Cartilage

Histology of Cartilage

Instructor Details

  • Instructor: Dr. Dinipre B. McGregor Youdubagha

  • Position: Assistant Professor, Histology and Cell Biology and Genetics

Learning Objectives

  • Describe cellular components of cartilage:

    • Origin, structure, and function of

    • Chondroblasts: Immature cartilage cells that produce the cartilage matrix and undergo mitosis.

    • Chondrocytes: Mature cartilage cells residing in lacunae, formed when chondroblasts become enveloped by the matrix.

  • Biochemical composition of cartilage extracellular matrix:

    • Focus on collagen types, proteoglycans, and glycosaminoglycans (GAGs).

  • Classification of cartilage into three types:

    • Compare structural features, functional roles, and anatomical distributions of

    • Hyaline

    • Elastic

    • Fibrocartilage

  • Developmental origin of cartilage:

    • Discuss growth mechanisms including interstitial and appositional growth.

Introduction to Cartilage

  • Definition:

    • Type of connective tissue that forms structural components of the skeleton and provides support to various organs.

  • Functions of cartilage:

    • Articulation:

    • Cushions and provides sliding regions in joints, facilitating bone movements.

    • Structural support:

    • Can bear mechanical stresses without distortion.

    • Bone formation:

    • Plays a vital role in the development and growth of long bones.

  • Contrast between bone and cartilage:

    • Cartilage is non-mineralized, whereas bone is mineralized.

Structure of Cartilage

  • Extracellular Matrix:

    • Firm and contains:

    • Types I & II collagen, elastic fibers, glycosaminoglycans, proteoglycans, glycoproteins, and ground substance.

  • Cells in cartilage:

    • Chondrocytes, Chondroblasts, Chondroclasts:

    • Chondrocytes: Mature, matrix-encased cells in lacunae.

    • Chondroblasts: Immature cells that manufacture the cartilage matrix and undergo mitosis.

    • Chondroclasts: Resorb cartilage and are more present in joint disease conditions.

  • Structural characteristics:

    • Cartilage is avascular, lacks nerves and lymphatics.

Types of Cartilage

Hyaline Cartilage

  • Matrix Composition:

    • Contains type II collagen fibers.

  • Characteristics:

    • Most abundant in the body.

    • Serves as a temporary skeleton in the fetus until replaced by bone.

  • Perichondrium:

    • Covers the hyaline cartilage except at the articular surfaces.

    • Consists of an outer fibrous layer (type I collagen, fibroblasts, blood vessels) and an inner cellular layer (chondrogenic cells, chondroblasts for new cells).

  • Functions:

    • Provides nutrients through the nearest blood supply and aids in the growth and maintenance of cartilage.

  • Location:

    • Found in articular surfaces, costal cartilages, trachea, bronchi, nasal septum, and epiphyseal plates.

  • Clinical correlations:

    • Osteoarthritis, achondroplasia, croup (laryngotracheitis).

Elastic Cartilage

  • Matrix Composition:

    • Contains elastic fibers and type II collagen.

  • Characteristics:

    • More flexible due to a higher concentration of elastic fibers.

  • Perichondrium:

    • Similar to hyaline cartilage with outer fibrous and inner cellular layers.

  • Location:

    • External ear, epiglottis, auditory tube.

  • Clinical correlations:

    • Relapsing polychondritis, trauma to auricle leading to cauliflower ear.

Fibrocartilage

  • Matrix Composition:

    • Features a parallel arrangement of type I collagen fibers with chondrocytes located singly in rows between collagen bundles.

  • Absence of Perichondrium:

  • Location:

    • In intervertebral discs, pubic symphysis, menisci, and TMJ.

  • Functions:

    • Provides cushioning, tensile strength, and resistance to tearing and compression.

  • Clinical correlations:

    • Intervertebral disc herniation, degenerative disc disease, meniscal tears.

Cartilage Growth Mechanisms

Chondrogenesis

  • Growth types:

    • Appositional Growth:

    • Adds new cartilage at the perichondrium through the activity of chondroblasts.

    • Interstitial Growth:

    • Expands cartilage volume from within; occurs via proliferation of chondrocytes in the isogenous group which secrete matrix, pushing cells apart.

  • Factors regulating chondrogenesis:

    • Consist of extracellular ligands, nuclear receptors, transcription factors, adhesion molecules, and matrix proteins.

Pathology of Cartilage

  • Chondrosarcoma:

    • A slow-growing malignant tumor of cartilage arising typically in the axial skeleton and long bones ends.

    • Features include high secretion of cartilage matrix, presence of transcription factor SOX-9, and typically affected hyaline cartilage.

  • Osteoarthritis:

    • Characterized by degeneration of articular cartilage, particularly affecting weight-bearing joints.

    • Treatments include hyaluronic acid injections and supplements such as glucosamine and chondroitin.

Matrix Composition Summary

Cartilage Types:

  • Hyaline Cartilage:

    • Type II collagen, aggrecan, chondrocytes, chondroblasts.

    • Arranged in isogenous groups, present in upper respiratory tract and articular surfaces.

  • Elastic Cartilage:

    • Type II collagen, elastic fibers, chondrocytes, chondroblasts.

    • Arranged in small isogenous groups, found in the external ear and epiglottis.

  • Fibrocartilage:

    • Type I and II collagen, chondrocytes, and fibroblasts.

    • Does not possess perichondrium, provides cushioning and tensile strength in intervertebral discs and pubic symphysis.

Hormonal Regulation of Chondrocytes

  • Hormones affecting chondrocyte activity:

    • Growth hormone, thyroxine, testosterone: Accelerate synthesis of glycosaminoglycans.

    • Cortisone, estradiol: Slow down synthesis of glycosaminoglycans.

    • Cartilage growth depends on somatropin and insulin-like growth factor (IGF).

Types of Joints Related to Cartilage

Synarthroses

  • Immovable joints composed of connective tissue, cartilage, or bone (e.g., union of the first rib to the sternum).

Diarthroses

  • Synovial joints permitting free movement, encapsulated in a two-layered capsule containing synovial fluid (hyaluronic acid, proteins).

  • Types of synovial cells:

    • Type A: Phagocytic.

    • Type B: Secretes synovial fluid.

Articular Cartilage Features

  • Hyaline cartilage without a perichondrium, present in the articular surfaces.

  • Importance in development:

    • Resists tensional and compressive forces, serving as a model for endochondral ossification.

Clinical Aspects of Cartilage

  • Osteoarthritis:

    • Degenerative joint disease characterized by the breakdown of cartilage leading to joint pain and reduced mobility. Treatment involves injections and dietary supplements that may support cartilage health.

Conclusion

  • Comprehensive understanding of cartilage histology, growth, types, and pathology is essential for connecting anatomy and clinical practices in medicine.