Cartilage

Summary

Cartilage is a type of connective tissue that adapts to the pushing and pulling required for mechanical movement. It is composed of chondrocytes (cartilage cells) and a specialized extracellular matrix (ECM). There are three types of cartilage: hyaline cartilage (the most predominant type, e.g., in the nasal septum), fibrocartilage (e.g., in intervertebral discs), and elastic cartilage (e.g., in the auricle). Cartilage does not contain vessels and receives nutrients via diffusion by a cartilaginous membrane (perichondrium) or joint fluid (synovial fluid). During embryonic skeletal development, numerous bones are initially generated as cartilage models, which then ossify in utero or after birth.

Components

Cartilage cells

Extracellular matrix (ECM) of cartilage

Perichondrium

Articular cartilage and fibrocartilage are not surrounded by perichondrium.References:[1][2][3]

Overview and structure of cartilage types

Hyaline cartilage Elastic cartilage Articular cartilage (specialized hyaline cartilage) Fibrocartilage
Main features
  • Resilience to friction
  • Most abundant type of cartilage, but weakest
  • High resilience to bending
  • Very flexible
  • Shock absorption
  • Uniform distribution of force over the joint surface
  • Resilience to tensile forces
  • Hard and nonelastic; strongest type
Histology
Perichondrium
  • Present
  • Not present
Examples
  • Cartilaginous rings of the trachea
  • Nasal septum
  • Cartilaginous precursor of bone (primordial bones)
  • Covers articular surfaces of synovial joints

All types of cartilage have high resilience to compression and contain type II collagen fibers (Type II: cartwolage.)

Chronic wear and tear decreases proteoglycan synthesis, making articular cartilage less elastic and more friable. Progressive cartilage degeneration results in joint space narrowing and reactive subchondral bone sclerosis (thickening), which occurs in the initial stages of osteoarthritis. Osteophyte formation is caused by an insufficient attempt at cartilage repair and is a characteristic of moderate to severe osteoarthritis.

References:[1][4]

Development, growth, and regeneration

Chondrogenesis

Chondrogenesis is a process of development, growth, and regeneration of cartilage. It is triggered by mechanical stimuli (e.g., load and hydrostatic pressure changes) and/or chemical stimuli (e.g., fibroblast growth factors, thyroid hormones, cytokines). The following sequence of steps can occur during chondrogenesis.

  1. Mesenchymal osteoprogenitor cells accumulate and differentiate into chondroblasts.
  2. Chondroblasts proliferate and produce the ECM of the cartilage. They are increasingly pushed apart as a result of ECM formation (located between the chondroblasts).
  3. The cartilage model is formed by chondrocyte proliferation and ECM secretion (interstitial growth).
  4. Mesenchymal cells at the periphery of the cartilage model differentiate into fibroblasts, which form a connective tissue capsule (perichondrium or periosteum).
  5. Undifferentiated cells remain in the inner layer of the perichondrium and can differentiate into chondroblasts.
  6. Cartilage growth can occur via new layers of cartilage tissue produced by osteoprogenitor cells in the perichondrium (appositional growth) or via the regeneration of damaged chondrocytes.

Regeneration

In individuals with achondroplasia, chondrocytes fail to proliferate particularly at the epiphyseal growth plates of long bones, resulting in impaired endochondral ossification and short limbs. Intermembranous ossification, which occurs in the frontal and parietal bones, is not affected, resulting in a large head relative to the limbs.

Clinical significance

  • 1. Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences; 2016.
  • 2. Faculty of Biological Sciences, University of Leeds. The Cell: A Molecular Approach. https://www.histology.leeds.ac.uk/bone/cartilage.php. Accessed November 18, 2018.
  • 3. Hewitt AT, Varner HH, Silver MH, Martin GR. The role of chondronectin and cartilage proteoglycan in the attachment of chondrocytes to collagen. Prog Clin Biol Res. 1982; 110 Pt B: pp. 25–33. pmid: 7167575.
  • 4. David Eyre. Collagen of articular cartilage. Arthritis Res. 2002; 4(1): p. 30. doi: 10.1186/ar380.
last updated 06/28/2019
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