Cartilage

Abstract

The skeletal system of the human body is composed of bones and special connective tissue structures such as cartilage, ligaments, and tendons. To meet the mechanical requirements of movement, cartilage serves as an elastic tissue that adapts to pushing and pulling. It is composed of chondrocytes (cartilage cells) and a specialized extracellular matrix (ECM). There are three types of cartilage: hyaline cartilage (most predominant type, e.g., in the nasal septum), fibrocartilage (e.g., in intervertebral discs), and elastic cartilage (e.g., in the auricle). Cartilage is free of vessels and is nourished through diffusion by its cartilaginous membrane (perichondrium) or joint fluid (synovial fluid). In 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

  • Definition: Connective tissue surrounding elastic and extra-articular cartilage that consists of an outer fibrous layer and an inner chondrogenic layer.
    • The outer fibrous layer contains blood vessels, lymphatics, and nerves, which nourish and drain cartilage.
    • The inner chondrogenic layer contains progenitor cells, which regenerate cartilage.

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

Overview and structure of cartilage types

Overview of cartilage types

  1. Hyaline cartilage (including articular cartilage)
  2. Fibrocartilage
  3. Elastic cartilage
Hyaline cartilage Elastic cartilage Articular cartilage (specialized hyaline cartilage) Fibrocartilage
Main feature
  • Resilience to compression and friction
  • Most abundant type of cartilage
  • High resilience to compression and bending
  • Very flexible
  • High resilience to compression
  • Shock absorption
  • Uniform distribution of force over the joint surface
  • Resilience to compression and high-tensile strength
  • Hard and nonelastic
Histology
Perichondrium
  • Present
  • Present
  • Not present
  • Not present
Examples
  • Cartilaginous rings of the trachea
  • Nasal septum
  • Cartilaginous precursor of bone (primordial bones)
  • Covers articular surfaces of synovial joints

Chronic wear and tear may decrease proteoglycan synthesis, which makes articular cartilage less elastic and more friable. Degradation of cartilage results in joint space narrowing and thickening of the subchondral bone (i.e., subchondral sclerosis). Insufficient attempts of reparation result in osteophytes (also: bone spurs) and inflammation. These changes are known as osteoarthritis.

References:[1][4]

Development, growth, and regeneration

Chondrogenesis

Chondrogenesis is triggered by mechanical (e.g., load and hydrostatic pressure changes) and chemical stimuli (e.g., fibroblast growth factor FGF3, thyroid hormones, cytokines).

  1. Mesenchymal osteoprogenitor cells accumulate and differentiate into chondroblasts.
  2. Chondroblasts proliferate and produce the ECM of the cartilage and are increasingly pushed apart as a result of ECM formation (located between the chondroblasts).
  3. The cartilaginous model grows through chondroblast proliferation and ECM formation (interstitial growth).
  4. After completion of proliferation or cartilage growth, chondrocytes develop from chondroblasts and are bundled into isogenic groups.
  5. Mesenchymal cells at the periphery of the cartilage model differentiate into fibroblasts, which form a connective tissue capsule (perichondrium or periosteum).
  6. Undifferentiated cells remain at the inner layer of the perichondrium and can differentiate into chondroblasts.
  7. The cartilage can either grow through new layers of cartilage tissue produced by osteoprogenitor cells in the perichondrium (appositional growth) or by regeneration of damaged chondrocytes.

Cartilage is regenerated from undifferentiated cartilage precursor cells of the perichondrium. Therefore, articular cartilage, which does not possess a perichondrium, cannot be regenerated!

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

In late adolescence, chondrocytes are no longer capable of cell division and cartilage stops growing. Therefore, regeneration of cartilage in adults is poor.

Clinical significance

  • 1. Standring S. Gray's Anatomy: The Anatomical Basis of Clinical Practice. Elsevier Health Sciences; 2015.
  • 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 11/18/2018
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