of Perichondrium

Provides nutrients and support, protects underlying cartilage, supports appositional growth, and supplies progenitor cells for repair.

Perichondrium Vascularity

Vascular and innervated with a small neural component.

Cause of Cartilage Compressibility

Due to water bound to GAGs and hyaluronan chains extending from proteoglycan cores, enclosed in a type II collagen meshwork.

ECM Interactions

Electrostatic bonds occur between negatively charged sulfated GAGs, proteoglycans, collagen II fibrils, hyaluronan, and water molecules.

Types of Cartilage

Hyaline cartilage, fibrocartilage, and elastic cartilage.

Structure of Hyaline Cartilage

Semi-rigid, glassy appearance; found in articular surfaces, nose, trachea, larynx, bronchi, costal cartilage, epiphyseal plates, and fetal skeleton.

Structure of Fibrocartilage

Hybrid of hyaline cartilage and dense irregular CT; found in intervertebral discs, pubic symphysis, menisci, and ligament insertions; contains type I and II collagen; highly acidophilic.

Structure of Elastic Cartilage

Contains elastic fibers; more stretchable and flexible than hyaline cartilage; found in epiglottis, auricle, auditory canals, Eustachian tube, and upper respiratory tract.

Hyaline Cartilage Matrix Composition

Type II collagen, hyaluronan, sulfated GAGs, aggrecan, chondronectin, and other glycoproteins; highly hydrated with basophilic matrix.

Aggrecan

Major proteoglycan in hyaline cartilage with ~150 GAG chains (chondroitin sulfate and keratin sulfate); binds water giving gelatinous property.

Chondronectin

Structural, multi-adhesive glycoprotein in hyaline cartilage; binds GAGs, collagen, and integrins to anchor chondrocytes to ECM.

Chondrocytes

Only cells in cartilage; embedded in lacunae; synthesize and maintain the ECM.

Matrix Zones in Cartilage

Territorial matrix surrounds chondrocytes, rich in GAGs and basophilic; interterritorial matrix surrounds territorial zone, less basophilic and richer in collagen.

Osteoarthritis

Degeneration of articular cartilage with age, especially in weight-bearing joints (knees, hips); involves matrix loss and cartilage dysfunction.

Cartilage Calcification

Occurs in aging costal cartilage and xiphoid process; involves mineral deposition and resembles endochondral ossification.

Elastic Cartilage Fibers

Contains elastic fibers for flexibility and recoil; also has type II collagen and aggrecan.

Elastic Cartilage Locations

Found in epiglottis, auricle of the ear, external auditory canal walls, Eustachian tube, and upper respiratory tract.

Elastic Cartilage Perichondrium

Present and stains with orcein and aldehyde; provides flexible support to soft tissues.

Fibrocartilage Composition

Combination of hyaline cartilage and dense irregular CT; lacks perichondrium; has fibroblasts and chondrocytes in parallel isogenous groups or isolated.

Fibrocartilage Fibers

Predominantly type I collagen, with some type II; contributes to acidophilic nature and tensile strength.

Function of Fibrocartilage

Provides cushioning, tensile strength, and resistance to tearing and compression; found in load-bearing areas.

Cartilage Origin

All cartilage types originate from mesenchyme (mesoderm-derived).

Chondrogenesis Sequence

Mesenchymal cells round up → proliferate and differentiate into chondroblasts → secrete ECM → become embedded in lacunae as chondrocytes → isogenous groups form by division within lacunae.

Formation of Isogenous Groups

Result from proliferation of chondroblasts within lacunae before maturing into chondrocytes.

Interstitial Growth

Expansion of cartilage from within by dividing chondrocytes; enlarges existing matrix.

Appositional Growth

Growth from the periphery by differentiation of progenitor cells from the inner perichondrium into chondroblasts.

Postnatal Growth

Appositional growth is the primary mode after birth.

Articular Cartilage Repair

Articular cartilage lacks perichondrium, so repair is from within; slow and often incomplete.

Cartilage Repair Mechanism

Perichondrium supplies progenitor cells for regeneration but often produces dense CT instead of new cartilage.

Limitations of Cartilage Regeneration

Due to avascularity and low metabolism; fibrocartilage has the poorest repair capacity.

Chondrogenesis Summary Steps

1. Mesenchymal cells round up and retract 2. Rapid mitosis and differentiation into chondroblasts 3. Matrix secretion → lacuna formation 4. Further proliferation → isogenous groups 5. Maturation → widely spaced chondrocytes producing matrix