Lecture on Cartilage

Cartilage Study Notes

Lecture Objectives

  • At the end of this lecture, the student should be able to:

    1. Define why cartilage is classified as a connective tissue.

    2. Describe the 3 components of cartilage.

    3. Classify cartilage into its 3 types.

    4. Describe the characteristics of each type of cartilage.

    5. Relate the structure of the 3 types of cartilage to the function of the locations where each is found.

    6. Describe the processes by which cartilage grows.

Introduction to Cartilage

  • Classification as a connective tissue:

    • Structure:

    • Cells: Specialized cells called chondroblasts and chondrocytes.

    • Extracellular matrix: Includes fibers and ground substance.

    • Comparison with other connective tissues: Cartilage differs significantly in structure and function.

    • Function: Provides support and flexibility in various parts of the body.

    • Development: Cartilage forms during embryonic development and continues through growth.

Classification of the Types of Cartilage and Their Locations

  • Types of Cartilage:

    1. Hyaline Cartilage

    2. Elastic Cartilage

    3. Fibrocartilage

Cartilage Locations

  • Hyaline Cartilage:

    • Articular cartilage of joints.

    • Costal cartilage (ribs).

    • Cartilage in the nose and trachea.

    • Growth plates (epiphyseal plates) in long bones.

  • Elastic Cartilage:

    • External ear (pinna).

    • Epiglottis and Eustachian tube.

  • Fibrocartilage:

    • Intervertebral discs.

    • Pubic symphysis.

    • Meniscus of knee joints.

    • Labrum of glenohumeral joint.

General Characteristics of Cartilage

  • Matrix:

    • Semi-solid ground substance.

    • High water content to resist compression.

    • Encloses cells in spaces called lacunae.

    • Fibers of collagen and elastin.

  • Cells:

    • Chondroblasts: Secrete fibers and matrix.

    • Chondrocytes: Mature chondroblasts that reside in lacunae and maintain the matrix.

  • Distinguishing Features from Other Connective Tissues:

    • Lacks nerve endings.

    • Lacks vasculature (blood vessels).

    • Lacks lymphatics.

Matrix of Cartilage

  • Glycosaminoglycans (GAGs):

    • Large carbohydrates formed from repeating disaccharides.

    • Hydrophilic, attracting water and forming a hydrated environment.

    • Examples include chondroitin sulfate, keratin sulfate, and hyaluronate (hyaluronic acid).

  • Hyaluronic Acid:

    • Composed of long chains of repeating disaccharides.

    • Functions include binding water and providing lubrication, especially in joints.

  • Role of Hyaline Cartilage:

    • It covers the ends of long bones, providing cushioning due to its resistance to wear and tear.

    • Referred to as “gristle cartilage” for its textural properties.

Hyaline Cartilage Characteristics

  • Description:

    • Amorphous but firm matrix with a network of type II collagen fibers.

    • Produces matrix primarily by chondroblasts and is maintained by chondrocytes in lacunae.

  • Function:

    • Provides support, reinforcement, and cushioning; resists compressive stress.

  • Locations:

    • Forms the embryonic skeleton, covers bone ends in joints, costal cartilages of ribs, and other specific locations.

  • Nutrition:

    • Diffusion from the perichondrium and synovial fluid where perichondrium is absent (as in articular cartilage).

Clinical Application: Osteoarthritis

  • Osteoarthritis:

    • Chronic condition affecting the hyaline cartilage of joints, leading to degeneration.

    • Common in weight-bearing joints (knees, hips) and often presents symptoms of pain and inflammation due to cartilage breakdown.

    • Involves secretion of matrix metalloproteinases from macrophages in response to cartilage wear-and-tear.

Elastic Cartilage Characteristics

  • Description:

    • Similar structure to hyaline cartilage but contains more elastic fibers.

  • Function:

    • Maintains shape while allowing flexibility.

  • Locations:

    • Comprises structures like the external ear and epiglottis.

Fibrocartilage Characteristics

  • Description:

    • Matrix similar to dense regular connective tissue but with distinct characteristics; type I and type II collagen fibers are predominant.

  • Locations:

    • Found in intervertebral discs, pubic symphysis, menisci of knee joints, and other joints.

  • Function:

    • Provides tensile strength, shock absorption, and support under stress.

Growth of Cartilage

  • Types of Growth:

    1. Appositional Growth:

    • Cartilage-forming cells in the perichondrium secrete matrix against the external face, leading to new layers forming on the surface.

    1. Interstitial Growth:

    • Chondrocytes within lacunae divide and secrete new matrix, expanding the cartilage from within.

Summary Table of Cartilage Types

  • Hyaline Cartilage:

    • Matrix: Homogeneous, dominant type II collagen and aggrecan.

    • Cells: Chondrocytes and chondroblasts.

    • Presentation: Usually isolated or in small isogenous groups.

    • Perichondrium Presence: Yes.

    • Locations: Respiratory tract, joints, fetal skeleton.

    • Function: Smooth, low-friction surfaces, and structural support.

  • Elastic Cartilage:

    • Matrix: Contains elastic fibers along with type II collagen and aggrecan.

    • Cells: Chondrocytes and chondroblasts.

    • Arrangement: Isolated or in small isogenous groups.

    • Perichondrium: Yes.

    • Locations: External ear, epiglottis.

    • Function: Flexible support.

  • Fibrocartilage:

    • Matrix: Type I collagen and type II collagen fibers, less rigid than hyaline.

    • Cells: Chondrocytes and fibroblasts.

    • Arrangement: Aligned in rows or isogenous groups.

    • Perichondrium Presence: No.

    • Locations: Intervertebral discs and pubic symphysis.

    • Function: Tensile strength and resistance to tearing and compression.